DOCBOOKS := z8530book.xml \
kernel-hacking.xml kernel-locking.xml \
- writing_usb_driver.xml networking.xml \
+ networking.xml \
filesystems.xml lsm.xml kgdb.xml \
- gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
+ libata.xml mtdnand.xml librs.xml rapidio.xml \
s390-drivers.xml scsi.xml \
- sh.xml w1.xml \
- writing_musb_glue_layer.xml
+ sh.xml w1.xml
ifeq ($(DOCBOOKS),)
+++ /dev/null
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
-
-<book id="USB-Gadget-API">
- <bookinfo>
- <title>USB Gadget API for Linux</title>
- <date>20 August 2004</date>
- <edition>20 August 2004</edition>
-
- <legalnotice>
- <para>
- This documentation is free software; you can redistribute
- it and/or modify it under the terms of the GNU General Public
- License as published by the Free Software Foundation; either
- version 2 of the License, or (at your option) any later
- version.
- </para>
-
- <para>
- This program is distributed in the hope that it will be
- useful, but WITHOUT ANY WARRANTY; without even the implied
- warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
- See the GNU General Public License for more details.
- </para>
-
- <para>
- You should have received a copy of the GNU General Public
- License along with this program; if not, write to the Free
- Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
- MA 02111-1307 USA
- </para>
-
- <para>
- For more details see the file COPYING in the source
- distribution of Linux.
- </para>
- </legalnotice>
- <copyright>
- <year>2003-2004</year>
- <holder>David Brownell</holder>
- </copyright>
-
- <author>
- <firstname>David</firstname>
- <surname>Brownell</surname>
- <affiliation>
- <address><email>dbrownell@users.sourceforge.net</email></address>
- </affiliation>
- </author>
- </bookinfo>
-
-<toc></toc>
-
-<chapter id="intro"><title>Introduction</title>
-
-<para>This document presents a Linux-USB "Gadget"
-kernel mode
-API, for use within peripherals and other USB devices
-that embed Linux.
-It provides an overview of the API structure,
-and shows how that fits into a system development project.
-This is the first such API released on Linux to address
-a number of important problems, including: </para>
-
-<itemizedlist>
- <listitem><para>Supports USB 2.0, for high speed devices which
- can stream data at several dozen megabytes per second.
- </para></listitem>
- <listitem><para>Handles devices with dozens of endpoints just as
- well as ones with just two fixed-function ones. Gadget drivers
- can be written so they're easy to port to new hardware.
- </para></listitem>
- <listitem><para>Flexible enough to expose more complex USB device
- capabilities such as multiple configurations, multiple interfaces,
- composite devices,
- and alternate interface settings.
- </para></listitem>
- <listitem><para>USB "On-The-Go" (OTG) support, in conjunction
- with updates to the Linux-USB host side.
- </para></listitem>
- <listitem><para>Sharing data structures and API models with the
- Linux-USB host side API. This helps the OTG support, and
- looks forward to more-symmetric frameworks (where the same
- I/O model is used by both host and device side drivers).
- </para></listitem>
- <listitem><para>Minimalist, so it's easier to support new device
- controller hardware. I/O processing doesn't imply large
- demands for memory or CPU resources.
- </para></listitem>
-</itemizedlist>
-
-
-<para>Most Linux developers will not be able to use this API, since they
-have USB "host" hardware in a PC, workstation, or server.
-Linux users with embedded systems are more likely to
-have USB peripheral hardware.
-To distinguish drivers running inside such hardware from the
-more familiar Linux "USB device drivers",
-which are host side proxies for the real USB devices,
-a different term is used:
-the drivers inside the peripherals are "USB gadget drivers".
-In USB protocol interactions, the device driver is the master
-(or "client driver")
-and the gadget driver is the slave (or "function driver").
-</para>
-
-<para>The gadget API resembles the host side Linux-USB API in that both
-use queues of request objects to package I/O buffers, and those requests
-may be submitted or canceled.
-They share common definitions for the standard USB
-<emphasis>Chapter 9</emphasis> messages, structures, and constants.
-Also, both APIs bind and unbind drivers to devices.
-The APIs differ in detail, since the host side's current
-URB framework exposes a number of implementation details
-and assumptions that are inappropriate for a gadget API.
-While the model for control transfers and configuration
-management is necessarily different (one side is a hardware-neutral master,
-the other is a hardware-aware slave), the endpoint I/0 API used here
-should also be usable for an overhead-reduced host side API.
-</para>
-
-</chapter>
-
-<chapter id="structure"><title>Structure of Gadget Drivers</title>
-
-<para>A system running inside a USB peripheral
-normally has at least three layers inside the kernel to handle
-USB protocol processing, and may have additional layers in
-user space code.
-The "gadget" API is used by the middle layer to interact
-with the lowest level (which directly handles hardware).
-</para>
-
-<para>In Linux, from the bottom up, these layers are:
-</para>
-
-<variablelist>
-
- <varlistentry>
- <term><emphasis>USB Controller Driver</emphasis></term>
-
- <listitem>
- <para>This is the lowest software level.
- It is the only layer that talks to hardware,
- through registers, fifos, dma, irqs, and the like.
- The <filename><linux/usb/gadget.h></filename> API abstracts
- the peripheral controller endpoint hardware.
- That hardware is exposed through endpoint objects, which accept
- streams of IN/OUT buffers, and through callbacks that interact
- with gadget drivers.
- Since normal USB devices only have one upstream
- port, they only have one of these drivers.
- The controller driver can support any number of different
- gadget drivers, but only one of them can be used at a time.
- </para>
-
- <para>Examples of such controller hardware include
- the PCI-based NetChip 2280 USB 2.0 high speed controller,
- the SA-11x0 or PXA-25x UDC (found within many PDAs),
- and a variety of other products.
- </para>
-
- </listitem></varlistentry>
-
- <varlistentry>
- <term><emphasis>Gadget Driver</emphasis></term>
-
- <listitem>
- <para>The lower boundary of this driver implements hardware-neutral
- USB functions, using calls to the controller driver.
- Because such hardware varies widely in capabilities and restrictions,
- and is used in embedded environments where space is at a premium,
- the gadget driver is often configured at compile time
- to work with endpoints supported by one particular controller.
- Gadget drivers may be portable to several different controllers,
- using conditional compilation.
- (Recent kernels substantially simplify the work involved in
- supporting new hardware, by <emphasis>autoconfiguring</emphasis>
- endpoints automatically for many bulk-oriented drivers.)
- Gadget driver responsibilities include:
- </para>
- <itemizedlist>
- <listitem><para>handling setup requests (ep0 protocol responses)
- possibly including class-specific functionality
- </para></listitem>
- <listitem><para>returning configuration and string descriptors
- </para></listitem>
- <listitem><para>(re)setting configurations and interface
- altsettings, including enabling and configuring endpoints
- </para></listitem>
- <listitem><para>handling life cycle events, such as managing
- bindings to hardware,
- USB suspend/resume, remote wakeup,
- and disconnection from the USB host.
- </para></listitem>
- <listitem><para>managing IN and OUT transfers on all currently
- enabled endpoints
- </para></listitem>
- </itemizedlist>
-
- <para>
- Such drivers may be modules of proprietary code, although
- that approach is discouraged in the Linux community.
- </para>
- </listitem></varlistentry>
-
- <varlistentry>
- <term><emphasis>Upper Level</emphasis></term>
-
- <listitem>
- <para>Most gadget drivers have an upper boundary that connects
- to some Linux driver or framework in Linux.
- Through that boundary flows the data which the gadget driver
- produces and/or consumes through protocol transfers over USB.
- Examples include:
- </para>
- <itemizedlist>
- <listitem><para>user mode code, using generic (gadgetfs)
- or application specific files in
- <filename>/dev</filename>
- </para></listitem>
- <listitem><para>networking subsystem (for network gadgets,
- like the CDC Ethernet Model gadget driver)
- </para></listitem>
- <listitem><para>data capture drivers, perhaps video4Linux or
- a scanner driver; or test and measurement hardware.
- </para></listitem>
- <listitem><para>input subsystem (for HID gadgets)
- </para></listitem>
- <listitem><para>sound subsystem (for audio gadgets)
- </para></listitem>
- <listitem><para>file system (for PTP gadgets)
- </para></listitem>
- <listitem><para>block i/o subsystem (for usb-storage gadgets)
- </para></listitem>
- <listitem><para>... and more </para></listitem>
- </itemizedlist>
- </listitem></varlistentry>
-
- <varlistentry>
- <term><emphasis>Additional Layers</emphasis></term>
-
- <listitem>
- <para>Other layers may exist.
- These could include kernel layers, such as network protocol stacks,
- as well as user mode applications building on standard POSIX
- system call APIs such as
- <emphasis>open()</emphasis>, <emphasis>close()</emphasis>,
- <emphasis>read()</emphasis> and <emphasis>write()</emphasis>.
- On newer systems, POSIX Async I/O calls may be an option.
- Such user mode code will not necessarily be subject to
- the GNU General Public License (GPL).
- </para>
- </listitem></varlistentry>
-
-
-</variablelist>
-
-<para>OTG-capable systems will also need to include a standard Linux-USB
-host side stack,
-with <emphasis>usbcore</emphasis>,
-one or more <emphasis>Host Controller Drivers</emphasis> (HCDs),
-<emphasis>USB Device Drivers</emphasis> to support
-the OTG "Targeted Peripheral List",
-and so forth.
-There will also be an <emphasis>OTG Controller Driver</emphasis>,
-which is visible to gadget and device driver developers only indirectly.
-That helps the host and device side USB controllers implement the
-two new OTG protocols (HNP and SRP).
-Roles switch (host to peripheral, or vice versa) using HNP
-during USB suspend processing, and SRP can be viewed as a
-more battery-friendly kind of device wakeup protocol.
-</para>
-
-<para>Over time, reusable utilities are evolving to help make some
-gadget driver tasks simpler.
-For example, building configuration descriptors from vectors of
-descriptors for the configurations interfaces and endpoints is
-now automated, and many drivers now use autoconfiguration to
-choose hardware endpoints and initialize their descriptors.
-
-A potential example of particular interest
-is code implementing standard USB-IF protocols for
-HID, networking, storage, or audio classes.
-Some developers are interested in KDB or KGDB hooks, to let
-target hardware be remotely debugged.
-Most such USB protocol code doesn't need to be hardware-specific,
-any more than network protocols like X11, HTTP, or NFS are.
-Such gadget-side interface drivers should eventually be combined,
-to implement composite devices.
-</para>
-
-</chapter>
-
-
-<chapter id="api"><title>Kernel Mode Gadget API</title>
-
-<para>Gadget drivers declare themselves through a
-<emphasis>struct usb_gadget_driver</emphasis>, which is responsible for
-most parts of enumeration for a <emphasis>struct usb_gadget</emphasis>.
-The response to a set_configuration usually involves
-enabling one or more of the <emphasis>struct usb_ep</emphasis> objects
-exposed by the gadget, and submitting one or more
-<emphasis>struct usb_request</emphasis> buffers to transfer data.
-Understand those four data types, and their operations, and
-you will understand how this API works.
-</para>
-
-<note><title>Incomplete Data Type Descriptions</title>
-
-<para>This documentation was prepared using the standard Linux
-kernel <filename>docproc</filename> tool, which turns text
-and in-code comments into SGML DocBook and then into usable
-formats such as HTML or PDF.
-Other than the "Chapter 9" data types, most of the significant
-data types and functions are described here.
-</para>
-
-<para>However, docproc does not understand all the C constructs
-that are used, so some relevant information is likely omitted from
-what you are reading.
-One example of such information is endpoint autoconfiguration.
-You'll have to read the header file, and use example source
-code (such as that for "Gadget Zero"), to fully understand the API.
-</para>
-
-<para>The part of the API implementing some basic
-driver capabilities is specific to the version of the
-Linux kernel that's in use.
-The 2.6 kernel includes a <emphasis>driver model</emphasis>
-framework that has no analogue on earlier kernels;
-so those parts of the gadget API are not fully portable.
-(They are implemented on 2.4 kernels, but in a different way.)
-The driver model state is another part of this API that is
-ignored by the kerneldoc tools.
-</para>
-</note>
-
-<para>The core API does not expose
-every possible hardware feature, only the most widely available ones.
-There are significant hardware features, such as device-to-device DMA
-(without temporary storage in a memory buffer)
-that would be added using hardware-specific APIs.
-</para>
-
-<para>This API allows drivers to use conditional compilation to handle
-endpoint capabilities of different hardware, but doesn't require that.
-Hardware tends to have arbitrary restrictions, relating to
-transfer types, addressing, packet sizes, buffering, and availability.
-As a rule, such differences only matter for "endpoint zero" logic
-that handles device configuration and management.
-The API supports limited run-time
-detection of capabilities, through naming conventions for endpoints.
-Many drivers will be able to at least partially autoconfigure
-themselves.
-In particular, driver init sections will often have endpoint
-autoconfiguration logic that scans the hardware's list of endpoints
-to find ones matching the driver requirements
-(relying on those conventions), to eliminate some of the most
-common reasons for conditional compilation.
-</para>
-
-<para>Like the Linux-USB host side API, this API exposes
-the "chunky" nature of USB messages: I/O requests are in terms
-of one or more "packets", and packet boundaries are visible to drivers.
-Compared to RS-232 serial protocols, USB resembles
-synchronous protocols like HDLC
-(N bytes per frame, multipoint addressing, host as the primary
-station and devices as secondary stations)
-more than asynchronous ones
-(tty style: 8 data bits per frame, no parity, one stop bit).
-So for example the controller drivers won't buffer
-two single byte writes into a single two-byte USB IN packet,
-although gadget drivers may do so when they implement
-protocols where packet boundaries (and "short packets")
-are not significant.
-</para>
-
-<sect1 id="lifecycle"><title>Driver Life Cycle</title>
-
-<para>Gadget drivers make endpoint I/O requests to hardware without
-needing to know many details of the hardware, but driver
-setup/configuration code needs to handle some differences.
-Use the API like this:
-</para>
-
-<orderedlist numeration='arabic'>
-
-<listitem><para>Register a driver for the particular device side
-usb controller hardware,
-such as the net2280 on PCI (USB 2.0),
-sa11x0 or pxa25x as found in Linux PDAs,
-and so on.
-At this point the device is logically in the USB ch9 initial state
-("attached"), drawing no power and not usable
-(since it does not yet support enumeration).
-Any host should not see the device, since it's not
-activated the data line pullup used by the host to
-detect a device, even if VBUS power is available.
-</para></listitem>
-
-<listitem><para>Register a gadget driver that implements some higher level
-device function. That will then bind() to a usb_gadget, which
-activates the data line pullup sometime after detecting VBUS.
-</para></listitem>
-
-<listitem><para>The hardware driver can now start enumerating.
-The steps it handles are to accept USB power and set_address requests.
-Other steps are handled by the gadget driver.
-If the gadget driver module is unloaded before the host starts to
-enumerate, steps before step 7 are skipped.
-</para></listitem>
-
-<listitem><para>The gadget driver's setup() call returns usb descriptors,
-based both on what the bus interface hardware provides and on the
-functionality being implemented.
-That can involve alternate settings or configurations,
-unless the hardware prevents such operation.
-For OTG devices, each configuration descriptor includes
-an OTG descriptor.
-</para></listitem>
-
-<listitem><para>The gadget driver handles the last step of enumeration,
-when the USB host issues a set_configuration call.
-It enables all endpoints used in that configuration,
-with all interfaces in their default settings.
-That involves using a list of the hardware's endpoints, enabling each
-endpoint according to its descriptor.
-It may also involve using <function>usb_gadget_vbus_draw</function>
-to let more power be drawn from VBUS, as allowed by that configuration.
-For OTG devices, setting a configuration may also involve reporting
-HNP capabilities through a user interface.
-</para></listitem>
-
-<listitem><para>Do real work and perform data transfers, possibly involving
-changes to interface settings or switching to new configurations, until the
-device is disconnect()ed from the host.
-Queue any number of transfer requests to each endpoint.
-It may be suspended and resumed several times before being disconnected.
-On disconnect, the drivers go back to step 3 (above).
-</para></listitem>
-
-<listitem><para>When the gadget driver module is being unloaded,
-the driver unbind() callback is issued. That lets the controller
-driver be unloaded.
-</para></listitem>
-
-</orderedlist>
-
-<para>Drivers will normally be arranged so that just loading the
-gadget driver module (or statically linking it into a Linux kernel)
-allows the peripheral device to be enumerated, but some drivers
-will defer enumeration until some higher level component (like
-a user mode daemon) enables it.
-Note that at this lowest level there are no policies about how
-ep0 configuration logic is implemented,
-except that it should obey USB specifications.
-Such issues are in the domain of gadget drivers,
-including knowing about implementation constraints
-imposed by some USB controllers
-or understanding that composite devices might happen to
-be built by integrating reusable components.
-</para>
-
-<para>Note that the lifecycle above can be slightly different
-for OTG devices.
-Other than providing an additional OTG descriptor in each
-configuration, only the HNP-related differences are particularly
-visible to driver code.
-They involve reporting requirements during the SET_CONFIGURATION
-request, and the option to invoke HNP during some suspend callbacks.
-Also, SRP changes the semantics of
-<function>usb_gadget_wakeup</function>
-slightly.
-</para>
-
-</sect1>
-
-<sect1 id="ch9"><title>USB 2.0 Chapter 9 Types and Constants</title>
-
-<para>Gadget drivers
-rely on common USB structures and constants
-defined in the
-<filename><linux/usb/ch9.h></filename>
-header file, which is standard in Linux 2.6 kernels.
-These are the same types and constants used by host
-side drivers (and usbcore).
-</para>
-
-!Iinclude/linux/usb/ch9.h
-</sect1>
-
-<sect1 id="core"><title>Core Objects and Methods</title>
-
-<para>These are declared in
-<filename><linux/usb/gadget.h></filename>,
-and are used by gadget drivers to interact with
-USB peripheral controller drivers.
-</para>
-
- <!-- yeech, this is ugly in nsgmls PDF output.
-
- the PDF bookmark and refentry output nesting is wrong,
- and the member/argument documentation indents ugly.
-
- plus something (docproc?) adds whitespace before the
- descriptive paragraph text, so it can't line up right
- unless the explanations are trivial.
- -->
-
-!Iinclude/linux/usb/gadget.h
-</sect1>
-
-<sect1 id="utils"><title>Optional Utilities</title>
-
-<para>The core API is sufficient for writing a USB Gadget Driver,
-but some optional utilities are provided to simplify common tasks.
-These utilities include endpoint autoconfiguration.
-</para>
-
-!Edrivers/usb/gadget/usbstring.c
-!Edrivers/usb/gadget/config.c
-<!-- !Edrivers/usb/gadget/epautoconf.c -->
-</sect1>
-
-<sect1 id="composite"><title>Composite Device Framework</title>
-
-<para>The core API is sufficient for writing drivers for composite
-USB devices (with more than one function in a given configuration),
-and also multi-configuration devices (also more than one function,
-but not necessarily sharing a given configuration).
-There is however an optional framework which makes it easier to
-reuse and combine functions.
-</para>
-
-<para>Devices using this framework provide a <emphasis>struct
-usb_composite_driver</emphasis>, which in turn provides one or
-more <emphasis>struct usb_configuration</emphasis> instances.
-Each such configuration includes at least one
-<emphasis>struct usb_function</emphasis>, which packages a user
-visible role such as "network link" or "mass storage device".
-Management functions may also exist, such as "Device Firmware
-Upgrade".
-</para>
-
-!Iinclude/linux/usb/composite.h
-!Edrivers/usb/gadget/composite.c
-
-</sect1>
-
-<sect1 id="functions"><title>Composite Device Functions</title>
-
-<para>At this writing, a few of the current gadget drivers have
-been converted to this framework.
-Near-term plans include converting all of them, except for "gadgetfs".
-</para>
-
-!Edrivers/usb/gadget/function/f_acm.c
-!Edrivers/usb/gadget/function/f_ecm.c
-!Edrivers/usb/gadget/function/f_subset.c
-!Edrivers/usb/gadget/function/f_obex.c
-!Edrivers/usb/gadget/function/f_serial.c
-
-</sect1>
-
-
-</chapter>
-
-<chapter id="controllers"><title>Peripheral Controller Drivers</title>
-
-<para>The first hardware supporting this API was the NetChip 2280
-controller, which supports USB 2.0 high speed and is based on PCI.
-This is the <filename>net2280</filename> driver module.
-The driver supports Linux kernel versions 2.4 and 2.6;
-contact NetChip Technologies for development boards and product
-information.
-</para>
-
-<para>Other hardware working in the "gadget" framework includes:
-Intel's PXA 25x and IXP42x series processors
-(<filename>pxa2xx_udc</filename>),
-Toshiba TC86c001 "Goku-S" (<filename>goku_udc</filename>),
-Renesas SH7705/7727 (<filename>sh_udc</filename>),
-MediaQ 11xx (<filename>mq11xx_udc</filename>),
-Hynix HMS30C7202 (<filename>h7202_udc</filename>),
-National 9303/4 (<filename>n9604_udc</filename>),
-Texas Instruments OMAP (<filename>omap_udc</filename>),
-Sharp LH7A40x (<filename>lh7a40x_udc</filename>),
-and more.
-Most of those are full speed controllers.
-</para>
-
-<para>At this writing, there are people at work on drivers in
-this framework for several other USB device controllers,
-with plans to make many of them be widely available.
-</para>
-
-<!-- !Edrivers/usb/gadget/net2280.c -->
-
-<para>A partial USB simulator,
-the <filename>dummy_hcd</filename> driver, is available.
-It can act like a net2280, a pxa25x, or an sa11x0 in terms
-of available endpoints and device speeds; and it simulates
-control, bulk, and to some extent interrupt transfers.
-That lets you develop some parts of a gadget driver on a normal PC,
-without any special hardware, and perhaps with the assistance
-of tools such as GDB running with User Mode Linux.
-At least one person has expressed interest in adapting that
-approach, hooking it up to a simulator for a microcontroller.
-Such simulators can help debug subsystems where the runtime hardware
-is unfriendly to software development, or is not yet available.
-</para>
-
-<para>Support for other controllers is expected to be developed
-and contributed
-over time, as this driver framework evolves.
-</para>
-
-</chapter>
-
-<chapter id="gadget"><title>Gadget Drivers</title>
-
-<para>In addition to <emphasis>Gadget Zero</emphasis>
-(used primarily for testing and development with drivers
-for usb controller hardware), other gadget drivers exist.
-</para>
-
-<para>There's an <emphasis>ethernet</emphasis> gadget
-driver, which implements one of the most useful
-<emphasis>Communications Device Class</emphasis> (CDC) models.
-One of the standards for cable modem interoperability even
-specifies the use of this ethernet model as one of two
-mandatory options.
-Gadgets using this code look to a USB host as if they're
-an Ethernet adapter.
-It provides access to a network where the gadget's CPU is one host,
-which could easily be bridging, routing, or firewalling
-access to other networks.
-Since some hardware can't fully implement the CDC Ethernet
-requirements, this driver also implements a "good parts only"
-subset of CDC Ethernet.
-(That subset doesn't advertise itself as CDC Ethernet,
-to avoid creating problems.)
-</para>
-
-<para>Support for Microsoft's <emphasis>RNDIS</emphasis>
-protocol has been contributed by Pengutronix and Auerswald GmbH.
-This is like CDC Ethernet, but it runs on more slightly USB hardware
-(but less than the CDC subset).
-However, its main claim to fame is being able to connect directly to
-recent versions of Windows, using drivers that Microsoft bundles
-and supports, making it much simpler to network with Windows.
-</para>
-
-<para>There is also support for user mode gadget drivers,
-using <emphasis>gadgetfs</emphasis>.
-This provides a <emphasis>User Mode API</emphasis> that presents
-each endpoint as a single file descriptor. I/O is done using
-normal <emphasis>read()</emphasis> and <emphasis>read()</emphasis> calls.
-Familiar tools like GDB and pthreads can be used to
-develop and debug user mode drivers, so that once a robust
-controller driver is available many applications for it
-won't require new kernel mode software.
-Linux 2.6 <emphasis>Async I/O (AIO)</emphasis>
-support is available, so that user mode software
-can stream data with only slightly more overhead
-than a kernel driver.
-</para>
-
-<para>There's a USB Mass Storage class driver, which provides
-a different solution for interoperability with systems such
-as MS-Windows and MacOS.
-That <emphasis>Mass Storage</emphasis> driver uses a
-file or block device as backing store for a drive,
-like the <filename>loop</filename> driver.
-The USB host uses the BBB, CB, or CBI versions of the mass
-storage class specification, using transparent SCSI commands
-to access the data from the backing store.
-</para>
-
-<para>There's a "serial line" driver, useful for TTY style
-operation over USB.
-The latest version of that driver supports CDC ACM style
-operation, like a USB modem, and so on most hardware it can
-interoperate easily with MS-Windows.
-One interesting use of that driver is in boot firmware (like a BIOS),
-which can sometimes use that model with very small systems without
-real serial lines.
-</para>
-
-<para>Support for other kinds of gadget is expected to
-be developed and contributed
-over time, as this driver framework evolves.
-</para>
-
-</chapter>
-
-<chapter id="otg"><title>USB On-The-GO (OTG)</title>
-
-<para>USB OTG support on Linux 2.6 was initially developed
-by Texas Instruments for
-<ulink url="http://www.omap.com">OMAP</ulink> 16xx and 17xx
-series processors.
-Other OTG systems should work in similar ways, but the
-hardware level details could be very different.
-</para>
-
-<para>Systems need specialized hardware support to implement OTG,
-notably including a special <emphasis>Mini-AB</emphasis> jack
-and associated transceiver to support <emphasis>Dual-Role</emphasis>
-operation:
-they can act either as a host, using the standard
-Linux-USB host side driver stack,
-or as a peripheral, using this "gadget" framework.
-To do that, the system software relies on small additions
-to those programming interfaces,
-and on a new internal component (here called an "OTG Controller")
-affecting which driver stack connects to the OTG port.
-In each role, the system can re-use the existing pool of
-hardware-neutral drivers, layered on top of the controller
-driver interfaces (<emphasis>usb_bus</emphasis> or
-<emphasis>usb_gadget</emphasis>).
-Such drivers need at most minor changes, and most of the calls
-added to support OTG can also benefit non-OTG products.
-</para>
-
-<itemizedlist>
- <listitem><para>Gadget drivers test the <emphasis>is_otg</emphasis>
- flag, and use it to determine whether or not to include
- an OTG descriptor in each of their configurations.
- </para></listitem>
- <listitem><para>Gadget drivers may need changes to support the
- two new OTG protocols, exposed in new gadget attributes
- such as <emphasis>b_hnp_enable</emphasis> flag.
- HNP support should be reported through a user interface
- (two LEDs could suffice), and is triggered in some cases
- when the host suspends the peripheral.
- SRP support can be user-initiated just like remote wakeup,
- probably by pressing the same button.
- </para></listitem>
- <listitem><para>On the host side, USB device drivers need
- to be taught to trigger HNP at appropriate moments, using
- <function>usb_suspend_device()</function>.
- That also conserves battery power, which is useful even
- for non-OTG configurations.
- </para></listitem>
- <listitem><para>Also on the host side, a driver must support the
- OTG "Targeted Peripheral List". That's just a whitelist,
- used to reject peripherals not supported with a given
- Linux OTG host.
- <emphasis>This whitelist is product-specific;
- each product must modify <filename>otg_whitelist.h</filename>
- to match its interoperability specification.
- </emphasis>
- </para>
- <para>Non-OTG Linux hosts, like PCs and workstations,
- normally have some solution for adding drivers, so that
- peripherals that aren't recognized can eventually be supported.
- That approach is unreasonable for consumer products that may
- never have their firmware upgraded, and where it's usually
- unrealistic to expect traditional PC/workstation/server kinds
- of support model to work.
- For example, it's often impractical to change device firmware
- once the product has been distributed, so driver bugs can't
- normally be fixed if they're found after shipment.
- </para></listitem>
-</itemizedlist>
-
-<para>
-Additional changes are needed below those hardware-neutral
-<emphasis>usb_bus</emphasis> and <emphasis>usb_gadget</emphasis>
-driver interfaces; those aren't discussed here in any detail.
-Those affect the hardware-specific code for each USB Host or Peripheral
-controller, and how the HCD initializes (since OTG can be active only
-on a single port).
-They also involve what may be called an <emphasis>OTG Controller
-Driver</emphasis>, managing the OTG transceiver and the OTG state
-machine logic as well as much of the root hub behavior for the
-OTG port.
-The OTG controller driver needs to activate and deactivate USB
-controllers depending on the relevant device role.
-Some related changes were needed inside usbcore, so that it
-can identify OTG-capable devices and respond appropriately
-to HNP or SRP protocols.
-</para>
-
-</chapter>
-
-</book>
-<!--
- vim:syntax=sgml:sw=4
--->
+++ /dev/null
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
-
-<book id="Writing-MUSB-Glue-Layer">
- <bookinfo>
- <title>Writing an MUSB Glue Layer</title>
-
- <authorgroup>
- <author>
- <firstname>Apelete</firstname>
- <surname>Seketeli</surname>
- <affiliation>
- <address>
- <email>apelete at seketeli.net</email>
- </address>
- </affiliation>
- </author>
- </authorgroup>
-
- <copyright>
- <year>2014</year>
- <holder>Apelete Seketeli</holder>
- </copyright>
-
- <legalnotice>
- <para>
- This documentation is free software; you can redistribute it
- and/or modify it under the terms of the GNU General Public
- License as published by the Free Software Foundation; either
- version 2 of the License, or (at your option) any later version.
- </para>
-
- <para>
- This documentation is distributed in the hope that it will be
- useful, but WITHOUT ANY WARRANTY; without even the implied
- warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
- See the GNU General Public License for more details.
- </para>
-
- <para>
- You should have received a copy of the GNU General Public License
- along with this documentation; if not, write to the Free Software
- Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
- 02111-1307 USA
- </para>
-
- <para>
- For more details see the file COPYING in the Linux kernel source
- tree.
- </para>
- </legalnotice>
- </bookinfo>
-
-<toc></toc>
-
- <chapter id="introduction">
- <title>Introduction</title>
- <para>
- The Linux MUSB subsystem is part of the larger Linux USB
- subsystem. It provides support for embedded USB Device Controllers
- (UDC) that do not use Universal Host Controller Interface (UHCI)
- or Open Host Controller Interface (OHCI).
- </para>
- <para>
- Instead, these embedded UDC rely on the USB On-the-Go (OTG)
- specification which they implement at least partially. The silicon
- reference design used in most cases is the Multipoint USB
- Highspeed Dual-Role Controller (MUSB HDRC) found in the Mentor
- Graphics Inventra™ design.
- </para>
- <para>
- As a self-taught exercise I have written an MUSB glue layer for
- the Ingenic JZ4740 SoC, modelled after the many MUSB glue layers
- in the kernel source tree. This layer can be found at
- drivers/usb/musb/jz4740.c. In this documentation I will walk
- through the basics of the jz4740.c glue layer, explaining the
- different pieces and what needs to be done in order to write your
- own device glue layer.
- </para>
- </chapter>
-
- <chapter id="linux-musb-basics">
- <title>Linux MUSB Basics</title>
- <para>
- To get started on the topic, please read USB On-the-Go Basics (see
- Resources) which provides an introduction of USB OTG operation at
- the hardware level. A couple of wiki pages by Texas Instruments
- and Analog Devices also provide an overview of the Linux kernel
- MUSB configuration, albeit focused on some specific devices
- provided by these companies. Finally, getting acquainted with the
- USB specification at USB home page may come in handy, with
- practical instance provided through the Writing USB Device Drivers
- documentation (again, see Resources).
- </para>
- <para>
- Linux USB stack is a layered architecture in which the MUSB
- controller hardware sits at the lowest. The MUSB controller driver
- abstract the MUSB controller hardware to the Linux USB stack.
- </para>
- <programlisting>
- ------------------------
- | | <------- drivers/usb/gadget
- | Linux USB Core Stack | <------- drivers/usb/host
- | | <------- drivers/usb/core
- ------------------------
- ⬍
- --------------------------
- | | <------ drivers/usb/musb/musb_gadget.c
- | MUSB Controller driver | <------ drivers/usb/musb/musb_host.c
- | | <------ drivers/usb/musb/musb_core.c
- --------------------------
- ⬍
- ---------------------------------
- | MUSB Platform Specific Driver |
- | | <-- drivers/usb/musb/jz4740.c
- | aka "Glue Layer" |
- ---------------------------------
- ⬍
- ---------------------------------
- | MUSB Controller Hardware |
- ---------------------------------
- </programlisting>
- <para>
- As outlined above, the glue layer is actually the platform
- specific code sitting in between the controller driver and the
- controller hardware.
- </para>
- <para>
- Just like a Linux USB driver needs to register itself with the
- Linux USB subsystem, the MUSB glue layer needs first to register
- itself with the MUSB controller driver. This will allow the
- controller driver to know about which device the glue layer
- supports and which functions to call when a supported device is
- detected or released; remember we are talking about an embedded
- controller chip here, so no insertion or removal at run-time.
- </para>
- <para>
- All of this information is passed to the MUSB controller driver
- through a platform_driver structure defined in the glue layer as:
- </para>
- <programlisting linenumbering="numbered">
-static struct platform_driver jz4740_driver = {
- .probe = jz4740_probe,
- .remove = jz4740_remove,
- .driver = {
- .name = "musb-jz4740",
- },
-};
- </programlisting>
- <para>
- The probe and remove function pointers are called when a matching
- device is detected and, respectively, released. The name string
- describes the device supported by this glue layer. In the current
- case it matches a platform_device structure declared in
- arch/mips/jz4740/platform.c. Note that we are not using device
- tree bindings here.
- </para>
- <para>
- In order to register itself to the controller driver, the glue
- layer goes through a few steps, basically allocating the
- controller hardware resources and initialising a couple of
- circuits. To do so, it needs to keep track of the information used
- throughout these steps. This is done by defining a private
- jz4740_glue structure:
- </para>
- <programlisting linenumbering="numbered">
-struct jz4740_glue {
- struct device *dev;
- struct platform_device *musb;
- struct clk *clk;
-};
- </programlisting>
- <para>
- The dev and musb members are both device structure variables. The
- first one holds generic information about the device, since it's
- the basic device structure, and the latter holds information more
- closely related to the subsystem the device is registered to. The
- clk variable keeps information related to the device clock
- operation.
- </para>
- <para>
- Let's go through the steps of the probe function that leads the
- glue layer to register itself to the controller driver.
- </para>
- <para>
- N.B.: For the sake of readability each function will be split in
- logical parts, each part being shown as if it was independent from
- the others.
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_probe(struct platform_device *pdev)
-{
- struct platform_device *musb;
- struct jz4740_glue *glue;
- struct clk *clk;
- int ret;
-
- glue = devm_kzalloc(&pdev->dev, sizeof(*glue), GFP_KERNEL);
- if (!glue)
- return -ENOMEM;
-
- musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO);
- if (!musb) {
- dev_err(&pdev->dev, "failed to allocate musb device\n");
- return -ENOMEM;
- }
-
- clk = devm_clk_get(&pdev->dev, "udc");
- if (IS_ERR(clk)) {
- dev_err(&pdev->dev, "failed to get clock\n");
- ret = PTR_ERR(clk);
- goto err_platform_device_put;
- }
-
- ret = clk_prepare_enable(clk);
- if (ret) {
- dev_err(&pdev->dev, "failed to enable clock\n");
- goto err_platform_device_put;
- }
-
- musb->dev.parent = &pdev->dev;
-
- glue->dev = &pdev->dev;
- glue->musb = musb;
- glue->clk = clk;
-
- return 0;
-
-err_platform_device_put:
- platform_device_put(musb);
- return ret;
-}
- </programlisting>
- <para>
- The first few lines of the probe function allocate and assign the
- glue, musb and clk variables. The GFP_KERNEL flag (line 8) allows
- the allocation process to sleep and wait for memory, thus being
- usable in a blocking situation. The PLATFORM_DEVID_AUTO flag (line
- 12) allows automatic allocation and management of device IDs in
- order to avoid device namespace collisions with explicit IDs. With
- devm_clk_get() (line 18) the glue layer allocates the clock -- the
- <literal>devm_</literal> prefix indicates that clk_get() is
- managed: it automatically frees the allocated clock resource data
- when the device is released -- and enable it.
- </para>
- <para>
- Then comes the registration steps:
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_probe(struct platform_device *pdev)
-{
- struct musb_hdrc_platform_data *pdata = &jz4740_musb_platform_data;
-
- pdata->platform_ops = &jz4740_musb_ops;
-
- platform_set_drvdata(pdev, glue);
-
- ret = platform_device_add_resources(musb, pdev->resource,
- pdev->num_resources);
- if (ret) {
- dev_err(&pdev->dev, "failed to add resources\n");
- goto err_clk_disable;
- }
-
- ret = platform_device_add_data(musb, pdata, sizeof(*pdata));
- if (ret) {
- dev_err(&pdev->dev, "failed to add platform_data\n");
- goto err_clk_disable;
- }
-
- return 0;
-
-err_clk_disable:
- clk_disable_unprepare(clk);
-err_platform_device_put:
- platform_device_put(musb);
- return ret;
-}
- </programlisting>
- <para>
- The first step is to pass the device data privately held by the
- glue layer on to the controller driver through
- platform_set_drvdata() (line 7). Next is passing on the device
- resources information, also privately held at that point, through
- platform_device_add_resources() (line 9).
- </para>
- <para>
- Finally comes passing on the platform specific data to the
- controller driver (line 16). Platform data will be discussed in
- <link linkend="device-platform-data">Chapter 4</link>, but here
- we are looking at the platform_ops function pointer (line 5) in
- musb_hdrc_platform_data structure (line 3). This function
- pointer allows the MUSB controller driver to know which function
- to call for device operation:
- </para>
- <programlisting linenumbering="numbered">
-static const struct musb_platform_ops jz4740_musb_ops = {
- .init = jz4740_musb_init,
- .exit = jz4740_musb_exit,
-};
- </programlisting>
- <para>
- Here we have the minimal case where only init and exit functions
- are called by the controller driver when needed. Fact is the
- JZ4740 MUSB controller is a basic controller, lacking some
- features found in other controllers, otherwise we may also have
- pointers to a few other functions like a power management function
- or a function to switch between OTG and non-OTG modes, for
- instance.
- </para>
- <para>
- At that point of the registration process, the controller driver
- actually calls the init function:
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_musb_init(struct musb *musb)
-{
- musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
- if (!musb->xceiv) {
- pr_err("HS UDC: no transceiver configured\n");
- return -ENODEV;
- }
-
- /* Silicon does not implement ConfigData register.
- * Set dyn_fifo to avoid reading EP config from hardware.
- */
- musb->dyn_fifo = true;
-
- musb->isr = jz4740_musb_interrupt;
-
- return 0;
-}
- </programlisting>
- <para>
- The goal of jz4740_musb_init() is to get hold of the transceiver
- driver data of the MUSB controller hardware and pass it on to the
- MUSB controller driver, as usual. The transceiver is the circuitry
- inside the controller hardware responsible for sending/receiving
- the USB data. Since it is an implementation of the physical layer
- of the OSI model, the transceiver is also referred to as PHY.
- </para>
- <para>
- Getting hold of the MUSB PHY driver data is done with
- usb_get_phy() which returns a pointer to the structure
- containing the driver instance data. The next couple of
- instructions (line 12 and 14) are used as a quirk and to setup
- IRQ handling respectively. Quirks and IRQ handling will be
- discussed later in <link linkend="device-quirks">Chapter
- 5</link> and <link linkend="handling-irqs">Chapter 3</link>.
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_musb_exit(struct musb *musb)
-{
- usb_put_phy(musb->xceiv);
-
- return 0;
-}
- </programlisting>
- <para>
- Acting as the counterpart of init, the exit function releases the
- MUSB PHY driver when the controller hardware itself is about to be
- released.
- </para>
- <para>
- Again, note that init and exit are fairly simple in this case due
- to the basic set of features of the JZ4740 controller hardware.
- When writing an musb glue layer for a more complex controller
- hardware, you might need to take care of more processing in those
- two functions.
- </para>
- <para>
- Returning from the init function, the MUSB controller driver jumps
- back into the probe function:
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_probe(struct platform_device *pdev)
-{
- ret = platform_device_add(musb);
- if (ret) {
- dev_err(&pdev->dev, "failed to register musb device\n");
- goto err_clk_disable;
- }
-
- return 0;
-
-err_clk_disable:
- clk_disable_unprepare(clk);
-err_platform_device_put:
- platform_device_put(musb);
- return ret;
-}
- </programlisting>
- <para>
- This is the last part of the device registration process where the
- glue layer adds the controller hardware device to Linux kernel
- device hierarchy: at this stage, all known information about the
- device is passed on to the Linux USB core stack.
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_remove(struct platform_device *pdev)
-{
- struct jz4740_glue *glue = platform_get_drvdata(pdev);
-
- platform_device_unregister(glue->musb);
- clk_disable_unprepare(glue->clk);
-
- return 0;
-}
- </programlisting>
- <para>
- Acting as the counterpart of probe, the remove function unregister
- the MUSB controller hardware (line 5) and disable the clock (line
- 6), allowing it to be gated.
- </para>
- </chapter>
-
- <chapter id="handling-irqs">
- <title>Handling IRQs</title>
- <para>
- Additionally to the MUSB controller hardware basic setup and
- registration, the glue layer is also responsible for handling the
- IRQs:
- </para>
- <programlisting linenumbering="numbered">
-static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
-{
- unsigned long flags;
- irqreturn_t retval = IRQ_NONE;
- struct musb *musb = __hci;
-
- spin_lock_irqsave(&musb->lock, flags);
-
- musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
- musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
- musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
-
- /*
- * The controller is gadget only, the state of the host mode IRQ bits is
- * undefined. Mask them to make sure that the musb driver core will
- * never see them set
- */
- musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
- MUSB_INTR_RESET | MUSB_INTR_SOF;
-
- if (musb->int_usb || musb->int_tx || musb->int_rx)
- retval = musb_interrupt(musb);
-
- spin_unlock_irqrestore(&musb->lock, flags);
-
- return retval;
-}
- </programlisting>
- <para>
- Here the glue layer mostly has to read the relevant hardware
- registers and pass their values on to the controller driver which
- will handle the actual event that triggered the IRQ.
- </para>
- <para>
- The interrupt handler critical section is protected by the
- spin_lock_irqsave() and counterpart spin_unlock_irqrestore()
- functions (line 7 and 24 respectively), which prevent the
- interrupt handler code to be run by two different threads at the
- same time.
- </para>
- <para>
- Then the relevant interrupt registers are read (line 9 to 11):
- </para>
- <itemizedlist>
- <listitem>
- <para>
- MUSB_INTRUSB: indicates which USB interrupts are currently
- active,
- </para>
- </listitem>
- <listitem>
- <para>
- MUSB_INTRTX: indicates which of the interrupts for TX
- endpoints are currently active,
- </para>
- </listitem>
- <listitem>
- <para>
- MUSB_INTRRX: indicates which of the interrupts for TX
- endpoints are currently active.
- </para>
- </listitem>
- </itemizedlist>
- <para>
- Note that musb_readb() is used to read 8-bit registers at most,
- while musb_readw() allows us to read at most 16-bit registers.
- There are other functions that can be used depending on the size
- of your device registers. See musb_io.h for more information.
- </para>
- <para>
- Instruction on line 18 is another quirk specific to the JZ4740
- USB device controller, which will be discussed later in <link
- linkend="device-quirks">Chapter 5</link>.
- </para>
- <para>
- The glue layer still needs to register the IRQ handler though.
- Remember the instruction on line 14 of the init function:
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_musb_init(struct musb *musb)
-{
- musb->isr = jz4740_musb_interrupt;
-
- return 0;
-}
- </programlisting>
- <para>
- This instruction sets a pointer to the glue layer IRQ handler
- function, in order for the controller hardware to call the handler
- back when an IRQ comes from the controller hardware. The interrupt
- handler is now implemented and registered.
- </para>
- </chapter>
-
- <chapter id="device-platform-data">
- <title>Device Platform Data</title>
- <para>
- In order to write an MUSB glue layer, you need to have some data
- describing the hardware capabilities of your controller hardware,
- which is called the platform data.
- </para>
- <para>
- Platform data is specific to your hardware, though it may cover a
- broad range of devices, and is generally found somewhere in the
- arch/ directory, depending on your device architecture.
- </para>
- <para>
- For instance, platform data for the JZ4740 SoC is found in
- arch/mips/jz4740/platform.c. In the platform.c file each device of
- the JZ4740 SoC is described through a set of structures.
- </para>
- <para>
- Here is the part of arch/mips/jz4740/platform.c that covers the
- USB Device Controller (UDC):
- </para>
- <programlisting linenumbering="numbered">
-/* USB Device Controller */
-struct platform_device jz4740_udc_xceiv_device = {
- .name = "usb_phy_gen_xceiv",
- .id = 0,
-};
-
-static struct resource jz4740_udc_resources[] = {
- [0] = {
- .start = JZ4740_UDC_BASE_ADDR,
- .end = JZ4740_UDC_BASE_ADDR + 0x10000 - 1,
- .flags = IORESOURCE_MEM,
- },
- [1] = {
- .start = JZ4740_IRQ_UDC,
- .end = JZ4740_IRQ_UDC,
- .flags = IORESOURCE_IRQ,
- .name = "mc",
- },
-};
-
-struct platform_device jz4740_udc_device = {
- .name = "musb-jz4740",
- .id = -1,
- .dev = {
- .dma_mask = &jz4740_udc_device.dev.coherent_dma_mask,
- .coherent_dma_mask = DMA_BIT_MASK(32),
- },
- .num_resources = ARRAY_SIZE(jz4740_udc_resources),
- .resource = jz4740_udc_resources,
-};
- </programlisting>
- <para>
- The jz4740_udc_xceiv_device platform device structure (line 2)
- describes the UDC transceiver with a name and id number.
- </para>
- <para>
- At the time of this writing, note that
- "usb_phy_gen_xceiv" is the specific name to be used for
- all transceivers that are either built-in with reference USB IP or
- autonomous and doesn't require any PHY programming. You will need
- to set CONFIG_NOP_USB_XCEIV=y in the kernel configuration to make
- use of the corresponding transceiver driver. The id field could be
- set to -1 (equivalent to PLATFORM_DEVID_NONE), -2 (equivalent to
- PLATFORM_DEVID_AUTO) or start with 0 for the first device of this
- kind if we want a specific id number.
- </para>
- <para>
- The jz4740_udc_resources resource structure (line 7) defines the
- UDC registers base addresses.
- </para>
- <para>
- The first array (line 9 to 11) defines the UDC registers base
- memory addresses: start points to the first register memory
- address, end points to the last register memory address and the
- flags member defines the type of resource we are dealing with. So
- IORESOURCE_MEM is used to define the registers memory addresses.
- The second array (line 14 to 17) defines the UDC IRQ registers
- addresses. Since there is only one IRQ register available for the
- JZ4740 UDC, start and end point at the same address. The
- IORESOURCE_IRQ flag tells that we are dealing with IRQ resources,
- and the name "mc" is in fact hard-coded in the MUSB core
- in order for the controller driver to retrieve this IRQ resource
- by querying it by its name.
- </para>
- <para>
- Finally, the jz4740_udc_device platform device structure (line 21)
- describes the UDC itself.
- </para>
- <para>
- The "musb-jz4740" name (line 22) defines the MUSB
- driver that is used for this device; remember this is in fact
- the name that we used in the jz4740_driver platform driver
- structure in <link linkend="linux-musb-basics">Chapter
- 2</link>. The id field (line 23) is set to -1 (equivalent to
- PLATFORM_DEVID_NONE) since we do not need an id for the device:
- the MUSB controller driver was already set to allocate an
- automatic id in <link linkend="linux-musb-basics">Chapter
- 2</link>. In the dev field we care for DMA related information
- here. The dma_mask field (line 25) defines the width of the DMA
- mask that is going to be used, and coherent_dma_mask (line 26)
- has the same purpose but for the alloc_coherent DMA mappings: in
- both cases we are using a 32 bits mask. Then the resource field
- (line 29) is simply a pointer to the resource structure defined
- before, while the num_resources field (line 28) keeps track of
- the number of arrays defined in the resource structure (in this
- case there were two resource arrays defined before).
- </para>
- <para>
- With this quick overview of the UDC platform data at the arch/
- level now done, let's get back to the MUSB glue layer specific
- platform data in drivers/usb/musb/jz4740.c:
- </para>
- <programlisting linenumbering="numbered">
-static struct musb_hdrc_config jz4740_musb_config = {
- /* Silicon does not implement USB OTG. */
- .multipoint = 0,
- /* Max EPs scanned, driver will decide which EP can be used. */
- .num_eps = 4,
- /* RAMbits needed to configure EPs from table */
- .ram_bits = 9,
- .fifo_cfg = jz4740_musb_fifo_cfg,
- .fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg),
-};
-
-static struct musb_hdrc_platform_data jz4740_musb_platform_data = {
- .mode = MUSB_PERIPHERAL,
- .config = &jz4740_musb_config,
-};
- </programlisting>
- <para>
- First the glue layer configures some aspects of the controller
- driver operation related to the controller hardware specifics.
- This is done through the jz4740_musb_config musb_hdrc_config
- structure.
- </para>
- <para>
- Defining the OTG capability of the controller hardware, the
- multipoint member (line 3) is set to 0 (equivalent to false)
- since the JZ4740 UDC is not OTG compatible. Then num_eps (line
- 5) defines the number of USB endpoints of the controller
- hardware, including endpoint 0: here we have 3 endpoints +
- endpoint 0. Next is ram_bits (line 7) which is the width of the
- RAM address bus for the MUSB controller hardware. This
- information is needed when the controller driver cannot
- automatically configure endpoints by reading the relevant
- controller hardware registers. This issue will be discussed when
- we get to device quirks in <link linkend="device-quirks">Chapter
- 5</link>. Last two fields (line 8 and 9) are also about device
- quirks: fifo_cfg points to the USB endpoints configuration table
- and fifo_cfg_size keeps track of the size of the number of
- entries in that configuration table. More on that later in <link
- linkend="device-quirks">Chapter 5</link>.
- </para>
- <para>
- Then this configuration is embedded inside
- jz4740_musb_platform_data musb_hdrc_platform_data structure (line
- 11): config is a pointer to the configuration structure itself,
- and mode tells the controller driver if the controller hardware
- may be used as MUSB_HOST only, MUSB_PERIPHERAL only or MUSB_OTG
- which is a dual mode.
- </para>
- <para>
- Remember that jz4740_musb_platform_data is then used to convey
- platform data information as we have seen in the probe function
- in <link linkend="linux-musb-basics">Chapter 2</link>
- </para>
- </chapter>
-
- <chapter id="device-quirks">
- <title>Device Quirks</title>
- <para>
- Completing the platform data specific to your device, you may also
- need to write some code in the glue layer to work around some
- device specific limitations. These quirks may be due to some
- hardware bugs, or simply be the result of an incomplete
- implementation of the USB On-the-Go specification.
- </para>
- <para>
- The JZ4740 UDC exhibits such quirks, some of which we will discuss
- here for the sake of insight even though these might not be found
- in the controller hardware you are working on.
- </para>
- <para>
- Let's get back to the init function first:
- </para>
- <programlisting linenumbering="numbered">
-static int jz4740_musb_init(struct musb *musb)
-{
- musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
- if (!musb->xceiv) {
- pr_err("HS UDC: no transceiver configured\n");
- return -ENODEV;
- }
-
- /* Silicon does not implement ConfigData register.
- * Set dyn_fifo to avoid reading EP config from hardware.
- */
- musb->dyn_fifo = true;
-
- musb->isr = jz4740_musb_interrupt;
-
- return 0;
-}
- </programlisting>
- <para>
- Instruction on line 12 helps the MUSB controller driver to work
- around the fact that the controller hardware is missing registers
- that are used for USB endpoints configuration.
- </para>
- <para>
- Without these registers, the controller driver is unable to read
- the endpoints configuration from the hardware, so we use line 12
- instruction to bypass reading the configuration from silicon, and
- rely on a hard-coded table that describes the endpoints
- configuration instead:
- </para>
- <programlisting linenumbering="numbered">
-static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = {
-{ .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, },
-{ .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, },
-{ .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, },
-};
- </programlisting>
- <para>
- Looking at the configuration table above, we see that each
- endpoints is described by three fields: hw_ep_num is the endpoint
- number, style is its direction (either FIFO_TX for the controller
- driver to send packets in the controller hardware, or FIFO_RX to
- receive packets from hardware), and maxpacket defines the maximum
- size of each data packet that can be transmitted over that
- endpoint. Reading from the table, the controller driver knows that
- endpoint 1 can be used to send and receive USB data packets of 512
- bytes at once (this is in fact a bulk in/out endpoint), and
- endpoint 2 can be used to send data packets of 64 bytes at once
- (this is in fact an interrupt endpoint).
- </para>
- <para>
- Note that there is no information about endpoint 0 here: that one
- is implemented by default in every silicon design, with a
- predefined configuration according to the USB specification. For
- more examples of endpoint configuration tables, see musb_core.c.
- </para>
- <para>
- Let's now get back to the interrupt handler function:
- </para>
- <programlisting linenumbering="numbered">
-static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
-{
- unsigned long flags;
- irqreturn_t retval = IRQ_NONE;
- struct musb *musb = __hci;
-
- spin_lock_irqsave(&musb->lock, flags);
-
- musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
- musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
- musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
-
- /*
- * The controller is gadget only, the state of the host mode IRQ bits is
- * undefined. Mask them to make sure that the musb driver core will
- * never see them set
- */
- musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
- MUSB_INTR_RESET | MUSB_INTR_SOF;
-
- if (musb->int_usb || musb->int_tx || musb->int_rx)
- retval = musb_interrupt(musb);
-
- spin_unlock_irqrestore(&musb->lock, flags);
-
- return retval;
-}
- </programlisting>
- <para>
- Instruction on line 18 above is a way for the controller driver to
- work around the fact that some interrupt bits used for USB host
- mode operation are missing in the MUSB_INTRUSB register, thus left
- in an undefined hardware state, since this MUSB controller
- hardware is used in peripheral mode only. As a consequence, the
- glue layer masks these missing bits out to avoid parasite
- interrupts by doing a logical AND operation between the value read
- from MUSB_INTRUSB and the bits that are actually implemented in
- the register.
- </para>
- <para>
- These are only a couple of the quirks found in the JZ4740 USB
- device controller. Some others were directly addressed in the MUSB
- core since the fixes were generic enough to provide a better
- handling of the issues for others controller hardware eventually.
- </para>
- </chapter>
-
- <chapter id="conclusion">
- <title>Conclusion</title>
- <para>
- Writing a Linux MUSB glue layer should be a more accessible task,
- as this documentation tries to show the ins and outs of this
- exercise.
- </para>
- <para>
- The JZ4740 USB device controller being fairly simple, I hope its
- glue layer serves as a good example for the curious mind. Used
- with the current MUSB glue layers, this documentation should
- provide enough guidance to get started; should anything gets out
- of hand, the linux-usb mailing list archive is another helpful
- resource to browse through.
- </para>
- </chapter>
-
- <chapter id="acknowledgements">
- <title>Acknowledgements</title>
- <para>
- Many thanks to Lars-Peter Clausen and Maarten ter Huurne for
- answering my questions while I was writing the JZ4740 glue layer
- and for helping me out getting the code in good shape.
- </para>
- <para>
- I would also like to thank the Qi-Hardware community at large for
- its cheerful guidance and support.
- </para>
- </chapter>
-
- <chapter id="resources">
- <title>Resources</title>
- <para>
- USB Home Page:
- <ulink url="http://www.usb.org">http://www.usb.org</ulink>
- </para>
- <para>
- linux-usb Mailing List Archives:
- <ulink url="http://marc.info/?l=linux-usb">http://marc.info/?l=linux-usb</ulink>
- </para>
- <para>
- USB On-the-Go Basics:
- <ulink url="http://www.maximintegrated.com/app-notes/index.mvp/id/1822">http://www.maximintegrated.com/app-notes/index.mvp/id/1822</ulink>
- </para>
- <para>
- Writing USB Device Drivers:
- <ulink url="https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html">https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html</ulink>
- </para>
- <para>
- Texas Instruments USB Configuration Wiki Page:
- <ulink url="http://processors.wiki.ti.com/index.php/Usbgeneralpage">http://processors.wiki.ti.com/index.php/Usbgeneralpage</ulink>
- </para>
- <para>
- Analog Devices Blackfin MUSB Configuration:
- <ulink url="http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb">http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb</ulink>
- </para>
- </chapter>
-
-</book>
+++ /dev/null
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
-
-<book id="USBDeviceDriver">
- <bookinfo>
- <title>Writing USB Device Drivers</title>
-
- <authorgroup>
- <author>
- <firstname>Greg</firstname>
- <surname>Kroah-Hartman</surname>
- <affiliation>
- <address>
- <email>greg@kroah.com</email>
- </address>
- </affiliation>
- </author>
- </authorgroup>
-
- <copyright>
- <year>2001-2002</year>
- <holder>Greg Kroah-Hartman</holder>
- </copyright>
-
- <legalnotice>
- <para>
- This documentation is free software; you can redistribute
- it and/or modify it under the terms of the GNU General Public
- License as published by the Free Software Foundation; either
- version 2 of the License, or (at your option) any later
- version.
- </para>
-
- <para>
- This program is distributed in the hope that it will be
- useful, but WITHOUT ANY WARRANTY; without even the implied
- warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
- See the GNU General Public License for more details.
- </para>
-
- <para>
- You should have received a copy of the GNU General Public
- License along with this program; if not, write to the Free
- Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
- MA 02111-1307 USA
- </para>
-
- <para>
- For more details see the file COPYING in the source
- distribution of Linux.
- </para>
-
- <para>
- This documentation is based on an article published in
- Linux Journal Magazine, October 2001, Issue 90.
- </para>
- </legalnotice>
- </bookinfo>
-
-<toc></toc>
-
- <chapter id="intro">
- <title>Introduction</title>
- <para>
- The Linux USB subsystem has grown from supporting only two different
- types of devices in the 2.2.7 kernel (mice and keyboards), to over 20
- different types of devices in the 2.4 kernel. Linux currently supports
- almost all USB class devices (standard types of devices like keyboards,
- mice, modems, printers and speakers) and an ever-growing number of
- vendor-specific devices (such as USB to serial converters, digital
- cameras, Ethernet devices and MP3 players). For a full list of the
- different USB devices currently supported, see Resources.
- </para>
- <para>
- The remaining kinds of USB devices that do not have support on Linux are
- almost all vendor-specific devices. Each vendor decides to implement a
- custom protocol to talk to their device, so a custom driver usually needs
- to be created. Some vendors are open with their USB protocols and help
- with the creation of Linux drivers, while others do not publish them, and
- developers are forced to reverse-engineer. See Resources for some links
- to handy reverse-engineering tools.
- </para>
- <para>
- Because each different protocol causes a new driver to be created, I have
- written a generic USB driver skeleton, modelled after the pci-skeleton.c
- file in the kernel source tree upon which many PCI network drivers have
- been based. This USB skeleton can be found at drivers/usb/usb-skeleton.c
- in the kernel source tree. In this article I will walk through the basics
- of the skeleton driver, explaining the different pieces and what needs to
- be done to customize it to your specific device.
- </para>
- </chapter>
-
- <chapter id="basics">
- <title>Linux USB Basics</title>
- <para>
- If you are going to write a Linux USB driver, please become familiar with
- the USB protocol specification. It can be found, along with many other
- useful documents, at the USB home page (see Resources). An excellent
- introduction to the Linux USB subsystem can be found at the USB Working
- Devices List (see Resources). It explains how the Linux USB subsystem is
- structured and introduces the reader to the concept of USB urbs
- (USB Request Blocks), which are essential to USB drivers.
- </para>
- <para>
- The first thing a Linux USB driver needs to do is register itself with
- the Linux USB subsystem, giving it some information about which devices
- the driver supports and which functions to call when a device supported
- by the driver is inserted or removed from the system. All of this
- information is passed to the USB subsystem in the usb_driver structure.
- The skeleton driver declares a usb_driver as:
- </para>
- <programlisting>
-static struct usb_driver skel_driver = {
- .name = "skeleton",
- .probe = skel_probe,
- .disconnect = skel_disconnect,
- .fops = &skel_fops,
- .minor = USB_SKEL_MINOR_BASE,
- .id_table = skel_table,
-};
- </programlisting>
- <para>
- The variable name is a string that describes the driver. It is used in
- informational messages printed to the system log. The probe and
- disconnect function pointers are called when a device that matches the
- information provided in the id_table variable is either seen or removed.
- </para>
- <para>
- The fops and minor variables are optional. Most USB drivers hook into
- another kernel subsystem, such as the SCSI, network or TTY subsystem.
- These types of drivers register themselves with the other kernel
- subsystem, and any user-space interactions are provided through that
- interface. But for drivers that do not have a matching kernel subsystem,
- such as MP3 players or scanners, a method of interacting with user space
- is needed. The USB subsystem provides a way to register a minor device
- number and a set of file_operations function pointers that enable this
- user-space interaction. The skeleton driver needs this kind of interface,
- so it provides a minor starting number and a pointer to its
- file_operations functions.
- </para>
- <para>
- The USB driver is then registered with a call to usb_register, usually in
- the driver's init function, as shown here:
- </para>
- <programlisting>
-static int __init usb_skel_init(void)
-{
- int result;
-
- /* register this driver with the USB subsystem */
- result = usb_register(&skel_driver);
- if (result < 0) {
- err("usb_register failed for the "__FILE__ "driver."
- "Error number %d", result);
- return -1;
- }
-
- return 0;
-}
-module_init(usb_skel_init);
- </programlisting>
- <para>
- When the driver is unloaded from the system, it needs to deregister
- itself with the USB subsystem. This is done with the usb_deregister
- function:
- </para>
- <programlisting>
-static void __exit usb_skel_exit(void)
-{
- /* deregister this driver with the USB subsystem */
- usb_deregister(&skel_driver);
-}
-module_exit(usb_skel_exit);
- </programlisting>
- <para>
- To enable the linux-hotplug system to load the driver automatically when
- the device is plugged in, you need to create a MODULE_DEVICE_TABLE. The
- following code tells the hotplug scripts that this module supports a
- single device with a specific vendor and product ID:
- </para>
- <programlisting>
-/* table of devices that work with this driver */
-static struct usb_device_id skel_table [] = {
- { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
- { } /* Terminating entry */
-};
-MODULE_DEVICE_TABLE (usb, skel_table);
- </programlisting>
- <para>
- There are other macros that can be used in describing a usb_device_id for
- drivers that support a whole class of USB drivers. See usb.h for more
- information on this.
- </para>
- </chapter>
-
- <chapter id="device">
- <title>Device operation</title>
- <para>
- When a device is plugged into the USB bus that matches the device ID
- pattern that your driver registered with the USB core, the probe function
- is called. The usb_device structure, interface number and the interface ID
- are passed to the function:
- </para>
- <programlisting>
-static int skel_probe(struct usb_interface *interface,
- const struct usb_device_id *id)
- </programlisting>
- <para>
- The driver now needs to verify that this device is actually one that it
- can accept. If so, it returns 0.
- If not, or if any error occurs during initialization, an errorcode
- (such as <literal>-ENOMEM</literal> or <literal>-ENODEV</literal>)
- is returned from the probe function.
- </para>
- <para>
- In the skeleton driver, we determine what end points are marked as bulk-in
- and bulk-out. We create buffers to hold the data that will be sent and
- received from the device, and a USB urb to write data to the device is
- initialized.
- </para>
- <para>
- Conversely, when the device is removed from the USB bus, the disconnect
- function is called with the device pointer. The driver needs to clean any
- private data that has been allocated at this time and to shut down any
- pending urbs that are in the USB system.
- </para>
- <para>
- Now that the device is plugged into the system and the driver is bound to
- the device, any of the functions in the file_operations structure that
- were passed to the USB subsystem will be called from a user program trying
- to talk to the device. The first function called will be open, as the
- program tries to open the device for I/O. We increment our private usage
- count and save a pointer to our internal structure in the file
- structure. This is done so that future calls to file operations will
- enable the driver to determine which device the user is addressing. All
- of this is done with the following code:
- </para>
- <programlisting>
-/* increment our usage count for the module */
-++skel->open_count;
-
-/* save our object in the file's private structure */
-file->private_data = dev;
- </programlisting>
- <para>
- After the open function is called, the read and write functions are called
- to receive and send data to the device. In the skel_write function, we
- receive a pointer to some data that the user wants to send to the device
- and the size of the data. The function determines how much data it can
- send to the device based on the size of the write urb it has created (this
- size depends on the size of the bulk out end point that the device has).
- Then it copies the data from user space to kernel space, points the urb to
- the data and submits the urb to the USB subsystem. This can be seen in
- the following code:
- </para>
- <programlisting>
-/* we can only write as much as 1 urb will hold */
-bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
-
-/* copy the data from user space into our urb */
-copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
-
-/* set up our urb */
-usb_fill_bulk_urb(skel->write_urb,
- skel->dev,
- usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr),
- skel->write_urb->transfer_buffer,
- bytes_written,
- skel_write_bulk_callback,
- skel);
-
-/* send the data out the bulk port */
-result = usb_submit_urb(skel->write_urb);
-if (result) {
- err("Failed submitting write urb, error %d", result);
-}
- </programlisting>
- <para>
- When the write urb is filled up with the proper information using the
- usb_fill_bulk_urb function, we point the urb's completion callback to call our
- own skel_write_bulk_callback function. This function is called when the
- urb is finished by the USB subsystem. The callback function is called in
- interrupt context, so caution must be taken not to do very much processing
- at that time. Our implementation of skel_write_bulk_callback merely
- reports if the urb was completed successfully or not and then returns.
- </para>
- <para>
- The read function works a bit differently from the write function in that
- we do not use an urb to transfer data from the device to the driver.
- Instead we call the usb_bulk_msg function, which can be used to send or
- receive data from a device without having to create urbs and handle
- urb completion callback functions. We call the usb_bulk_msg function,
- giving it a buffer into which to place any data received from the device
- and a timeout value. If the timeout period expires without receiving any
- data from the device, the function will fail and return an error message.
- This can be shown with the following code:
- </para>
- <programlisting>
-/* do an immediate bulk read to get data from the device */
-retval = usb_bulk_msg (skel->dev,
- usb_rcvbulkpipe (skel->dev,
- skel->bulk_in_endpointAddr),
- skel->bulk_in_buffer,
- skel->bulk_in_size,
- &count, HZ*10);
-/* if the read was successful, copy the data to user space */
-if (!retval) {
- if (copy_to_user (buffer, skel->bulk_in_buffer, count))
- retval = -EFAULT;
- else
- retval = count;
-}
- </programlisting>
- <para>
- The usb_bulk_msg function can be very useful for doing single reads or
- writes to a device; however, if you need to read or write constantly to a
- device, it is recommended to set up your own urbs and submit them to the
- USB subsystem.
- </para>
- <para>
- When the user program releases the file handle that it has been using to
- talk to the device, the release function in the driver is called. In this
- function we decrement our private usage count and wait for possible
- pending writes:
- </para>
- <programlisting>
-/* decrement our usage count for the device */
---skel->open_count;
- </programlisting>
- <para>
- One of the more difficult problems that USB drivers must be able to handle
- smoothly is the fact that the USB device may be removed from the system at
- any point in time, even if a program is currently talking to it. It needs
- to be able to shut down any current reads and writes and notify the
- user-space programs that the device is no longer there. The following
- code (function <function>skel_delete</function>)
- is an example of how to do this: </para>
- <programlisting>
-static inline void skel_delete (struct usb_skel *dev)
-{
- kfree (dev->bulk_in_buffer);
- if (dev->bulk_out_buffer != NULL)
- usb_free_coherent (dev->udev, dev->bulk_out_size,
- dev->bulk_out_buffer,
- dev->write_urb->transfer_dma);
- usb_free_urb (dev->write_urb);
- kfree (dev);
-}
- </programlisting>
- <para>
- If a program currently has an open handle to the device, we reset the flag
- <literal>device_present</literal>. For
- every read, write, release and other functions that expect a device to be
- present, the driver first checks this flag to see if the device is
- still present. If not, it releases that the device has disappeared, and a
- -ENODEV error is returned to the user-space program. When the release
- function is eventually called, it determines if there is no device
- and if not, it does the cleanup that the skel_disconnect
- function normally does if there are no open files on the device (see
- Listing 5).
- </para>
- </chapter>
-
- <chapter id="iso">
- <title>Isochronous Data</title>
- <para>
- This usb-skeleton driver does not have any examples of interrupt or
- isochronous data being sent to or from the device. Interrupt data is sent
- almost exactly as bulk data is, with a few minor exceptions. Isochronous
- data works differently with continuous streams of data being sent to or
- from the device. The audio and video camera drivers are very good examples
- of drivers that handle isochronous data and will be useful if you also
- need to do this.
- </para>
- </chapter>
-
- <chapter id="Conclusion">
- <title>Conclusion</title>
- <para>
- Writing Linux USB device drivers is not a difficult task as the
- usb-skeleton driver shows. This driver, combined with the other current
- USB drivers, should provide enough examples to help a beginning author
- create a working driver in a minimal amount of time. The linux-usb-devel
- mailing list archives also contain a lot of helpful information.
- </para>
- </chapter>
-
- <chapter id="resources">
- <title>Resources</title>
- <para>
- The Linux USB Project: <ulink url="http://www.linux-usb.org">http://www.linux-usb.org/</ulink>
- </para>
- <para>
- Linux Hotplug Project: <ulink url="http://linux-hotplug.sourceforge.net">http://linux-hotplug.sourceforge.net/</ulink>
- </para>
- <para>
- Linux USB Working Devices List: <ulink url="http://www.qbik.ch/usb/devices">http://www.qbik.ch/usb/devices/</ulink>
- </para>
- <para>
- linux-usb-devel Mailing List Archives: <ulink url="http://marc.theaimsgroup.com/?l=linux-usb-devel">http://marc.theaimsgroup.com/?l=linux-usb-devel</ulink>
- </para>
- <para>
- Programming Guide for Linux USB Device Drivers: <ulink url="http://usb.cs.tum.edu/usbdoc">http://usb.cs.tum.edu/usbdoc</ulink>
- </para>
- <para>
- USB Home Page: <ulink url="http://www.usb.org">http://www.usb.org</ulink>
- </para>
- </chapter>
-
-</book>
regulator
iio/index
input
- usb
+ usb/index
pci
spi
i2c
+++ /dev/null
-===========================
-The Linux-USB Host Side API
-===========================
-
-Introduction to USB on Linux
-============================
-
-A Universal Serial Bus (USB) is used to connect a host, such as a PC or
-workstation, to a number of peripheral devices. USB uses a tree
-structure, with the host as the root (the system's master), hubs as
-interior nodes, and peripherals as leaves (and slaves). Modern PCs
-support several such trees of USB devices, usually
-a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy
-USB 2.0 (480 MBit/s) busses just in case.
-
-That master/slave asymmetry was designed-in for a number of reasons, one
-being ease of use. It is not physically possible to mistake upstream and
-downstream or it does not matter with a type C plug (or they are built into the
-peripheral). Also, the host software doesn't need to deal with
-distributed auto-configuration since the pre-designated master node
-manages all that.
-
-Kernel developers added USB support to Linux early in the 2.2 kernel
-series and have been developing it further since then. Besides support
-for each new generation of USB, various host controllers gained support,
-new drivers for peripherals have been added and advanced features for latency
-measurement and improved power management introduced.
-
-Linux can run inside USB devices as well as on the hosts that control
-the devices. But USB device drivers running inside those peripherals
-don't do the same things as the ones running inside hosts, so they've
-been given a different name: *gadget drivers*. This document does not
-cover gadget drivers.
-
-USB Host-Side API Model
-=======================
-
-Host-side drivers for USB devices talk to the "usbcore" APIs. There are
-two. One is intended for *general-purpose* drivers (exposed through
-driver frameworks), and the other is for drivers that are *part of the
-core*. Such core drivers include the *hub* driver (which manages trees
-of USB devices) and several different kinds of *host controller
-drivers*, which control individual busses.
-
-The device model seen by USB drivers is relatively complex.
-
-- USB supports four kinds of data transfers (control, bulk, interrupt,
- and isochronous). Two of them (control and bulk) use bandwidth as
- it's available, while the other two (interrupt and isochronous) are
- scheduled to provide guaranteed bandwidth.
-
-- The device description model includes one or more "configurations"
- per device, only one of which is active at a time. Devices are supposed
- to be capable of operating at lower than their top
- speeds and may provide a BOS descriptor showing the lowest speed they
- remain fully operational at.
-
-- From USB 3.0 on configurations have one or more "functions", which
- provide a common functionality and are grouped together for purposes
- of power management.
-
-- Configurations or functions have one or more "interfaces", each of which may have
- "alternate settings". Interfaces may be standardized by USB "Class"
- specifications, or may be specific to a vendor or device.
-
- USB device drivers actually bind to interfaces, not devices. Think of
- them as "interface drivers", though you may not see many devices
- where the distinction is important. *Most USB devices are simple,
- with only one function, one configuration, one interface, and one alternate
- setting.*
-
-- Interfaces have one or more "endpoints", each of which supports one
- type and direction of data transfer such as "bulk out" or "interrupt
- in". The entire configuration may have up to sixteen endpoints in
- each direction, allocated as needed among all the interfaces.
-
-- Data transfer on USB is packetized; each endpoint has a maximum
- packet size. Drivers must often be aware of conventions such as
- flagging the end of bulk transfers using "short" (including zero
- length) packets.
-
-- The Linux USB API supports synchronous calls for control and bulk
- messages. It also supports asynchronous calls for all kinds of data
- transfer, using request structures called "URBs" (USB Request
- Blocks).
-
-Accordingly, the USB Core API exposed to device drivers covers quite a
-lot of territory. You'll probably need to consult the USB 3.0
-specification, available online from www.usb.org at no cost, as well as
-class or device specifications.
-
-The only host-side drivers that actually touch hardware (reading/writing
-registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs
-provide the same functionality through the same API. In practice, that's
-becoming more true, but there are still differences
-that crop up especially with fault handling on the less common controllers.
-Different controllers don't
-necessarily report the same aspects of failures, and recovery from
-faults (including software-induced ones like unlinking an URB) isn't yet
-fully consistent. Device driver authors should make a point of doing
-disconnect testing (while the device is active) with each different host
-controller driver, to make sure drivers don't have bugs of their own as
-well as to make sure they aren't relying on some HCD-specific behavior.
-
-USB-Standard Types
-==================
-
-In ``<linux/usb/ch9.h>`` you will find the USB data types defined in
-chapter 9 of the USB specification. These data types are used throughout
-USB, and in APIs including this host side API, gadget APIs, and usbfs.
-
-.. kernel-doc:: include/linux/usb/ch9.h
- :internal:
-
-Host-Side Data Types and Macros
-===============================
-
-The host side API exposes several layers to drivers, some of which are
-more necessary than others. These support lifecycle models for host side
-drivers and devices, and support passing buffers through usbcore to some
-HCD that performs the I/O for the device driver.
-
-.. kernel-doc:: include/linux/usb.h
- :internal:
-
-USB Core APIs
-=============
-
-There are two basic I/O models in the USB API. The most elemental one is
-asynchronous: drivers submit requests in the form of an URB, and the
-URB's completion callback handles the next step. All USB transfer types
-support that model, although there are special cases for control URBs
-(which always have setup and status stages, but may not have a data
-stage) and isochronous URBs (which allow large packets and include
-per-packet fault reports). Built on top of that is synchronous API
-support, where a driver calls a routine that allocates one or more URBs,
-submits them, and waits until they complete. There are synchronous
-wrappers for single-buffer control and bulk transfers (which are awkward
-to use in some driver disconnect scenarios), and for scatterlist based
-streaming i/o (bulk or interrupt).
-
-USB drivers need to provide buffers that can be used for DMA, although
-they don't necessarily need to provide the DMA mapping themselves. There
-are APIs to use used when allocating DMA buffers, which can prevent use
-of bounce buffers on some systems. In some cases, drivers may be able to
-rely on 64bit DMA to eliminate another kind of bounce buffer.
-
-.. kernel-doc:: drivers/usb/core/urb.c
- :export:
-
-.. kernel-doc:: drivers/usb/core/message.c
- :export:
-
-.. kernel-doc:: drivers/usb/core/file.c
- :export:
-
-.. kernel-doc:: drivers/usb/core/driver.c
- :export:
-
-.. kernel-doc:: drivers/usb/core/usb.c
- :export:
-
-.. kernel-doc:: drivers/usb/core/hub.c
- :export:
-
-Host Controller APIs
-====================
-
-These APIs are only for use by host controller drivers, most of which
-implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI
-was one of the first interfaces, designed by Intel and also used by VIA;
-it doesn't do much in hardware. OHCI was designed later, to have the
-hardware do more work (bigger transfers, tracking protocol state, and so
-on). EHCI was designed with USB 2.0; its design has features that
-resemble OHCI (hardware does much more work) as well as UHCI (some parts
-of ISO support, TD list processing). XHCI was designed with USB 3.0. It
-continues to shift support for functionality into hardware.
-
-There are host controllers other than the "big three", although most PCI
-based controllers (and a few non-PCI based ones) use one of those
-interfaces. Not all host controllers use DMA; some use PIO, and there is
-also a simulator and a virtual host controller to pipe USB over the network.
-
-The same basic APIs are available to drivers for all those controllers.
-For historical reasons they are in two layers: :c:type:`struct
-usb_bus <usb_bus>` is a rather thin layer that became available
-in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>`
-is a more featureful layer
-that lets HCDs share common code, to shrink driver size and
-significantly reduce hcd-specific behaviors.
-
-.. kernel-doc:: drivers/usb/core/hcd.c
- :export:
-
-.. kernel-doc:: drivers/usb/core/hcd-pci.c
- :export:
-
-.. kernel-doc:: drivers/usb/core/buffer.c
- :internal:
-
-The USB Filesystem (usbfs)
-==========================
-
-This chapter presents the Linux *usbfs*. You may prefer to avoid writing
-new kernel code for your USB driver; that's the problem that usbfs set
-out to solve. User mode device drivers are usually packaged as
-applications or libraries, and may use usbfs through some programming
-library that wraps it. Such libraries include
-`libusb <http://libusb.sourceforge.net>`__ for C/C++, and
-`jUSB <http://jUSB.sourceforge.net>`__ for Java.
-
- **Note**
-
- This particular documentation is incomplete, especially with respect
- to the asynchronous mode. As of kernel 2.5.66 the code and this
- (new) documentation need to be cross-reviewed.
-
-Configure usbfs into Linux kernels by enabling the *USB filesystem*
-option (CONFIG_USB_DEVICEFS), and you get basic support for user mode
-USB device drivers. Until relatively recently it was often (confusingly)
-called *usbdevfs* although it wasn't solving what *devfs* was. Every USB
-device will appear in usbfs, regardless of whether or not it has a
-kernel driver.
-
-What files are in "usbfs"?
---------------------------
-
-Conventionally mounted at ``/proc/bus/usb``, usbfs features include:
-
-- ``/proc/bus/usb/devices`` ... a text file showing each of the USB
- devices on known to the kernel, and their configuration descriptors.
- You can also poll() this to learn about new devices.
-
-- ``/proc/bus/usb/BBB/DDD`` ... magic files exposing the each device's
- configuration descriptors, and supporting a series of ioctls for
- making device requests, including I/O to devices. (Purely for access
- by programs.)
-
-Each bus is given a number (BBB) based on when it was enumerated; within
-each bus, each device is given a similar number (DDD). Those BBB/DDD
-paths are not "stable" identifiers; expect them to change even if you
-always leave the devices plugged in to the same hub port. *Don't even
-think of saving these in application configuration files.* Stable
-identifiers are available, for user mode applications that want to use
-them. HID and networking devices expose these stable IDs, so that for
-example you can be sure that you told the right UPS to power down its
-second server. "usbfs" doesn't (yet) expose those IDs.
-
-Mounting and Access Control
----------------------------
-
-There are a number of mount options for usbfs, which will be of most
-interest to you if you need to override the default access control
-policy. That policy is that only root may read or write device files
-(``/proc/bus/BBB/DDD``) although anyone may read the ``devices`` or
-``drivers`` files. I/O requests to the device also need the
-CAP_SYS_RAWIO capability,
-
-The significance of that is that by default, all user mode device
-drivers need super-user privileges. You can change modes or ownership in
-a driver setup when the device hotplugs, or maye just start the driver
-right then, as a privileged server (or some activity within one). That's
-the most secure approach for multi-user systems, but for single user
-systems ("trusted" by that user) it's more convenient just to grant
-everyone all access (using the *devmode=0666* option) so the driver can
-start whenever it's needed.
-
-The mount options for usbfs, usable in /etc/fstab or in command line
-invocations of *mount*, are:
-
-*busgid*\ =NNNNN
- Controls the GID used for the /proc/bus/usb/BBB directories.
- (Default: 0)
-
-*busmode*\ =MMM
- Controls the file mode used for the /proc/bus/usb/BBB directories.
- (Default: 0555)
-
-*busuid*\ =NNNNN
- Controls the UID used for the /proc/bus/usb/BBB directories.
- (Default: 0)
-
-*devgid*\ =NNNNN
- Controls the GID used for the /proc/bus/usb/BBB/DDD files. (Default:
- 0)
-
-*devmode*\ =MMM
- Controls the file mode used for the /proc/bus/usb/BBB/DDD files.
- (Default: 0644)
-
-*devuid*\ =NNNNN
- Controls the UID used for the /proc/bus/usb/BBB/DDD files. (Default:
- 0)
-
-*listgid*\ =NNNNN
- Controls the GID used for the /proc/bus/usb/devices and drivers
- files. (Default: 0)
-
-*listmode*\ =MMM
- Controls the file mode used for the /proc/bus/usb/devices and
- drivers files. (Default: 0444)
-
-*listuid*\ =NNNNN
- Controls the UID used for the /proc/bus/usb/devices and drivers
- files. (Default: 0)
-
-Note that many Linux distributions hard-wire the mount options for usbfs
-in their init scripts, such as ``/etc/rc.d/rc.sysinit``, rather than
-making it easy to set this per-system policy in ``/etc/fstab``.
-
-/proc/bus/usb/devices
----------------------
-
-This file is handy for status viewing tools in user mode, which can scan
-the text format and ignore most of it. More detailed device status
-(including class and vendor status) is available from device-specific
-files. For information about the current format of this file, see the
-``Documentation/usb/proc_usb_info.txt`` file in your Linux kernel
-sources.
-
-This file, in combination with the poll() system call, can also be used
-to detect when devices are added or removed:
-
-::
-
- int fd;
- struct pollfd pfd;
-
- fd = open("/proc/bus/usb/devices", O_RDONLY);
- pfd = { fd, POLLIN, 0 };
- for (;;) {
- /* The first time through, this call will return immediately. */
- poll(&pfd, 1, -1);
-
- /* To see what's changed, compare the file's previous and current
- contents or scan the filesystem. (Scanning is more precise.) */
- }
-
-Note that this behavior is intended to be used for informational and
-debug purposes. It would be more appropriate to use programs such as
-udev or HAL to initialize a device or start a user-mode helper program,
-for instance.
-
-/proc/bus/usb/BBB/DDD
----------------------
-
-Use these files in one of these basic ways:
-
-*They can be read,* producing first the device descriptor (18 bytes) and
-then the descriptors for the current configuration. See the USB 2.0 spec
-for details about those binary data formats. You'll need to convert most
-multibyte values from little endian format to your native host byte
-order, although a few of the fields in the device descriptor (both of
-the BCD-encoded fields, and the vendor and product IDs) will be
-byteswapped for you. Note that configuration descriptors include
-descriptors for interfaces, altsettings, endpoints, and maybe additional
-class descriptors.
-
-*Perform USB operations* using *ioctl()* requests to make endpoint I/O
-requests (synchronously or asynchronously) or manage the device. These
-requests need the CAP_SYS_RAWIO capability, as well as filesystem
-access permissions. Only one ioctl request can be made on one of these
-device files at a time. This means that if you are synchronously reading
-an endpoint from one thread, you won't be able to write to a different
-endpoint from another thread until the read completes. This works for
-*half duplex* protocols, but otherwise you'd use asynchronous i/o
-requests.
-
-Life Cycle of User Mode Drivers
--------------------------------
-
-Such a driver first needs to find a device file for a device it knows
-how to handle. Maybe it was told about it because a ``/sbin/hotplug``
-event handling agent chose that driver to handle the new device. Or
-maybe it's an application that scans all the /proc/bus/usb device files,
-and ignores most devices. In either case, it should :c:func:`read()`
-all the descriptors from the device file, and check them against what it
-knows how to handle. It might just reject everything except a particular
-vendor and product ID, or need a more complex policy.
-
-Never assume there will only be one such device on the system at a time!
-If your code can't handle more than one device at a time, at least
-detect when there's more than one, and have your users choose which
-device to use.
-
-Once your user mode driver knows what device to use, it interacts with
-it in either of two styles. The simple style is to make only control
-requests; some devices don't need more complex interactions than those.
-(An example might be software using vendor-specific control requests for
-some initialization or configuration tasks, with a kernel driver for the
-rest.)
-
-More likely, you need a more complex style driver: one using non-control
-endpoints, reading or writing data and claiming exclusive use of an
-interface. *Bulk* transfers are easiest to use, but only their sibling
-*interrupt* transfers work with low speed devices. Both interrupt and
-*isochronous* transfers offer service guarantees because their bandwidth
-is reserved. Such "periodic" transfers are awkward to use through usbfs,
-unless you're using the asynchronous calls. However, interrupt transfers
-can also be used in a synchronous "one shot" style.
-
-Your user-mode driver should never need to worry about cleaning up
-request state when the device is disconnected, although it should close
-its open file descriptors as soon as it starts seeing the ENODEV errors.
-
-The ioctl() Requests
---------------------
-
-To use these ioctls, you need to include the following headers in your
-userspace program:
-
-::
-
- #include <linux/usb.h>
- #include <linux/usbdevice_fs.h>
- #include <asm/byteorder.h>
-
-The standard USB device model requests, from "Chapter 9" of the USB 2.0
-specification, are automatically included from the ``<linux/usb/ch9.h>``
-header.
-
-Unless noted otherwise, the ioctl requests described here will update
-the modification time on the usbfs file to which they are applied
-(unless they fail). A return of zero indicates success; otherwise, a
-standard USB error code is returned. (These are documented in
-``Documentation/usb/error-codes.txt`` in your kernel sources.)
-
-Each of these files multiplexes access to several I/O streams, one per
-endpoint. Each device has one control endpoint (endpoint zero) which
-supports a limited RPC style RPC access. Devices are configured by
-hub_wq (in the kernel) setting a device-wide *configuration* that
-affects things like power consumption and basic functionality. The
-endpoints are part of USB *interfaces*, which may have *altsettings*
-affecting things like which endpoints are available. Many devices only
-have a single configuration and interface, so drivers for them will
-ignore configurations and altsettings.
-
-Management/Status Requests
-~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-A number of usbfs requests don't deal very directly with device I/O.
-They mostly relate to device management and status. These are all
-synchronous requests.
-
-USBDEVFS_CLAIMINTERFACE
- This is used to force usbfs to claim a specific interface, which has
- not previously been claimed by usbfs or any other kernel driver. The
- ioctl parameter is an integer holding the number of the interface
- (bInterfaceNumber from descriptor).
-
- Note that if your driver doesn't claim an interface before trying to
- use one of its endpoints, and no other driver has bound to it, then
- the interface is automatically claimed by usbfs.
-
- This claim will be released by a RELEASEINTERFACE ioctl, or by
- closing the file descriptor. File modification time is not updated
- by this request.
-
-USBDEVFS_CONNECTINFO
- Says whether the device is lowspeed. The ioctl parameter points to a
- structure like this:
-
- ::
-
- struct usbdevfs_connectinfo {
- unsigned int devnum;
- unsigned char slow;
- };
-
- File modification time is not updated by this request.
-
- *You can't tell whether a "not slow" device is connected at high
- speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should
- know the devnum value already, it's the DDD value of the device file
- name.
-
-USBDEVFS_GETDRIVER
- Returns the name of the kernel driver bound to a given interface (a
- string). Parameter is a pointer to this structure, which is
- modified:
-
- ::
-
- struct usbdevfs_getdriver {
- unsigned int interface;
- char driver[USBDEVFS_MAXDRIVERNAME + 1];
- };
-
- File modification time is not updated by this request.
-
-USBDEVFS_IOCTL
- Passes a request from userspace through to a kernel driver that has
- an ioctl entry in the *struct usb_driver* it registered.
-
- ::
-
- struct usbdevfs_ioctl {
- int ifno;
- int ioctl_code;
- void *data;
- };
-
- /* user mode call looks like this.
- * 'request' becomes the driver->ioctl() 'code' parameter.
- * the size of 'param' is encoded in 'request', and that data
- * is copied to or from the driver->ioctl() 'buf' parameter.
- */
- static int
- usbdev_ioctl (int fd, int ifno, unsigned request, void *param)
- {
- struct usbdevfs_ioctl wrapper;
-
- wrapper.ifno = ifno;
- wrapper.ioctl_code = request;
- wrapper.data = param;
-
- return ioctl (fd, USBDEVFS_IOCTL, &wrapper);
- }
-
- File modification time is not updated by this request.
-
- This request lets kernel drivers talk to user mode code through
- filesystem operations even when they don't create a character or
- block special device. It's also been used to do things like ask
- devices what device special file should be used. Two pre-defined
- ioctls are used to disconnect and reconnect kernel drivers, so that
- user mode code can completely manage binding and configuration of
- devices.
-
-USBDEVFS_RELEASEINTERFACE
- This is used to release the claim usbfs made on interface, either
- implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the
- file descriptor is closed. The ioctl parameter is an integer holding
- the number of the interface (bInterfaceNumber from descriptor); File
- modification time is not updated by this request.
-
- **Warning**
-
- *No security check is made to ensure that the task which made
- the claim is the one which is releasing it. This means that user
- mode driver may interfere other ones.*
-
-USBDEVFS_RESETEP
- Resets the data toggle value for an endpoint (bulk or interrupt) to
- DATA0. The ioctl parameter is an integer endpoint number (1 to 15,
- as identified in the endpoint descriptor), with USB_DIR_IN added
- if the device's endpoint sends data to the host.
-
- **Warning**
-
- *Avoid using this request. It should probably be removed.* Using
- it typically means the device and driver will lose toggle
- synchronization. If you really lost synchronization, you likely
- need to completely handshake with the device, using a request
- like CLEAR_HALT or SET_INTERFACE.
-
-USBDEVFS_DROP_PRIVILEGES
- This is used to relinquish the ability to do certain operations
- which are considered to be privileged on a usbfs file descriptor.
- This includes claiming arbitrary interfaces, resetting a device on
- which there are currently claimed interfaces from other users, and
- issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask
- of interfaces the user is allowed to claim on this file descriptor.
- You may issue this ioctl more than one time to narrow said mask.
-
-Synchronous I/O Support
-~~~~~~~~~~~~~~~~~~~~~~~
-
-Synchronous requests involve the kernel blocking until the user mode
-request completes, either by finishing successfully or by reporting an
-error. In most cases this is the simplest way to use usbfs, although as
-noted above it does prevent performing I/O to more than one endpoint at
-a time.
-
-USBDEVFS_BULK
- Issues a bulk read or write request to the device. The ioctl
- parameter is a pointer to this structure:
-
- ::
-
- struct usbdevfs_bulktransfer {
- unsigned int ep;
- unsigned int len;
- unsigned int timeout; /* in milliseconds */
- void *data;
- };
-
- The "ep" value identifies a bulk endpoint number (1 to 15, as
- identified in an endpoint descriptor), masked with USB_DIR_IN when
- referring to an endpoint which sends data to the host from the
- device. The length of the data buffer is identified by "len"; Recent
- kernels support requests up to about 128KBytes. *FIXME say how read
- length is returned, and how short reads are handled.*.
-
-USBDEVFS_CLEAR_HALT
- Clears endpoint halt (stall) and resets the endpoint toggle. This is
- only meaningful for bulk or interrupt endpoints. The ioctl parameter
- is an integer endpoint number (1 to 15, as identified in an endpoint
- descriptor), masked with USB_DIR_IN when referring to an endpoint
- which sends data to the host from the device.
-
- Use this on bulk or interrupt endpoints which have stalled,
- returning *-EPIPE* status to a data transfer request. Do not issue
- the control request directly, since that could invalidate the host's
- record of the data toggle.
-
-USBDEVFS_CONTROL
- Issues a control request to the device. The ioctl parameter points
- to a structure like this:
-
- ::
-
- struct usbdevfs_ctrltransfer {
- __u8 bRequestType;
- __u8 bRequest;
- __u16 wValue;
- __u16 wIndex;
- __u16 wLength;
- __u32 timeout; /* in milliseconds */
- void *data;
- };
-
- The first eight bytes of this structure are the contents of the
- SETUP packet to be sent to the device; see the USB 2.0 specification
- for details. The bRequestType value is composed by combining a
- USB_TYPE_\* value, a USB_DIR_\* value, and a USB_RECIP_\*
- value (from *<linux/usb.h>*). If wLength is nonzero, it describes
- the length of the data buffer, which is either written to the device
- (USB_DIR_OUT) or read from the device (USB_DIR_IN).
-
- At this writing, you can't transfer more than 4 KBytes of data to or
- from a device; usbfs has a limit, and some host controller drivers
- have a limit. (That's not usually a problem.) *Also* there's no way
- to say it's not OK to get a short read back from the device.
-
-USBDEVFS_RESET
- Does a USB level device reset. The ioctl parameter is ignored. After
- the reset, this rebinds all device interfaces. File modification
- time is not updated by this request.
-
- **Warning**
-
- *Avoid using this call* until some usbcore bugs get fixed, since
- it does not fully synchronize device, interface, and driver (not
- just usbfs) state.
-
-USBDEVFS_SETINTERFACE
- Sets the alternate setting for an interface. The ioctl parameter is
- a pointer to a structure like this:
-
- ::
-
- struct usbdevfs_setinterface {
- unsigned int interface;
- unsigned int altsetting;
- };
-
- File modification time is not updated by this request.
-
- Those struct members are from some interface descriptor applying to
- the current configuration. The interface number is the
- bInterfaceNumber value, and the altsetting number is the
- bAlternateSetting value. (This resets each endpoint in the
- interface.)
-
-USBDEVFS_SETCONFIGURATION
- Issues the :c:func:`usb_set_configuration()` call for the
- device. The parameter is an integer holding the number of a
- configuration (bConfigurationValue from descriptor). File
- modification time is not updated by this request.
-
- **Warning**
-
- *Avoid using this call* until some usbcore bugs get fixed, since
- it does not fully synchronize device, interface, and driver (not
- just usbfs) state.
-
-Asynchronous I/O Support
-~~~~~~~~~~~~~~~~~~~~~~~~
-
-As mentioned above, there are situations where it may be important to
-initiate concurrent operations from user mode code. This is particularly
-important for periodic transfers (interrupt and isochronous), but it can
-be used for other kinds of USB requests too. In such cases, the
-asynchronous requests described here are essential. Rather than
-submitting one request and having the kernel block until it completes,
-the blocking is separate.
-
-These requests are packaged into a structure that resembles the URB used
-by kernel device drivers. (No POSIX Async I/O support here, sorry.) It
-identifies the endpoint type (USBDEVFS_URB_TYPE_\*), endpoint
-(number, masked with USB_DIR_IN as appropriate), buffer and length,
-and a user "context" value serving to uniquely identify each request.
-(It's usually a pointer to per-request data.) Flags can modify requests
-(not as many as supported for kernel drivers).
-
-Each request can specify a realtime signal number (between SIGRTMIN and
-SIGRTMAX, inclusive) to request a signal be sent when the request
-completes.
-
-When usbfs returns these urbs, the status value is updated, and the
-buffer may have been modified. Except for isochronous transfers, the
-actual_length is updated to say how many bytes were transferred; if the
-USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if
-fewer bytes were read than were requested then you get an error report.
-
-::
-
- struct usbdevfs_iso_packet_desc {
- unsigned int length;
- unsigned int actual_length;
- unsigned int status;
- };
-
- struct usbdevfs_urb {
- unsigned char type;
- unsigned char endpoint;
- int status;
- unsigned int flags;
- void *buffer;
- int buffer_length;
- int actual_length;
- int start_frame;
- int number_of_packets;
- int error_count;
- unsigned int signr;
- void *usercontext;
- struct usbdevfs_iso_packet_desc iso_frame_desc[];
- };
-
-For these asynchronous requests, the file modification time reflects
-when the request was initiated. This contrasts with their use with the
-synchronous requests, where it reflects when requests complete.
-
-USBDEVFS_DISCARDURB
- *TBS* File modification time is not updated by this request.
-
-USBDEVFS_DISCSIGNAL
- *TBS* File modification time is not updated by this request.
-
-USBDEVFS_REAPURB
- *TBS* File modification time is not updated by this request.
-
-USBDEVFS_REAPURBNDELAY
- *TBS* File modification time is not updated by this request.
-
-USBDEVFS_SUBMITURB
- *TBS*
--- /dev/null
+========================
+USB Gadget API for Linux
+========================
+
+:Author: David Brownell
+:Date: 20 August 2004
+
+Introduction
+============
+
+This document presents a Linux-USB "Gadget" kernel mode API, for use
+within peripherals and other USB devices that embed Linux. It provides
+an overview of the API structure, and shows how that fits into a system
+development project. This is the first such API released on Linux to
+address a number of important problems, including:
+
+- Supports USB 2.0, for high speed devices which can stream data at
+ several dozen megabytes per second.
+
+- Handles devices with dozens of endpoints just as well as ones with
+ just two fixed-function ones. Gadget drivers can be written so
+ they're easy to port to new hardware.
+
+- Flexible enough to expose more complex USB device capabilities such
+ as multiple configurations, multiple interfaces, composite devices,
+ and alternate interface settings.
+
+- USB "On-The-Go" (OTG) support, in conjunction with updates to the
+ Linux-USB host side.
+
+- Sharing data structures and API models with the Linux-USB host side
+ API. This helps the OTG support, and looks forward to more-symmetric
+ frameworks (where the same I/O model is used by both host and device
+ side drivers).
+
+- Minimalist, so it's easier to support new device controller hardware.
+ I/O processing doesn't imply large demands for memory or CPU
+ resources.
+
+Most Linux developers will not be able to use this API, since they have
+USB "host" hardware in a PC, workstation, or server. Linux users with
+embedded systems are more likely to have USB peripheral hardware. To
+distinguish drivers running inside such hardware from the more familiar
+Linux "USB device drivers", which are host side proxies for the real USB
+devices, a different term is used: the drivers inside the peripherals
+are "USB gadget drivers". In USB protocol interactions, the device
+driver is the master (or "client driver") and the gadget driver is the
+slave (or "function driver").
+
+The gadget API resembles the host side Linux-USB API in that both use
+queues of request objects to package I/O buffers, and those requests may
+be submitted or canceled. They share common definitions for the standard
+USB *Chapter 9* messages, structures, and constants. Also, both APIs
+bind and unbind drivers to devices. The APIs differ in detail, since the
+host side's current URB framework exposes a number of implementation
+details and assumptions that are inappropriate for a gadget API. While
+the model for control transfers and configuration management is
+necessarily different (one side is a hardware-neutral master, the other
+is a hardware-aware slave), the endpoint I/0 API used here should also
+be usable for an overhead-reduced host side API.
+
+Structure of Gadget Drivers
+===========================
+
+A system running inside a USB peripheral normally has at least three
+layers inside the kernel to handle USB protocol processing, and may have
+additional layers in user space code. The "gadget" API is used by the
+middle layer to interact with the lowest level (which directly handles
+hardware).
+
+In Linux, from the bottom up, these layers are:
+
+*USB Controller Driver*
+ This is the lowest software level. It is the only layer that talks
+ to hardware, through registers, fifos, dma, irqs, and the like. The
+ ``<linux/usb/gadget.h>`` API abstracts the peripheral controller
+ endpoint hardware. That hardware is exposed through endpoint
+ objects, which accept streams of IN/OUT buffers, and through
+ callbacks that interact with gadget drivers. Since normal USB
+ devices only have one upstream port, they only have one of these
+ drivers. The controller driver can support any number of different
+ gadget drivers, but only one of them can be used at a time.
+
+ Examples of such controller hardware include the PCI-based NetChip
+ 2280 USB 2.0 high speed controller, the SA-11x0 or PXA-25x UDC
+ (found within many PDAs), and a variety of other products.
+
+*Gadget Driver*
+ The lower boundary of this driver implements hardware-neutral USB
+ functions, using calls to the controller driver. Because such
+ hardware varies widely in capabilities and restrictions, and is used
+ in embedded environments where space is at a premium, the gadget
+ driver is often configured at compile time to work with endpoints
+ supported by one particular controller. Gadget drivers may be
+ portable to several different controllers, using conditional
+ compilation. (Recent kernels substantially simplify the work
+ involved in supporting new hardware, by *autoconfiguring* endpoints
+ automatically for many bulk-oriented drivers.) Gadget driver
+ responsibilities include:
+
+ - handling setup requests (ep0 protocol responses) possibly
+ including class-specific functionality
+
+ - returning configuration and string descriptors
+
+ - (re)setting configurations and interface altsettings, including
+ enabling and configuring endpoints
+
+ - handling life cycle events, such as managing bindings to
+ hardware, USB suspend/resume, remote wakeup, and disconnection
+ from the USB host.
+
+ - managing IN and OUT transfers on all currently enabled endpoints
+
+ Such drivers may be modules of proprietary code, although that
+ approach is discouraged in the Linux community.
+
+*Upper Level*
+ Most gadget drivers have an upper boundary that connects to some
+ Linux driver or framework in Linux. Through that boundary flows the
+ data which the gadget driver produces and/or consumes through
+ protocol transfers over USB. Examples include:
+
+ - user mode code, using generic (gadgetfs) or application specific
+ files in ``/dev``
+
+ - networking subsystem (for network gadgets, like the CDC Ethernet
+ Model gadget driver)
+
+ - data capture drivers, perhaps video4Linux or a scanner driver; or
+ test and measurement hardware.
+
+ - input subsystem (for HID gadgets)
+
+ - sound subsystem (for audio gadgets)
+
+ - file system (for PTP gadgets)
+
+ - block i/o subsystem (for usb-storage gadgets)
+
+ - ... and more
+
+*Additional Layers*
+ Other layers may exist. These could include kernel layers, such as
+ network protocol stacks, as well as user mode applications building
+ on standard POSIX system call APIs such as *open()*, *close()*,
+ *read()* and *write()*. On newer systems, POSIX Async I/O calls may
+ be an option. Such user mode code will not necessarily be subject to
+ the GNU General Public License (GPL).
+
+OTG-capable systems will also need to include a standard Linux-USB host
+side stack, with *usbcore*, one or more *Host Controller Drivers*
+(HCDs), *USB Device Drivers* to support the OTG "Targeted Peripheral
+List", and so forth. There will also be an *OTG Controller Driver*,
+which is visible to gadget and device driver developers only indirectly.
+That helps the host and device side USB controllers implement the two
+new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or
+vice versa) using HNP during USB suspend processing, and SRP can be
+viewed as a more battery-friendly kind of device wakeup protocol.
+
+Over time, reusable utilities are evolving to help make some gadget
+driver tasks simpler. For example, building configuration descriptors
+from vectors of descriptors for the configurations interfaces and
+endpoints is now automated, and many drivers now use autoconfiguration
+to choose hardware endpoints and initialize their descriptors. A
+potential example of particular interest is code implementing standard
+USB-IF protocols for HID, networking, storage, or audio classes. Some
+developers are interested in KDB or KGDB hooks, to let target hardware
+be remotely debugged. Most such USB protocol code doesn't need to be
+hardware-specific, any more than network protocols like X11, HTTP, or
+NFS are. Such gadget-side interface drivers should eventually be
+combined, to implement composite devices.
+
+Kernel Mode Gadget API
+======================
+
+Gadget drivers declare themselves through a *struct
+usb_gadget_driver*, which is responsible for most parts of enumeration
+for a *struct usb_gadget*. The response to a set_configuration usually
+involves enabling one or more of the *struct usb_ep* objects exposed by
+the gadget, and submitting one or more *struct usb_request* buffers to
+transfer data. Understand those four data types, and their operations,
+and you will understand how this API works.
+
+ **Note**
+
+ This documentation was prepared using the standard Linux kernel
+ ``docproc`` tool, which turns text and in-code comments into SGML
+ DocBook and then into usable formats such as HTML or PDF. Other than
+ the "Chapter 9" data types, most of the significant data types and
+ functions are described here.
+
+ However, docproc does not understand all the C constructs that are
+ used, so some relevant information is likely omitted from what you
+ are reading. One example of such information is endpoint
+ autoconfiguration. You'll have to read the header file, and use
+ example source code (such as that for "Gadget Zero"), to fully
+ understand the API.
+
+ The part of the API implementing some basic driver capabilities is
+ specific to the version of the Linux kernel that's in use. The 2.6
+ kernel includes a *driver model* framework that has no analogue on
+ earlier kernels; so those parts of the gadget API are not fully
+ portable. (They are implemented on 2.4 kernels, but in a different
+ way.) The driver model state is another part of this API that is
+ ignored by the kerneldoc tools.
+
+The core API does not expose every possible hardware feature, only the
+most widely available ones. There are significant hardware features,
+such as device-to-device DMA (without temporary storage in a memory
+buffer) that would be added using hardware-specific APIs.
+
+This API allows drivers to use conditional compilation to handle
+endpoint capabilities of different hardware, but doesn't require that.
+Hardware tends to have arbitrary restrictions, relating to transfer
+types, addressing, packet sizes, buffering, and availability. As a rule,
+such differences only matter for "endpoint zero" logic that handles
+device configuration and management. The API supports limited run-time
+detection of capabilities, through naming conventions for endpoints.
+Many drivers will be able to at least partially autoconfigure
+themselves. In particular, driver init sections will often have endpoint
+autoconfiguration logic that scans the hardware's list of endpoints to
+find ones matching the driver requirements (relying on those
+conventions), to eliminate some of the most common reasons for
+conditional compilation.
+
+Like the Linux-USB host side API, this API exposes the "chunky" nature
+of USB messages: I/O requests are in terms of one or more "packets", and
+packet boundaries are visible to drivers. Compared to RS-232 serial
+protocols, USB resembles synchronous protocols like HDLC (N bytes per
+frame, multipoint addressing, host as the primary station and devices as
+secondary stations) more than asynchronous ones (tty style: 8 data bits
+per frame, no parity, one stop bit). So for example the controller
+drivers won't buffer two single byte writes into a single two-byte USB
+IN packet, although gadget drivers may do so when they implement
+protocols where packet boundaries (and "short packets") are not
+significant.
+
+Driver Life Cycle
+-----------------
+
+Gadget drivers make endpoint I/O requests to hardware without needing to
+know many details of the hardware, but driver setup/configuration code
+needs to handle some differences. Use the API like this:
+
+1. Register a driver for the particular device side usb controller
+ hardware, such as the net2280 on PCI (USB 2.0), sa11x0 or pxa25x as
+ found in Linux PDAs, and so on. At this point the device is logically
+ in the USB ch9 initial state ("attached"), drawing no power and not
+ usable (since it does not yet support enumeration). Any host should
+ not see the device, since it's not activated the data line pullup
+ used by the host to detect a device, even if VBUS power is available.
+
+2. Register a gadget driver that implements some higher level device
+ function. That will then bind() to a usb_gadget, which activates the
+ data line pullup sometime after detecting VBUS.
+
+3. The hardware driver can now start enumerating. The steps it handles
+ are to accept USB power and set_address requests. Other steps are
+ handled by the gadget driver. If the gadget driver module is unloaded
+ before the host starts to enumerate, steps before step 7 are skipped.
+
+4. The gadget driver's setup() call returns usb descriptors, based both
+ on what the bus interface hardware provides and on the functionality
+ being implemented. That can involve alternate settings or
+ configurations, unless the hardware prevents such operation. For OTG
+ devices, each configuration descriptor includes an OTG descriptor.
+
+5. The gadget driver handles the last step of enumeration, when the USB
+ host issues a set_configuration call. It enables all endpoints used
+ in that configuration, with all interfaces in their default settings.
+ That involves using a list of the hardware's endpoints, enabling each
+ endpoint according to its descriptor. It may also involve using
+ :c:func:`usb_gadget_vbus_draw()` to let more power be drawn
+ from VBUS, as allowed by that configuration. For OTG devices, setting
+ a configuration may also involve reporting HNP capabilities through a
+ user interface.
+
+6. Do real work and perform data transfers, possibly involving changes
+ to interface settings or switching to new configurations, until the
+ device is disconnect()ed from the host. Queue any number of transfer
+ requests to each endpoint. It may be suspended and resumed several
+ times before being disconnected. On disconnect, the drivers go back
+ to step 3 (above).
+
+7. When the gadget driver module is being unloaded, the driver unbind()
+ callback is issued. That lets the controller driver be unloaded.
+
+Drivers will normally be arranged so that just loading the gadget driver
+module (or statically linking it into a Linux kernel) allows the
+peripheral device to be enumerated, but some drivers will defer
+enumeration until some higher level component (like a user mode daemon)
+enables it. Note that at this lowest level there are no policies about
+how ep0 configuration logic is implemented, except that it should obey
+USB specifications. Such issues are in the domain of gadget drivers,
+including knowing about implementation constraints imposed by some USB
+controllers or understanding that composite devices might happen to be
+built by integrating reusable components.
+
+Note that the lifecycle above can be slightly different for OTG devices.
+Other than providing an additional OTG descriptor in each configuration,
+only the HNP-related differences are particularly visible to driver
+code. They involve reporting requirements during the SET_CONFIGURATION
+request, and the option to invoke HNP during some suspend callbacks.
+Also, SRP changes the semantics of :c:func:`usb_gadget_wakeup()`
+slightly.
+
+USB 2.0 Chapter 9 Types and Constants
+-------------------------------------
+
+Gadget drivers rely on common USB structures and constants defined in
+the ``<linux/usb/ch9.h>`` header file, which is standard in Linux 2.6
+kernels. These are the same types and constants used by host side
+drivers (and usbcore).
+
+.. kernel-doc:: include/linux/usb/ch9.h
+ :internal:
+
+Core Objects and Methods
+------------------------
+
+These are declared in ``<linux/usb/gadget.h>``, and are used by gadget
+drivers to interact with USB peripheral controller drivers.
+
+.. kernel-doc:: include/linux/usb/gadget.h
+ :internal:
+
+Optional Utilities
+------------------
+
+The core API is sufficient for writing a USB Gadget Driver, but some
+optional utilities are provided to simplify common tasks. These
+utilities include endpoint autoconfiguration.
+
+.. kernel-doc:: drivers/usb/gadget/usbstring.c
+ :export:
+
+.. kernel-doc:: drivers/usb/gadget/config.c
+ :export:
+
+Composite Device Framework
+--------------------------
+
+The core API is sufficient for writing drivers for composite USB devices
+(with more than one function in a given configuration), and also
+multi-configuration devices (also more than one function, but not
+necessarily sharing a given configuration). There is however an optional
+framework which makes it easier to reuse and combine functions.
+
+Devices using this framework provide a *struct usb_composite_driver*,
+which in turn provides one or more *struct usb_configuration*
+instances. Each such configuration includes at least one *struct
+usb_function*, which packages a user visible role such as "network
+link" or "mass storage device". Management functions may also exist,
+such as "Device Firmware Upgrade".
+
+.. kernel-doc:: include/linux/usb/composite.h
+ :internal:
+
+.. kernel-doc:: drivers/usb/gadget/composite.c
+ :export:
+
+Composite Device Functions
+--------------------------
+
+At this writing, a few of the current gadget drivers have been converted
+to this framework. Near-term plans include converting all of them,
+except for "gadgetfs".
+
+.. kernel-doc:: drivers/usb/gadget/function/f_acm.c
+ :export:
+
+.. kernel-doc:: drivers/usb/gadget/function/f_ecm.c
+ :export:
+
+.. kernel-doc:: drivers/usb/gadget/function/f_subset.c
+ :export:
+
+.. kernel-doc:: drivers/usb/gadget/function/f_obex.c
+ :export:
+
+.. kernel-doc:: drivers/usb/gadget/function/f_serial.c
+ :export:
+
+Peripheral Controller Drivers
+=============================
+
+The first hardware supporting this API was the NetChip 2280 controller,
+which supports USB 2.0 high speed and is based on PCI. This is the
+``net2280`` driver module. The driver supports Linux kernel versions 2.4
+and 2.6; contact NetChip Technologies for development boards and product
+information.
+
+Other hardware working in the "gadget" framework includes: Intel's PXA
+25x and IXP42x series processors (``pxa2xx_udc``), Toshiba TC86c001
+"Goku-S" (``goku_udc``), Renesas SH7705/7727 (``sh_udc``), MediaQ 11xx
+(``mq11xx_udc``), Hynix HMS30C7202 (``h7202_udc``), National 9303/4
+(``n9604_udc``), Texas Instruments OMAP (``omap_udc``), Sharp LH7A40x
+(``lh7a40x_udc``), and more. Most of those are full speed controllers.
+
+At this writing, there are people at work on drivers in this framework
+for several other USB device controllers, with plans to make many of
+them be widely available.
+
+A partial USB simulator, the ``dummy_hcd`` driver, is available. It can
+act like a net2280, a pxa25x, or an sa11x0 in terms of available
+endpoints and device speeds; and it simulates control, bulk, and to some
+extent interrupt transfers. That lets you develop some parts of a gadget
+driver on a normal PC, without any special hardware, and perhaps with
+the assistance of tools such as GDB running with User Mode Linux. At
+least one person has expressed interest in adapting that approach,
+hooking it up to a simulator for a microcontroller. Such simulators can
+help debug subsystems where the runtime hardware is unfriendly to
+software development, or is not yet available.
+
+Support for other controllers is expected to be developed and
+contributed over time, as this driver framework evolves.
+
+Gadget Drivers
+==============
+
+In addition to *Gadget Zero* (used primarily for testing and development
+with drivers for usb controller hardware), other gadget drivers exist.
+
+There's an *ethernet* gadget driver, which implements one of the most
+useful *Communications Device Class* (CDC) models. One of the standards
+for cable modem interoperability even specifies the use of this ethernet
+model as one of two mandatory options. Gadgets using this code look to a
+USB host as if they're an Ethernet adapter. It provides access to a
+network where the gadget's CPU is one host, which could easily be
+bridging, routing, or firewalling access to other networks. Since some
+hardware can't fully implement the CDC Ethernet requirements, this
+driver also implements a "good parts only" subset of CDC Ethernet. (That
+subset doesn't advertise itself as CDC Ethernet, to avoid creating
+problems.)
+
+Support for Microsoft's *RNDIS* protocol has been contributed by
+Pengutronix and Auerswald GmbH. This is like CDC Ethernet, but it runs
+on more slightly USB hardware (but less than the CDC subset). However,
+its main claim to fame is being able to connect directly to recent
+versions of Windows, using drivers that Microsoft bundles and supports,
+making it much simpler to network with Windows.
+
+There is also support for user mode gadget drivers, using *gadgetfs*.
+This provides a *User Mode API* that presents each endpoint as a single
+file descriptor. I/O is done using normal *read()* and *read()* calls.
+Familiar tools like GDB and pthreads can be used to develop and debug
+user mode drivers, so that once a robust controller driver is available
+many applications for it won't require new kernel mode software. Linux
+2.6 *Async I/O (AIO)* support is available, so that user mode software
+can stream data with only slightly more overhead than a kernel driver.
+
+There's a USB Mass Storage class driver, which provides a different
+solution for interoperability with systems such as MS-Windows and MacOS.
+That *Mass Storage* driver uses a file or block device as backing store
+for a drive, like the ``loop`` driver. The USB host uses the BBB, CB, or
+CBI versions of the mass storage class specification, using transparent
+SCSI commands to access the data from the backing store.
+
+There's a "serial line" driver, useful for TTY style operation over USB.
+The latest version of that driver supports CDC ACM style operation, like
+a USB modem, and so on most hardware it can interoperate easily with
+MS-Windows. One interesting use of that driver is in boot firmware (like
+a BIOS), which can sometimes use that model with very small systems
+without real serial lines.
+
+Support for other kinds of gadget is expected to be developed and
+contributed over time, as this driver framework evolves.
+
+USB On-The-GO (OTG)
+===================
+
+USB OTG support on Linux 2.6 was initially developed by Texas
+Instruments for `OMAP <http://www.omap.com>`__ 16xx and 17xx series
+processors. Other OTG systems should work in similar ways, but the
+hardware level details could be very different.
+
+Systems need specialized hardware support to implement OTG, notably
+including a special *Mini-AB* jack and associated transceiver to support
+*Dual-Role* operation: they can act either as a host, using the standard
+Linux-USB host side driver stack, or as a peripheral, using this
+"gadget" framework. To do that, the system software relies on small
+additions to those programming interfaces, and on a new internal
+component (here called an "OTG Controller") affecting which driver stack
+connects to the OTG port. In each role, the system can re-use the
+existing pool of hardware-neutral drivers, layered on top of the
+controller driver interfaces (*usb_bus* or *usb_gadget*). Such drivers
+need at most minor changes, and most of the calls added to support OTG
+can also benefit non-OTG products.
+
+- Gadget drivers test the *is_otg* flag, and use it to determine
+ whether or not to include an OTG descriptor in each of their
+ configurations.
+
+- Gadget drivers may need changes to support the two new OTG protocols,
+ exposed in new gadget attributes such as *b_hnp_enable* flag. HNP
+ support should be reported through a user interface (two LEDs could
+ suffice), and is triggered in some cases when the host suspends the
+ peripheral. SRP support can be user-initiated just like remote
+ wakeup, probably by pressing the same button.
+
+- On the host side, USB device drivers need to be taught to trigger HNP
+ at appropriate moments, using :c:func:`usb_suspend_device()`.
+ That also conserves battery power, which is useful even for non-OTG
+ configurations.
+
+- Also on the host side, a driver must support the OTG "Targeted
+ Peripheral List". That's just a whitelist, used to reject peripherals
+ not supported with a given Linux OTG host. *This whitelist is
+ product-specific; each product must modify ``otg_whitelist.h`` to
+ match its interoperability specification.*
+
+ Non-OTG Linux hosts, like PCs and workstations, normally have some
+ solution for adding drivers, so that peripherals that aren't
+ recognized can eventually be supported. That approach is unreasonable
+ for consumer products that may never have their firmware upgraded,
+ and where it's usually unrealistic to expect traditional
+ PC/workstation/server kinds of support model to work. For example,
+ it's often impractical to change device firmware once the product has
+ been distributed, so driver bugs can't normally be fixed if they're
+ found after shipment.
+
+Additional changes are needed below those hardware-neutral *usb_bus*
+and *usb_gadget* driver interfaces; those aren't discussed here in any
+detail. Those affect the hardware-specific code for each USB Host or
+Peripheral controller, and how the HCD initializes (since OTG can be
+active only on a single port). They also involve what may be called an
+*OTG Controller Driver*, managing the OTG transceiver and the OTG state
+machine logic as well as much of the root hub behavior for the OTG port.
+The OTG controller driver needs to activate and deactivate USB
+controllers depending on the relevant device role. Some related changes
+were needed inside usbcore, so that it can identify OTG-capable devices
+and respond appropriately to HNP or SRP protocols.
--- /dev/null
+=============
+Linux USB API
+=============
+
+.. toctree::
+
+ usb
+ gadget
+ writing_usb_driver
+ writing_musb_glue_layer
+
+.. only:: subproject and html
+
+ Indices
+ =======
+
+ * :ref:`genindex`
--- /dev/null
+===========================
+The Linux-USB Host Side API
+===========================
+
+Introduction to USB on Linux
+============================
+
+A Universal Serial Bus (USB) is used to connect a host, such as a PC or
+workstation, to a number of peripheral devices. USB uses a tree
+structure, with the host as the root (the system's master), hubs as
+interior nodes, and peripherals as leaves (and slaves). Modern PCs
+support several such trees of USB devices, usually
+a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy
+USB 2.0 (480 MBit/s) busses just in case.
+
+That master/slave asymmetry was designed-in for a number of reasons, one
+being ease of use. It is not physically possible to mistake upstream and
+downstream or it does not matter with a type C plug (or they are built into the
+peripheral). Also, the host software doesn't need to deal with
+distributed auto-configuration since the pre-designated master node
+manages all that.
+
+Kernel developers added USB support to Linux early in the 2.2 kernel
+series and have been developing it further since then. Besides support
+for each new generation of USB, various host controllers gained support,
+new drivers for peripherals have been added and advanced features for latency
+measurement and improved power management introduced.
+
+Linux can run inside USB devices as well as on the hosts that control
+the devices. But USB device drivers running inside those peripherals
+don't do the same things as the ones running inside hosts, so they've
+been given a different name: *gadget drivers*. This document does not
+cover gadget drivers.
+
+USB Host-Side API Model
+=======================
+
+Host-side drivers for USB devices talk to the "usbcore" APIs. There are
+two. One is intended for *general-purpose* drivers (exposed through
+driver frameworks), and the other is for drivers that are *part of the
+core*. Such core drivers include the *hub* driver (which manages trees
+of USB devices) and several different kinds of *host controller
+drivers*, which control individual busses.
+
+The device model seen by USB drivers is relatively complex.
+
+- USB supports four kinds of data transfers (control, bulk, interrupt,
+ and isochronous). Two of them (control and bulk) use bandwidth as
+ it's available, while the other two (interrupt and isochronous) are
+ scheduled to provide guaranteed bandwidth.
+
+- The device description model includes one or more "configurations"
+ per device, only one of which is active at a time. Devices are supposed
+ to be capable of operating at lower than their top
+ speeds and may provide a BOS descriptor showing the lowest speed they
+ remain fully operational at.
+
+- From USB 3.0 on configurations have one or more "functions", which
+ provide a common functionality and are grouped together for purposes
+ of power management.
+
+- Configurations or functions have one or more "interfaces", each of which may have
+ "alternate settings". Interfaces may be standardized by USB "Class"
+ specifications, or may be specific to a vendor or device.
+
+ USB device drivers actually bind to interfaces, not devices. Think of
+ them as "interface drivers", though you may not see many devices
+ where the distinction is important. *Most USB devices are simple,
+ with only one function, one configuration, one interface, and one alternate
+ setting.*
+
+- Interfaces have one or more "endpoints", each of which supports one
+ type and direction of data transfer such as "bulk out" or "interrupt
+ in". The entire configuration may have up to sixteen endpoints in
+ each direction, allocated as needed among all the interfaces.
+
+- Data transfer on USB is packetized; each endpoint has a maximum
+ packet size. Drivers must often be aware of conventions such as
+ flagging the end of bulk transfers using "short" (including zero
+ length) packets.
+
+- The Linux USB API supports synchronous calls for control and bulk
+ messages. It also supports asynchronous calls for all kinds of data
+ transfer, using request structures called "URBs" (USB Request
+ Blocks).
+
+Accordingly, the USB Core API exposed to device drivers covers quite a
+lot of territory. You'll probably need to consult the USB 3.0
+specification, available online from www.usb.org at no cost, as well as
+class or device specifications.
+
+The only host-side drivers that actually touch hardware (reading/writing
+registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs
+provide the same functionality through the same API. In practice, that's
+becoming more true, but there are still differences
+that crop up especially with fault handling on the less common controllers.
+Different controllers don't
+necessarily report the same aspects of failures, and recovery from
+faults (including software-induced ones like unlinking an URB) isn't yet
+fully consistent. Device driver authors should make a point of doing
+disconnect testing (while the device is active) with each different host
+controller driver, to make sure drivers don't have bugs of their own as
+well as to make sure they aren't relying on some HCD-specific behavior.
+
+USB-Standard Types
+==================
+
+In ``<linux/usb/ch9.h>`` you will find the USB data types defined in
+chapter 9 of the USB specification. These data types are used throughout
+USB, and in APIs including this host side API, gadget APIs, and usbfs.
+
+.. kernel-doc:: include/linux/usb/ch9.h
+ :internal:
+
+Host-Side Data Types and Macros
+===============================
+
+The host side API exposes several layers to drivers, some of which are
+more necessary than others. These support lifecycle models for host side
+drivers and devices, and support passing buffers through usbcore to some
+HCD that performs the I/O for the device driver.
+
+.. kernel-doc:: include/linux/usb.h
+ :internal:
+
+USB Core APIs
+=============
+
+There are two basic I/O models in the USB API. The most elemental one is
+asynchronous: drivers submit requests in the form of an URB, and the
+URB's completion callback handles the next step. All USB transfer types
+support that model, although there are special cases for control URBs
+(which always have setup and status stages, but may not have a data
+stage) and isochronous URBs (which allow large packets and include
+per-packet fault reports). Built on top of that is synchronous API
+support, where a driver calls a routine that allocates one or more URBs,
+submits them, and waits until they complete. There are synchronous
+wrappers for single-buffer control and bulk transfers (which are awkward
+to use in some driver disconnect scenarios), and for scatterlist based
+streaming i/o (bulk or interrupt).
+
+USB drivers need to provide buffers that can be used for DMA, although
+they don't necessarily need to provide the DMA mapping themselves. There
+are APIs to use used when allocating DMA buffers, which can prevent use
+of bounce buffers on some systems. In some cases, drivers may be able to
+rely on 64bit DMA to eliminate another kind of bounce buffer.
+
+.. kernel-doc:: drivers/usb/core/urb.c
+ :export:
+
+.. kernel-doc:: drivers/usb/core/message.c
+ :export:
+
+.. kernel-doc:: drivers/usb/core/file.c
+ :export:
+
+.. kernel-doc:: drivers/usb/core/driver.c
+ :export:
+
+.. kernel-doc:: drivers/usb/core/usb.c
+ :export:
+
+.. kernel-doc:: drivers/usb/core/hub.c
+ :export:
+
+Host Controller APIs
+====================
+
+These APIs are only for use by host controller drivers, most of which
+implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI
+was one of the first interfaces, designed by Intel and also used by VIA;
+it doesn't do much in hardware. OHCI was designed later, to have the
+hardware do more work (bigger transfers, tracking protocol state, and so
+on). EHCI was designed with USB 2.0; its design has features that
+resemble OHCI (hardware does much more work) as well as UHCI (some parts
+of ISO support, TD list processing). XHCI was designed with USB 3.0. It
+continues to shift support for functionality into hardware.
+
+There are host controllers other than the "big three", although most PCI
+based controllers (and a few non-PCI based ones) use one of those
+interfaces. Not all host controllers use DMA; some use PIO, and there is
+also a simulator and a virtual host controller to pipe USB over the network.
+
+The same basic APIs are available to drivers for all those controllers.
+For historical reasons they are in two layers: :c:type:`struct
+usb_bus <usb_bus>` is a rather thin layer that became available
+in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>`
+is a more featureful layer
+that lets HCDs share common code, to shrink driver size and
+significantly reduce hcd-specific behaviors.
+
+.. kernel-doc:: drivers/usb/core/hcd.c
+ :export:
+
+.. kernel-doc:: drivers/usb/core/hcd-pci.c
+ :export:
+
+.. kernel-doc:: drivers/usb/core/buffer.c
+ :internal:
+
+The USB Filesystem (usbfs)
+==========================
+
+This chapter presents the Linux *usbfs*. You may prefer to avoid writing
+new kernel code for your USB driver; that's the problem that usbfs set
+out to solve. User mode device drivers are usually packaged as
+applications or libraries, and may use usbfs through some programming
+library that wraps it. Such libraries include
+`libusb <http://libusb.sourceforge.net>`__ for C/C++, and
+`jUSB <http://jUSB.sourceforge.net>`__ for Java.
+
+ **Note**
+
+ This particular documentation is incomplete, especially with respect
+ to the asynchronous mode. As of kernel 2.5.66 the code and this
+ (new) documentation need to be cross-reviewed.
+
+Configure usbfs into Linux kernels by enabling the *USB filesystem*
+option (CONFIG_USB_DEVICEFS), and you get basic support for user mode
+USB device drivers. Until relatively recently it was often (confusingly)
+called *usbdevfs* although it wasn't solving what *devfs* was. Every USB
+device will appear in usbfs, regardless of whether or not it has a
+kernel driver.
+
+What files are in "usbfs"?
+--------------------------
+
+Conventionally mounted at ``/proc/bus/usb``, usbfs features include:
+
+- ``/proc/bus/usb/devices`` ... a text file showing each of the USB
+ devices on known to the kernel, and their configuration descriptors.
+ You can also poll() this to learn about new devices.
+
+- ``/proc/bus/usb/BBB/DDD`` ... magic files exposing the each device's
+ configuration descriptors, and supporting a series of ioctls for
+ making device requests, including I/O to devices. (Purely for access
+ by programs.)
+
+Each bus is given a number (BBB) based on when it was enumerated; within
+each bus, each device is given a similar number (DDD). Those BBB/DDD
+paths are not "stable" identifiers; expect them to change even if you
+always leave the devices plugged in to the same hub port. *Don't even
+think of saving these in application configuration files.* Stable
+identifiers are available, for user mode applications that want to use
+them. HID and networking devices expose these stable IDs, so that for
+example you can be sure that you told the right UPS to power down its
+second server. "usbfs" doesn't (yet) expose those IDs.
+
+Mounting and Access Control
+---------------------------
+
+There are a number of mount options for usbfs, which will be of most
+interest to you if you need to override the default access control
+policy. That policy is that only root may read or write device files
+(``/proc/bus/BBB/DDD``) although anyone may read the ``devices`` or
+``drivers`` files. I/O requests to the device also need the
+CAP_SYS_RAWIO capability,
+
+The significance of that is that by default, all user mode device
+drivers need super-user privileges. You can change modes or ownership in
+a driver setup when the device hotplugs, or maye just start the driver
+right then, as a privileged server (or some activity within one). That's
+the most secure approach for multi-user systems, but for single user
+systems ("trusted" by that user) it's more convenient just to grant
+everyone all access (using the *devmode=0666* option) so the driver can
+start whenever it's needed.
+
+The mount options for usbfs, usable in /etc/fstab or in command line
+invocations of *mount*, are:
+
+*busgid*\ =NNNNN
+ Controls the GID used for the /proc/bus/usb/BBB directories.
+ (Default: 0)
+
+*busmode*\ =MMM
+ Controls the file mode used for the /proc/bus/usb/BBB directories.
+ (Default: 0555)
+
+*busuid*\ =NNNNN
+ Controls the UID used for the /proc/bus/usb/BBB directories.
+ (Default: 0)
+
+*devgid*\ =NNNNN
+ Controls the GID used for the /proc/bus/usb/BBB/DDD files. (Default:
+ 0)
+
+*devmode*\ =MMM
+ Controls the file mode used for the /proc/bus/usb/BBB/DDD files.
+ (Default: 0644)
+
+*devuid*\ =NNNNN
+ Controls the UID used for the /proc/bus/usb/BBB/DDD files. (Default:
+ 0)
+
+*listgid*\ =NNNNN
+ Controls the GID used for the /proc/bus/usb/devices and drivers
+ files. (Default: 0)
+
+*listmode*\ =MMM
+ Controls the file mode used for the /proc/bus/usb/devices and
+ drivers files. (Default: 0444)
+
+*listuid*\ =NNNNN
+ Controls the UID used for the /proc/bus/usb/devices and drivers
+ files. (Default: 0)
+
+Note that many Linux distributions hard-wire the mount options for usbfs
+in their init scripts, such as ``/etc/rc.d/rc.sysinit``, rather than
+making it easy to set this per-system policy in ``/etc/fstab``.
+
+/proc/bus/usb/devices
+---------------------
+
+This file is handy for status viewing tools in user mode, which can scan
+the text format and ignore most of it. More detailed device status
+(including class and vendor status) is available from device-specific
+files. For information about the current format of this file, see the
+``Documentation/usb/proc_usb_info.txt`` file in your Linux kernel
+sources.
+
+This file, in combination with the poll() system call, can also be used
+to detect when devices are added or removed:
+
+::
+
+ int fd;
+ struct pollfd pfd;
+
+ fd = open("/proc/bus/usb/devices", O_RDONLY);
+ pfd = { fd, POLLIN, 0 };
+ for (;;) {
+ /* The first time through, this call will return immediately. */
+ poll(&pfd, 1, -1);
+
+ /* To see what's changed, compare the file's previous and current
+ contents or scan the filesystem. (Scanning is more precise.) */
+ }
+
+Note that this behavior is intended to be used for informational and
+debug purposes. It would be more appropriate to use programs such as
+udev or HAL to initialize a device or start a user-mode helper program,
+for instance.
+
+/proc/bus/usb/BBB/DDD
+---------------------
+
+Use these files in one of these basic ways:
+
+*They can be read,* producing first the device descriptor (18 bytes) and
+then the descriptors for the current configuration. See the USB 2.0 spec
+for details about those binary data formats. You'll need to convert most
+multibyte values from little endian format to your native host byte
+order, although a few of the fields in the device descriptor (both of
+the BCD-encoded fields, and the vendor and product IDs) will be
+byteswapped for you. Note that configuration descriptors include
+descriptors for interfaces, altsettings, endpoints, and maybe additional
+class descriptors.
+
+*Perform USB operations* using *ioctl()* requests to make endpoint I/O
+requests (synchronously or asynchronously) or manage the device. These
+requests need the CAP_SYS_RAWIO capability, as well as filesystem
+access permissions. Only one ioctl request can be made on one of these
+device files at a time. This means that if you are synchronously reading
+an endpoint from one thread, you won't be able to write to a different
+endpoint from another thread until the read completes. This works for
+*half duplex* protocols, but otherwise you'd use asynchronous i/o
+requests.
+
+Life Cycle of User Mode Drivers
+-------------------------------
+
+Such a driver first needs to find a device file for a device it knows
+how to handle. Maybe it was told about it because a ``/sbin/hotplug``
+event handling agent chose that driver to handle the new device. Or
+maybe it's an application that scans all the /proc/bus/usb device files,
+and ignores most devices. In either case, it should :c:func:`read()`
+all the descriptors from the device file, and check them against what it
+knows how to handle. It might just reject everything except a particular
+vendor and product ID, or need a more complex policy.
+
+Never assume there will only be one such device on the system at a time!
+If your code can't handle more than one device at a time, at least
+detect when there's more than one, and have your users choose which
+device to use.
+
+Once your user mode driver knows what device to use, it interacts with
+it in either of two styles. The simple style is to make only control
+requests; some devices don't need more complex interactions than those.
+(An example might be software using vendor-specific control requests for
+some initialization or configuration tasks, with a kernel driver for the
+rest.)
+
+More likely, you need a more complex style driver: one using non-control
+endpoints, reading or writing data and claiming exclusive use of an
+interface. *Bulk* transfers are easiest to use, but only their sibling
+*interrupt* transfers work with low speed devices. Both interrupt and
+*isochronous* transfers offer service guarantees because their bandwidth
+is reserved. Such "periodic" transfers are awkward to use through usbfs,
+unless you're using the asynchronous calls. However, interrupt transfers
+can also be used in a synchronous "one shot" style.
+
+Your user-mode driver should never need to worry about cleaning up
+request state when the device is disconnected, although it should close
+its open file descriptors as soon as it starts seeing the ENODEV errors.
+
+The ioctl() Requests
+--------------------
+
+To use these ioctls, you need to include the following headers in your
+userspace program:
+
+::
+
+ #include <linux/usb.h>
+ #include <linux/usbdevice_fs.h>
+ #include <asm/byteorder.h>
+
+The standard USB device model requests, from "Chapter 9" of the USB 2.0
+specification, are automatically included from the ``<linux/usb/ch9.h>``
+header.
+
+Unless noted otherwise, the ioctl requests described here will update
+the modification time on the usbfs file to which they are applied
+(unless they fail). A return of zero indicates success; otherwise, a
+standard USB error code is returned. (These are documented in
+``Documentation/usb/error-codes.txt`` in your kernel sources.)
+
+Each of these files multiplexes access to several I/O streams, one per
+endpoint. Each device has one control endpoint (endpoint zero) which
+supports a limited RPC style RPC access. Devices are configured by
+hub_wq (in the kernel) setting a device-wide *configuration* that
+affects things like power consumption and basic functionality. The
+endpoints are part of USB *interfaces*, which may have *altsettings*
+affecting things like which endpoints are available. Many devices only
+have a single configuration and interface, so drivers for them will
+ignore configurations and altsettings.
+
+Management/Status Requests
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A number of usbfs requests don't deal very directly with device I/O.
+They mostly relate to device management and status. These are all
+synchronous requests.
+
+USBDEVFS_CLAIMINTERFACE
+ This is used to force usbfs to claim a specific interface, which has
+ not previously been claimed by usbfs or any other kernel driver. The
+ ioctl parameter is an integer holding the number of the interface
+ (bInterfaceNumber from descriptor).
+
+ Note that if your driver doesn't claim an interface before trying to
+ use one of its endpoints, and no other driver has bound to it, then
+ the interface is automatically claimed by usbfs.
+
+ This claim will be released by a RELEASEINTERFACE ioctl, or by
+ closing the file descriptor. File modification time is not updated
+ by this request.
+
+USBDEVFS_CONNECTINFO
+ Says whether the device is lowspeed. The ioctl parameter points to a
+ structure like this:
+
+ ::
+
+ struct usbdevfs_connectinfo {
+ unsigned int devnum;
+ unsigned char slow;
+ };
+
+ File modification time is not updated by this request.
+
+ *You can't tell whether a "not slow" device is connected at high
+ speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should
+ know the devnum value already, it's the DDD value of the device file
+ name.
+
+USBDEVFS_GETDRIVER
+ Returns the name of the kernel driver bound to a given interface (a
+ string). Parameter is a pointer to this structure, which is
+ modified:
+
+ ::
+
+ struct usbdevfs_getdriver {
+ unsigned int interface;
+ char driver[USBDEVFS_MAXDRIVERNAME + 1];
+ };
+
+ File modification time is not updated by this request.
+
+USBDEVFS_IOCTL
+ Passes a request from userspace through to a kernel driver that has
+ an ioctl entry in the *struct usb_driver* it registered.
+
+ ::
+
+ struct usbdevfs_ioctl {
+ int ifno;
+ int ioctl_code;
+ void *data;
+ };
+
+ /* user mode call looks like this.
+ * 'request' becomes the driver->ioctl() 'code' parameter.
+ * the size of 'param' is encoded in 'request', and that data
+ * is copied to or from the driver->ioctl() 'buf' parameter.
+ */
+ static int
+ usbdev_ioctl (int fd, int ifno, unsigned request, void *param)
+ {
+ struct usbdevfs_ioctl wrapper;
+
+ wrapper.ifno = ifno;
+ wrapper.ioctl_code = request;
+ wrapper.data = param;
+
+ return ioctl (fd, USBDEVFS_IOCTL, &wrapper);
+ }
+
+ File modification time is not updated by this request.
+
+ This request lets kernel drivers talk to user mode code through
+ filesystem operations even when they don't create a character or
+ block special device. It's also been used to do things like ask
+ devices what device special file should be used. Two pre-defined
+ ioctls are used to disconnect and reconnect kernel drivers, so that
+ user mode code can completely manage binding and configuration of
+ devices.
+
+USBDEVFS_RELEASEINTERFACE
+ This is used to release the claim usbfs made on interface, either
+ implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the
+ file descriptor is closed. The ioctl parameter is an integer holding
+ the number of the interface (bInterfaceNumber from descriptor); File
+ modification time is not updated by this request.
+
+ **Warning**
+
+ *No security check is made to ensure that the task which made
+ the claim is the one which is releasing it. This means that user
+ mode driver may interfere other ones.*
+
+USBDEVFS_RESETEP
+ Resets the data toggle value for an endpoint (bulk or interrupt) to
+ DATA0. The ioctl parameter is an integer endpoint number (1 to 15,
+ as identified in the endpoint descriptor), with USB_DIR_IN added
+ if the device's endpoint sends data to the host.
+
+ **Warning**
+
+ *Avoid using this request. It should probably be removed.* Using
+ it typically means the device and driver will lose toggle
+ synchronization. If you really lost synchronization, you likely
+ need to completely handshake with the device, using a request
+ like CLEAR_HALT or SET_INTERFACE.
+
+USBDEVFS_DROP_PRIVILEGES
+ This is used to relinquish the ability to do certain operations
+ which are considered to be privileged on a usbfs file descriptor.
+ This includes claiming arbitrary interfaces, resetting a device on
+ which there are currently claimed interfaces from other users, and
+ issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask
+ of interfaces the user is allowed to claim on this file descriptor.
+ You may issue this ioctl more than one time to narrow said mask.
+
+Synchronous I/O Support
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Synchronous requests involve the kernel blocking until the user mode
+request completes, either by finishing successfully or by reporting an
+error. In most cases this is the simplest way to use usbfs, although as
+noted above it does prevent performing I/O to more than one endpoint at
+a time.
+
+USBDEVFS_BULK
+ Issues a bulk read or write request to the device. The ioctl
+ parameter is a pointer to this structure:
+
+ ::
+
+ struct usbdevfs_bulktransfer {
+ unsigned int ep;
+ unsigned int len;
+ unsigned int timeout; /* in milliseconds */
+ void *data;
+ };
+
+ The "ep" value identifies a bulk endpoint number (1 to 15, as
+ identified in an endpoint descriptor), masked with USB_DIR_IN when
+ referring to an endpoint which sends data to the host from the
+ device. The length of the data buffer is identified by "len"; Recent
+ kernels support requests up to about 128KBytes. *FIXME say how read
+ length is returned, and how short reads are handled.*.
+
+USBDEVFS_CLEAR_HALT
+ Clears endpoint halt (stall) and resets the endpoint toggle. This is
+ only meaningful for bulk or interrupt endpoints. The ioctl parameter
+ is an integer endpoint number (1 to 15, as identified in an endpoint
+ descriptor), masked with USB_DIR_IN when referring to an endpoint
+ which sends data to the host from the device.
+
+ Use this on bulk or interrupt endpoints which have stalled,
+ returning *-EPIPE* status to a data transfer request. Do not issue
+ the control request directly, since that could invalidate the host's
+ record of the data toggle.
+
+USBDEVFS_CONTROL
+ Issues a control request to the device. The ioctl parameter points
+ to a structure like this:
+
+ ::
+
+ struct usbdevfs_ctrltransfer {
+ __u8 bRequestType;
+ __u8 bRequest;
+ __u16 wValue;
+ __u16 wIndex;
+ __u16 wLength;
+ __u32 timeout; /* in milliseconds */
+ void *data;
+ };
+
+ The first eight bytes of this structure are the contents of the
+ SETUP packet to be sent to the device; see the USB 2.0 specification
+ for details. The bRequestType value is composed by combining a
+ USB_TYPE_\* value, a USB_DIR_\* value, and a USB_RECIP_\*
+ value (from *<linux/usb.h>*). If wLength is nonzero, it describes
+ the length of the data buffer, which is either written to the device
+ (USB_DIR_OUT) or read from the device (USB_DIR_IN).
+
+ At this writing, you can't transfer more than 4 KBytes of data to or
+ from a device; usbfs has a limit, and some host controller drivers
+ have a limit. (That's not usually a problem.) *Also* there's no way
+ to say it's not OK to get a short read back from the device.
+
+USBDEVFS_RESET
+ Does a USB level device reset. The ioctl parameter is ignored. After
+ the reset, this rebinds all device interfaces. File modification
+ time is not updated by this request.
+
+ **Warning**
+
+ *Avoid using this call* until some usbcore bugs get fixed, since
+ it does not fully synchronize device, interface, and driver (not
+ just usbfs) state.
+
+USBDEVFS_SETINTERFACE
+ Sets the alternate setting for an interface. The ioctl parameter is
+ a pointer to a structure like this:
+
+ ::
+
+ struct usbdevfs_setinterface {
+ unsigned int interface;
+ unsigned int altsetting;
+ };
+
+ File modification time is not updated by this request.
+
+ Those struct members are from some interface descriptor applying to
+ the current configuration. The interface number is the
+ bInterfaceNumber value, and the altsetting number is the
+ bAlternateSetting value. (This resets each endpoint in the
+ interface.)
+
+USBDEVFS_SETCONFIGURATION
+ Issues the :c:func:`usb_set_configuration()` call for the
+ device. The parameter is an integer holding the number of a
+ configuration (bConfigurationValue from descriptor). File
+ modification time is not updated by this request.
+
+ **Warning**
+
+ *Avoid using this call* until some usbcore bugs get fixed, since
+ it does not fully synchronize device, interface, and driver (not
+ just usbfs) state.
+
+Asynchronous I/O Support
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+As mentioned above, there are situations where it may be important to
+initiate concurrent operations from user mode code. This is particularly
+important for periodic transfers (interrupt and isochronous), but it can
+be used for other kinds of USB requests too. In such cases, the
+asynchronous requests described here are essential. Rather than
+submitting one request and having the kernel block until it completes,
+the blocking is separate.
+
+These requests are packaged into a structure that resembles the URB used
+by kernel device drivers. (No POSIX Async I/O support here, sorry.) It
+identifies the endpoint type (USBDEVFS_URB_TYPE_\*), endpoint
+(number, masked with USB_DIR_IN as appropriate), buffer and length,
+and a user "context" value serving to uniquely identify each request.
+(It's usually a pointer to per-request data.) Flags can modify requests
+(not as many as supported for kernel drivers).
+
+Each request can specify a realtime signal number (between SIGRTMIN and
+SIGRTMAX, inclusive) to request a signal be sent when the request
+completes.
+
+When usbfs returns these urbs, the status value is updated, and the
+buffer may have been modified. Except for isochronous transfers, the
+actual_length is updated to say how many bytes were transferred; if the
+USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if
+fewer bytes were read than were requested then you get an error report.
+
+::
+
+ struct usbdevfs_iso_packet_desc {
+ unsigned int length;
+ unsigned int actual_length;
+ unsigned int status;
+ };
+
+ struct usbdevfs_urb {
+ unsigned char type;
+ unsigned char endpoint;
+ int status;
+ unsigned int flags;
+ void *buffer;
+ int buffer_length;
+ int actual_length;
+ int start_frame;
+ int number_of_packets;
+ int error_count;
+ unsigned int signr;
+ void *usercontext;
+ struct usbdevfs_iso_packet_desc iso_frame_desc[];
+ };
+
+For these asynchronous requests, the file modification time reflects
+when the request was initiated. This contrasts with their use with the
+synchronous requests, where it reflects when requests complete.
+
+USBDEVFS_DISCARDURB
+ *TBS* File modification time is not updated by this request.
+
+USBDEVFS_DISCSIGNAL
+ *TBS* File modification time is not updated by this request.
+
+USBDEVFS_REAPURB
+ *TBS* File modification time is not updated by this request.
+
+USBDEVFS_REAPURBNDELAY
+ *TBS* File modification time is not updated by this request.
+
+USBDEVFS_SUBMITURB
+ *TBS*
--- /dev/null
+==========================
+Writing an MUSB Glue Layer
+==========================
+
+:Author: Apelete Seketeli
+
+Introduction
+============
+
+The Linux MUSB subsystem is part of the larger Linux USB subsystem. It
+provides support for embedded USB Device Controllers (UDC) that do not
+use Universal Host Controller Interface (UHCI) or Open Host Controller
+Interface (OHCI).
+
+Instead, these embedded UDC rely on the USB On-the-Go (OTG)
+specification which they implement at least partially. The silicon
+reference design used in most cases is the Multipoint USB Highspeed
+Dual-Role Controller (MUSB HDRC) found in the Mentor Graphics Inventra™
+design.
+
+As a self-taught exercise I have written an MUSB glue layer for the
+Ingenic JZ4740 SoC, modelled after the many MUSB glue layers in the
+kernel source tree. This layer can be found at
+drivers/usb/musb/jz4740.c. In this documentation I will walk through the
+basics of the jz4740.c glue layer, explaining the different pieces and
+what needs to be done in order to write your own device glue layer.
+
+Linux MUSB Basics
+=================
+
+To get started on the topic, please read USB On-the-Go Basics (see
+Resources) which provides an introduction of USB OTG operation at the
+hardware level. A couple of wiki pages by Texas Instruments and Analog
+Devices also provide an overview of the Linux kernel MUSB configuration,
+albeit focused on some specific devices provided by these companies.
+Finally, getting acquainted with the USB specification at USB home page
+may come in handy, with practical instance provided through the Writing
+USB Device Drivers documentation (again, see Resources).
+
+Linux USB stack is a layered architecture in which the MUSB controller
+hardware sits at the lowest. The MUSB controller driver abstract the
+MUSB controller hardware to the Linux USB stack.
+
+::
+
+ ------------------------
+ | | <------- drivers/usb/gadget
+ | Linux USB Core Stack | <------- drivers/usb/host
+ | | <------- drivers/usb/core
+ ------------------------
+ ⬍
+ --------------------------
+ | | <------ drivers/usb/musb/musb_gadget.c
+ | MUSB Controller driver | <------ drivers/usb/musb/musb_host.c
+ | | <------ drivers/usb/musb/musb_core.c
+ --------------------------
+ ⬍
+ ---------------------------------
+ | MUSB Platform Specific Driver |
+ | | <-- drivers/usb/musb/jz4740.c
+ | aka "Glue Layer" |
+ ---------------------------------
+ ⬍
+ ---------------------------------
+ | MUSB Controller Hardware |
+ ---------------------------------
+
+
+As outlined above, the glue layer is actually the platform specific code
+sitting in between the controller driver and the controller hardware.
+
+Just like a Linux USB driver needs to register itself with the Linux USB
+subsystem, the MUSB glue layer needs first to register itself with the
+MUSB controller driver. This will allow the controller driver to know
+about which device the glue layer supports and which functions to call
+when a supported device is detected or released; remember we are talking
+about an embedded controller chip here, so no insertion or removal at
+run-time.
+
+All of this information is passed to the MUSB controller driver through
+a platform_driver structure defined in the glue layer as:
+
+::
+
+ static struct platform_driver jz4740_driver = {
+ .probe = jz4740_probe,
+ .remove = jz4740_remove,
+ .driver = {
+ .name = "musb-jz4740",
+ },
+ };
+
+
+The probe and remove function pointers are called when a matching device
+is detected and, respectively, released. The name string describes the
+device supported by this glue layer. In the current case it matches a
+platform_device structure declared in arch/mips/jz4740/platform.c. Note
+that we are not using device tree bindings here.
+
+In order to register itself to the controller driver, the glue layer
+goes through a few steps, basically allocating the controller hardware
+resources and initialising a couple of circuits. To do so, it needs to
+keep track of the information used throughout these steps. This is done
+by defining a private jz4740_glue structure:
+
+::
+
+ struct jz4740_glue {
+ struct device *dev;
+ struct platform_device *musb;
+ struct clk *clk;
+ };
+
+
+The dev and musb members are both device structure variables. The first
+one holds generic information about the device, since it's the basic
+device structure, and the latter holds information more closely related
+to the subsystem the device is registered to. The clk variable keeps
+information related to the device clock operation.
+
+Let's go through the steps of the probe function that leads the glue
+layer to register itself to the controller driver.
+
+N.B.: For the sake of readability each function will be split in logical
+parts, each part being shown as if it was independent from the others.
+
+::
+
+ static int jz4740_probe(struct platform_device *pdev)
+ {
+ struct platform_device *musb;
+ struct jz4740_glue *glue;
+ struct clk *clk;
+ int ret;
+
+ glue = devm_kzalloc(&pdev->dev, sizeof(*glue), GFP_KERNEL);
+ if (!glue)
+ return -ENOMEM;
+
+ musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO);
+ if (!musb) {
+ dev_err(&pdev->dev, "failed to allocate musb device\n");
+ return -ENOMEM;
+ }
+
+ clk = devm_clk_get(&pdev->dev, "udc");
+ if (IS_ERR(clk)) {
+ dev_err(&pdev->dev, "failed to get clock\n");
+ ret = PTR_ERR(clk);
+ goto err_platform_device_put;
+ }
+
+ ret = clk_prepare_enable(clk);
+ if (ret) {
+ dev_err(&pdev->dev, "failed to enable clock\n");
+ goto err_platform_device_put;
+ }
+
+ musb->dev.parent = &pdev->dev;
+
+ glue->dev = &pdev->dev;
+ glue->musb = musb;
+ glue->clk = clk;
+
+ return 0;
+
+ err_platform_device_put:
+ platform_device_put(musb);
+ return ret;
+ }
+
+
+The first few lines of the probe function allocate and assign the glue,
+musb and clk variables. The GFP_KERNEL flag (line 8) allows the
+allocation process to sleep and wait for memory, thus being usable in a
+blocking situation. The PLATFORM_DEVID_AUTO flag (line 12) allows
+automatic allocation and management of device IDs in order to avoid
+device namespace collisions with explicit IDs. With devm_clk_get()
+(line 18) the glue layer allocates the clock -- the ``devm_`` prefix
+indicates that clk_get() is managed: it automatically frees the
+allocated clock resource data when the device is released -- and enable
+it.
+
+Then comes the registration steps:
+
+::
+
+ static int jz4740_probe(struct platform_device *pdev)
+ {
+ struct musb_hdrc_platform_data *pdata = &jz4740_musb_platform_data;
+
+ pdata->platform_ops = &jz4740_musb_ops;
+
+ platform_set_drvdata(pdev, glue);
+
+ ret = platform_device_add_resources(musb, pdev->resource,
+ pdev->num_resources);
+ if (ret) {
+ dev_err(&pdev->dev, "failed to add resources\n");
+ goto err_clk_disable;
+ }
+
+ ret = platform_device_add_data(musb, pdata, sizeof(*pdata));
+ if (ret) {
+ dev_err(&pdev->dev, "failed to add platform_data\n");
+ goto err_clk_disable;
+ }
+
+ return 0;
+
+ err_clk_disable:
+ clk_disable_unprepare(clk);
+ err_platform_device_put:
+ platform_device_put(musb);
+ return ret;
+ }
+
+
+The first step is to pass the device data privately held by the glue
+layer on to the controller driver through platform_set_drvdata() (line
+7). Next is passing on the device resources information, also privately
+held at that point, through platform_device_add_resources() (line 9).
+
+Finally comes passing on the platform specific data to the controller
+driver (line 16). Platform data will be discussed in `Chapter
+4 <#device-platform-data>`__, but here we are looking at the
+platform_ops function pointer (line 5) in musb_hdrc_platform_data
+structure (line 3). This function pointer allows the MUSB controller
+driver to know which function to call for device operation:
+
+::
+
+ static const struct musb_platform_ops jz4740_musb_ops = {
+ .init = jz4740_musb_init,
+ .exit = jz4740_musb_exit,
+ };
+
+
+Here we have the minimal case where only init and exit functions are
+called by the controller driver when needed. Fact is the JZ4740 MUSB
+controller is a basic controller, lacking some features found in other
+controllers, otherwise we may also have pointers to a few other
+functions like a power management function or a function to switch
+between OTG and non-OTG modes, for instance.
+
+At that point of the registration process, the controller driver
+actually calls the init function:
+
+::
+
+ static int jz4740_musb_init(struct musb *musb)
+ {
+ musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
+ if (!musb->xceiv) {
+ pr_err("HS UDC: no transceiver configured\n");
+ return -ENODEV;
+ }
+
+ /* Silicon does not implement ConfigData register.
+ * Set dyn_fifo to avoid reading EP config from hardware.
+ */
+ musb->dyn_fifo = true;
+
+ musb->isr = jz4740_musb_interrupt;
+
+ return 0;
+ }
+
+
+The goal of jz4740_musb_init() is to get hold of the transceiver
+driver data of the MUSB controller hardware and pass it on to the MUSB
+controller driver, as usual. The transceiver is the circuitry inside the
+controller hardware responsible for sending/receiving the USB data.
+Since it is an implementation of the physical layer of the OSI model,
+the transceiver is also referred to as PHY.
+
+Getting hold of the MUSB PHY driver data is done with usb_get_phy()
+which returns a pointer to the structure containing the driver instance
+data. The next couple of instructions (line 12 and 14) are used as a
+quirk and to setup IRQ handling respectively. Quirks and IRQ handling
+will be discussed later in `Chapter 5 <#device-quirks>`__ and `Chapter
+3 <#handling-irqs>`__.
+
+::
+
+ static int jz4740_musb_exit(struct musb *musb)
+ {
+ usb_put_phy(musb->xceiv);
+
+ return 0;
+ }
+
+
+Acting as the counterpart of init, the exit function releases the MUSB
+PHY driver when the controller hardware itself is about to be released.
+
+Again, note that init and exit are fairly simple in this case due to the
+basic set of features of the JZ4740 controller hardware. When writing an
+musb glue layer for a more complex controller hardware, you might need
+to take care of more processing in those two functions.
+
+Returning from the init function, the MUSB controller driver jumps back
+into the probe function:
+
+::
+
+ static int jz4740_probe(struct platform_device *pdev)
+ {
+ ret = platform_device_add(musb);
+ if (ret) {
+ dev_err(&pdev->dev, "failed to register musb device\n");
+ goto err_clk_disable;
+ }
+
+ return 0;
+
+ err_clk_disable:
+ clk_disable_unprepare(clk);
+ err_platform_device_put:
+ platform_device_put(musb);
+ return ret;
+ }
+
+
+This is the last part of the device registration process where the glue
+layer adds the controller hardware device to Linux kernel device
+hierarchy: at this stage, all known information about the device is
+passed on to the Linux USB core stack.
+
+::
+
+ static int jz4740_remove(struct platform_device *pdev)
+ {
+ struct jz4740_glue *glue = platform_get_drvdata(pdev);
+
+ platform_device_unregister(glue->musb);
+ clk_disable_unprepare(glue->clk);
+
+ return 0;
+ }
+
+
+Acting as the counterpart of probe, the remove function unregister the
+MUSB controller hardware (line 5) and disable the clock (line 6),
+allowing it to be gated.
+
+Handling IRQs
+=============
+
+Additionally to the MUSB controller hardware basic setup and
+registration, the glue layer is also responsible for handling the IRQs:
+
+::
+
+ static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
+ {
+ unsigned long flags;
+ irqreturn_t retval = IRQ_NONE;
+ struct musb *musb = __hci;
+
+ spin_lock_irqsave(&musb->lock, flags);
+
+ musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
+ musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
+ musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
+
+ /*
+ * The controller is gadget only, the state of the host mode IRQ bits is
+ * undefined. Mask them to make sure that the musb driver core will
+ * never see them set
+ */
+ musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
+ MUSB_INTR_RESET | MUSB_INTR_SOF;
+
+ if (musb->int_usb || musb->int_tx || musb->int_rx)
+ retval = musb_interrupt(musb);
+
+ spin_unlock_irqrestore(&musb->lock, flags);
+
+ return retval;
+ }
+
+
+Here the glue layer mostly has to read the relevant hardware registers
+and pass their values on to the controller driver which will handle the
+actual event that triggered the IRQ.
+
+The interrupt handler critical section is protected by the
+spin_lock_irqsave() and counterpart spin_unlock_irqrestore()
+functions (line 7 and 24 respectively), which prevent the interrupt
+handler code to be run by two different threads at the same time.
+
+Then the relevant interrupt registers are read (line 9 to 11):
+
+- MUSB_INTRUSB: indicates which USB interrupts are currently active,
+
+- MUSB_INTRTX: indicates which of the interrupts for TX endpoints are
+ currently active,
+
+- MUSB_INTRRX: indicates which of the interrupts for TX endpoints are
+ currently active.
+
+Note that musb_readb() is used to read 8-bit registers at most, while
+musb_readw() allows us to read at most 16-bit registers. There are
+other functions that can be used depending on the size of your device
+registers. See musb_io.h for more information.
+
+Instruction on line 18 is another quirk specific to the JZ4740 USB
+device controller, which will be discussed later in `Chapter
+5 <#device-quirks>`__.
+
+The glue layer still needs to register the IRQ handler though. Remember
+the instruction on line 14 of the init function:
+
+::
+
+ static int jz4740_musb_init(struct musb *musb)
+ {
+ musb->isr = jz4740_musb_interrupt;
+
+ return 0;
+ }
+
+
+This instruction sets a pointer to the glue layer IRQ handler function,
+in order for the controller hardware to call the handler back when an
+IRQ comes from the controller hardware. The interrupt handler is now
+implemented and registered.
+
+Device Platform Data
+====================
+
+In order to write an MUSB glue layer, you need to have some data
+describing the hardware capabilities of your controller hardware, which
+is called the platform data.
+
+Platform data is specific to your hardware, though it may cover a broad
+range of devices, and is generally found somewhere in the arch/
+directory, depending on your device architecture.
+
+For instance, platform data for the JZ4740 SoC is found in
+arch/mips/jz4740/platform.c. In the platform.c file each device of the
+JZ4740 SoC is described through a set of structures.
+
+Here is the part of arch/mips/jz4740/platform.c that covers the USB
+Device Controller (UDC):
+
+::
+
+ /* USB Device Controller */
+ struct platform_device jz4740_udc_xceiv_device = {
+ .name = "usb_phy_gen_xceiv",
+ .id = 0,
+ };
+
+ static struct resource jz4740_udc_resources[] = {
+ [0] = {
+ .start = JZ4740_UDC_BASE_ADDR,
+ .end = JZ4740_UDC_BASE_ADDR + 0x10000 - 1,
+ .flags = IORESOURCE_MEM,
+ },
+ [1] = {
+ .start = JZ4740_IRQ_UDC,
+ .end = JZ4740_IRQ_UDC,
+ .flags = IORESOURCE_IRQ,
+ .name = "mc",
+ },
+ };
+
+ struct platform_device jz4740_udc_device = {
+ .name = "musb-jz4740",
+ .id = -1,
+ .dev = {
+ .dma_mask = &jz4740_udc_device.dev.coherent_dma_mask,
+ .coherent_dma_mask = DMA_BIT_MASK(32),
+ },
+ .num_resources = ARRAY_SIZE(jz4740_udc_resources),
+ .resource = jz4740_udc_resources,
+ };
+
+
+The jz4740_udc_xceiv_device platform device structure (line 2)
+describes the UDC transceiver with a name and id number.
+
+At the time of this writing, note that "usb_phy_gen_xceiv" is the
+specific name to be used for all transceivers that are either built-in
+with reference USB IP or autonomous and doesn't require any PHY
+programming. You will need to set CONFIG_NOP_USB_XCEIV=y in the
+kernel configuration to make use of the corresponding transceiver
+driver. The id field could be set to -1 (equivalent to
+PLATFORM_DEVID_NONE), -2 (equivalent to PLATFORM_DEVID_AUTO) or
+start with 0 for the first device of this kind if we want a specific id
+number.
+
+The jz4740_udc_resources resource structure (line 7) defines the UDC
+registers base addresses.
+
+The first array (line 9 to 11) defines the UDC registers base memory
+addresses: start points to the first register memory address, end points
+to the last register memory address and the flags member defines the
+type of resource we are dealing with. So IORESOURCE_MEM is used to
+define the registers memory addresses. The second array (line 14 to 17)
+defines the UDC IRQ registers addresses. Since there is only one IRQ
+register available for the JZ4740 UDC, start and end point at the same
+address. The IORESOURCE_IRQ flag tells that we are dealing with IRQ
+resources, and the name "mc" is in fact hard-coded in the MUSB core in
+order for the controller driver to retrieve this IRQ resource by
+querying it by its name.
+
+Finally, the jz4740_udc_device platform device structure (line 21)
+describes the UDC itself.
+
+The "musb-jz4740" name (line 22) defines the MUSB driver that is used
+for this device; remember this is in fact the name that we used in the
+jz4740_driver platform driver structure in `Chapter
+2 <#linux-musb-basics>`__. The id field (line 23) is set to -1
+(equivalent to PLATFORM_DEVID_NONE) since we do not need an id for the
+device: the MUSB controller driver was already set to allocate an
+automatic id in `Chapter 2 <#linux-musb-basics>`__. In the dev field we
+care for DMA related information here. The dma_mask field (line 25)
+defines the width of the DMA mask that is going to be used, and
+coherent_dma_mask (line 26) has the same purpose but for the
+alloc_coherent DMA mappings: in both cases we are using a 32 bits mask.
+Then the resource field (line 29) is simply a pointer to the resource
+structure defined before, while the num_resources field (line 28) keeps
+track of the number of arrays defined in the resource structure (in this
+case there were two resource arrays defined before).
+
+With this quick overview of the UDC platform data at the arch/ level now
+done, let's get back to the MUSB glue layer specific platform data in
+drivers/usb/musb/jz4740.c:
+
+::
+
+ static struct musb_hdrc_config jz4740_musb_config = {
+ /* Silicon does not implement USB OTG. */
+ .multipoint = 0,
+ /* Max EPs scanned, driver will decide which EP can be used. */
+ .num_eps = 4,
+ /* RAMbits needed to configure EPs from table */
+ .ram_bits = 9,
+ .fifo_cfg = jz4740_musb_fifo_cfg,
+ .fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg),
+ };
+
+ static struct musb_hdrc_platform_data jz4740_musb_platform_data = {
+ .mode = MUSB_PERIPHERAL,
+ .config = &jz4740_musb_config,
+ };
+
+
+First the glue layer configures some aspects of the controller driver
+operation related to the controller hardware specifics. This is done
+through the jz4740_musb_config musb_hdrc_config structure.
+
+Defining the OTG capability of the controller hardware, the multipoint
+member (line 3) is set to 0 (equivalent to false) since the JZ4740 UDC
+is not OTG compatible. Then num_eps (line 5) defines the number of USB
+endpoints of the controller hardware, including endpoint 0: here we have
+3 endpoints + endpoint 0. Next is ram_bits (line 7) which is the width
+of the RAM address bus for the MUSB controller hardware. This
+information is needed when the controller driver cannot automatically
+configure endpoints by reading the relevant controller hardware
+registers. This issue will be discussed when we get to device quirks in
+`Chapter 5 <#device-quirks>`__. Last two fields (line 8 and 9) are also
+about device quirks: fifo_cfg points to the USB endpoints configuration
+table and fifo_cfg_size keeps track of the size of the number of
+entries in that configuration table. More on that later in `Chapter
+5 <#device-quirks>`__.
+
+Then this configuration is embedded inside jz4740_musb_platform_data
+musb_hdrc_platform_data structure (line 11): config is a pointer to
+the configuration structure itself, and mode tells the controller driver
+if the controller hardware may be used as MUSB_HOST only,
+MUSB_PERIPHERAL only or MUSB_OTG which is a dual mode.
+
+Remember that jz4740_musb_platform_data is then used to convey
+platform data information as we have seen in the probe function in
+`Chapter 2 <#linux-musb-basics>`__
+
+Device Quirks
+=============
+
+Completing the platform data specific to your device, you may also need
+to write some code in the glue layer to work around some device specific
+limitations. These quirks may be due to some hardware bugs, or simply be
+the result of an incomplete implementation of the USB On-the-Go
+specification.
+
+The JZ4740 UDC exhibits such quirks, some of which we will discuss here
+for the sake of insight even though these might not be found in the
+controller hardware you are working on.
+
+Let's get back to the init function first:
+
+::
+
+ static int jz4740_musb_init(struct musb *musb)
+ {
+ musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
+ if (!musb->xceiv) {
+ pr_err("HS UDC: no transceiver configured\n");
+ return -ENODEV;
+ }
+
+ /* Silicon does not implement ConfigData register.
+ * Set dyn_fifo to avoid reading EP config from hardware.
+ */
+ musb->dyn_fifo = true;
+
+ musb->isr = jz4740_musb_interrupt;
+
+ return 0;
+ }
+
+
+Instruction on line 12 helps the MUSB controller driver to work around
+the fact that the controller hardware is missing registers that are used
+for USB endpoints configuration.
+
+Without these registers, the controller driver is unable to read the
+endpoints configuration from the hardware, so we use line 12 instruction
+to bypass reading the configuration from silicon, and rely on a
+hard-coded table that describes the endpoints configuration instead:
+
+::
+
+ static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = {
+ { .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, },
+ { .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, },
+ { .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, },
+ };
+
+
+Looking at the configuration table above, we see that each endpoints is
+described by three fields: hw_ep_num is the endpoint number, style is
+its direction (either FIFO_TX for the controller driver to send packets
+in the controller hardware, or FIFO_RX to receive packets from
+hardware), and maxpacket defines the maximum size of each data packet
+that can be transmitted over that endpoint. Reading from the table, the
+controller driver knows that endpoint 1 can be used to send and receive
+USB data packets of 512 bytes at once (this is in fact a bulk in/out
+endpoint), and endpoint 2 can be used to send data packets of 64 bytes
+at once (this is in fact an interrupt endpoint).
+
+Note that there is no information about endpoint 0 here: that one is
+implemented by default in every silicon design, with a predefined
+configuration according to the USB specification. For more examples of
+endpoint configuration tables, see musb_core.c.
+
+Let's now get back to the interrupt handler function:
+
+::
+
+ static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
+ {
+ unsigned long flags;
+ irqreturn_t retval = IRQ_NONE;
+ struct musb *musb = __hci;
+
+ spin_lock_irqsave(&musb->lock, flags);
+
+ musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
+ musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
+ musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
+
+ /*
+ * The controller is gadget only, the state of the host mode IRQ bits is
+ * undefined. Mask them to make sure that the musb driver core will
+ * never see them set
+ */
+ musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
+ MUSB_INTR_RESET | MUSB_INTR_SOF;
+
+ if (musb->int_usb || musb->int_tx || musb->int_rx)
+ retval = musb_interrupt(musb);
+
+ spin_unlock_irqrestore(&musb->lock, flags);
+
+ return retval;
+ }
+
+
+Instruction on line 18 above is a way for the controller driver to work
+around the fact that some interrupt bits used for USB host mode
+operation are missing in the MUSB_INTRUSB register, thus left in an
+undefined hardware state, since this MUSB controller hardware is used in
+peripheral mode only. As a consequence, the glue layer masks these
+missing bits out to avoid parasite interrupts by doing a logical AND
+operation between the value read from MUSB_INTRUSB and the bits that
+are actually implemented in the register.
+
+These are only a couple of the quirks found in the JZ4740 USB device
+controller. Some others were directly addressed in the MUSB core since
+the fixes were generic enough to provide a better handling of the issues
+for others controller hardware eventually.
+
+Conclusion
+==========
+
+Writing a Linux MUSB glue layer should be a more accessible task, as
+this documentation tries to show the ins and outs of this exercise.
+
+The JZ4740 USB device controller being fairly simple, I hope its glue
+layer serves as a good example for the curious mind. Used with the
+current MUSB glue layers, this documentation should provide enough
+guidance to get started; should anything gets out of hand, the linux-usb
+mailing list archive is another helpful resource to browse through.
+
+Acknowledgements
+================
+
+Many thanks to Lars-Peter Clausen and Maarten ter Huurne for answering
+my questions while I was writing the JZ4740 glue layer and for helping
+me out getting the code in good shape.
+
+I would also like to thank the Qi-Hardware community at large for its
+cheerful guidance and support.
+
+Resources
+=========
+
+USB Home Page: http://www.usb.org
+
+linux-usb Mailing List Archives: http://marc.info/?l=linux-usb
+
+USB On-the-Go Basics:
+http://www.maximintegrated.com/app-notes/index.mvp/id/1822
+
+Writing USB Device Drivers:
+https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html
+
+Texas Instruments USB Configuration Wiki Page:
+http://processors.wiki.ti.com/index.php/Usbgeneralpage
+
+Analog Devices Blackfin MUSB Configuration:
+http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb
--- /dev/null
+==========================
+Writing USB Device Drivers
+==========================
+
+:Author: Greg Kroah-Hartman
+
+Introduction
+============
+
+The Linux USB subsystem has grown from supporting only two different
+types of devices in the 2.2.7 kernel (mice and keyboards), to over 20
+different types of devices in the 2.4 kernel. Linux currently supports
+almost all USB class devices (standard types of devices like keyboards,
+mice, modems, printers and speakers) and an ever-growing number of
+vendor-specific devices (such as USB to serial converters, digital
+cameras, Ethernet devices and MP3 players). For a full list of the
+different USB devices currently supported, see Resources.
+
+The remaining kinds of USB devices that do not have support on Linux are
+almost all vendor-specific devices. Each vendor decides to implement a
+custom protocol to talk to their device, so a custom driver usually
+needs to be created. Some vendors are open with their USB protocols and
+help with the creation of Linux drivers, while others do not publish
+them, and developers are forced to reverse-engineer. See Resources for
+some links to handy reverse-engineering tools.
+
+Because each different protocol causes a new driver to be created, I
+have written a generic USB driver skeleton, modelled after the
+pci-skeleton.c file in the kernel source tree upon which many PCI
+network drivers have been based. This USB skeleton can be found at
+drivers/usb/usb-skeleton.c in the kernel source tree. In this article I
+will walk through the basics of the skeleton driver, explaining the
+different pieces and what needs to be done to customize it to your
+specific device.
+
+Linux USB Basics
+================
+
+If you are going to write a Linux USB driver, please become familiar
+with the USB protocol specification. It can be found, along with many
+other useful documents, at the USB home page (see Resources). An
+excellent introduction to the Linux USB subsystem can be found at the
+USB Working Devices List (see Resources). It explains how the Linux USB
+subsystem is structured and introduces the reader to the concept of USB
+urbs (USB Request Blocks), which are essential to USB drivers.
+
+The first thing a Linux USB driver needs to do is register itself with
+the Linux USB subsystem, giving it some information about which devices
+the driver supports and which functions to call when a device supported
+by the driver is inserted or removed from the system. All of this
+information is passed to the USB subsystem in the usb_driver structure.
+The skeleton driver declares a usb_driver as:
+
+::
+
+ static struct usb_driver skel_driver = {
+ .name = "skeleton",
+ .probe = skel_probe,
+ .disconnect = skel_disconnect,
+ .fops = &skel_fops,
+ .minor = USB_SKEL_MINOR_BASE,
+ .id_table = skel_table,
+ };
+
+
+The variable name is a string that describes the driver. It is used in
+informational messages printed to the system log. The probe and
+disconnect function pointers are called when a device that matches the
+information provided in the id_table variable is either seen or
+removed.
+
+The fops and minor variables are optional. Most USB drivers hook into
+another kernel subsystem, such as the SCSI, network or TTY subsystem.
+These types of drivers register themselves with the other kernel
+subsystem, and any user-space interactions are provided through that
+interface. But for drivers that do not have a matching kernel subsystem,
+such as MP3 players or scanners, a method of interacting with user space
+is needed. The USB subsystem provides a way to register a minor device
+number and a set of file_operations function pointers that enable this
+user-space interaction. The skeleton driver needs this kind of
+interface, so it provides a minor starting number and a pointer to its
+file_operations functions.
+
+The USB driver is then registered with a call to usb_register, usually
+in the driver's init function, as shown here:
+
+::
+
+ static int __init usb_skel_init(void)
+ {
+ int result;
+
+ /* register this driver with the USB subsystem */
+ result = usb_register(&skel_driver);
+ if (result < 0) {
+ err("usb_register failed for the "__FILE__ "driver."
+ "Error number %d", result);
+ return -1;
+ }
+
+ return 0;
+ }
+ module_init(usb_skel_init);
+
+
+When the driver is unloaded from the system, it needs to deregister
+itself with the USB subsystem. This is done with the usb_deregister
+function:
+
+::
+
+ static void __exit usb_skel_exit(void)
+ {
+ /* deregister this driver with the USB subsystem */
+ usb_deregister(&skel_driver);
+ }
+ module_exit(usb_skel_exit);
+
+
+To enable the linux-hotplug system to load the driver automatically when
+the device is plugged in, you need to create a MODULE_DEVICE_TABLE.
+The following code tells the hotplug scripts that this module supports a
+single device with a specific vendor and product ID:
+
+::
+
+ /* table of devices that work with this driver */
+ static struct usb_device_id skel_table [] = {
+ { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
+ { } /* Terminating entry */
+ };
+ MODULE_DEVICE_TABLE (usb, skel_table);
+
+
+There are other macros that can be used in describing a usb_device_id
+for drivers that support a whole class of USB drivers. See usb.h for
+more information on this.
+
+Device operation
+================
+
+When a device is plugged into the USB bus that matches the device ID
+pattern that your driver registered with the USB core, the probe
+function is called. The usb_device structure, interface number and the
+interface ID are passed to the function:
+
+::
+
+ static int skel_probe(struct usb_interface *interface,
+ const struct usb_device_id *id)
+
+
+The driver now needs to verify that this device is actually one that it
+can accept. If so, it returns 0. If not, or if any error occurs during
+initialization, an errorcode (such as ``-ENOMEM`` or ``-ENODEV``) is
+returned from the probe function.
+
+In the skeleton driver, we determine what end points are marked as
+bulk-in and bulk-out. We create buffers to hold the data that will be
+sent and received from the device, and a USB urb to write data to the
+device is initialized.
+
+Conversely, when the device is removed from the USB bus, the disconnect
+function is called with the device pointer. The driver needs to clean
+any private data that has been allocated at this time and to shut down
+any pending urbs that are in the USB system.
+
+Now that the device is plugged into the system and the driver is bound
+to the device, any of the functions in the file_operations structure
+that were passed to the USB subsystem will be called from a user program
+trying to talk to the device. The first function called will be open, as
+the program tries to open the device for I/O. We increment our private
+usage count and save a pointer to our internal structure in the file
+structure. This is done so that future calls to file operations will
+enable the driver to determine which device the user is addressing. All
+of this is done with the following code:
+
+::
+
+ /* increment our usage count for the module */
+ ++skel->open_count;
+
+ /* save our object in the file's private structure */
+ file->private_data = dev;
+
+
+After the open function is called, the read and write functions are
+called to receive and send data to the device. In the skel_write
+function, we receive a pointer to some data that the user wants to send
+to the device and the size of the data. The function determines how much
+data it can send to the device based on the size of the write urb it has
+created (this size depends on the size of the bulk out end point that
+the device has). Then it copies the data from user space to kernel
+space, points the urb to the data and submits the urb to the USB
+subsystem. This can be seen in the following code:
+
+::
+
+ /* we can only write as much as 1 urb will hold */
+ bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
+
+ /* copy the data from user space into our urb */
+ copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
+
+ /* set up our urb */
+ usb_fill_bulk_urb(skel->write_urb,
+ skel->dev,
+ usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr),
+ skel->write_urb->transfer_buffer,
+ bytes_written,
+ skel_write_bulk_callback,
+ skel);
+
+ /* send the data out the bulk port */
+ result = usb_submit_urb(skel->write_urb);
+ if (result) {
+ err("Failed submitting write urb, error %d", result);
+ }
+
+
+When the write urb is filled up with the proper information using the
+usb_fill_bulk_urb function, we point the urb's completion callback to
+call our own skel_write_bulk_callback function. This function is
+called when the urb is finished by the USB subsystem. The callback
+function is called in interrupt context, so caution must be taken not to
+do very much processing at that time. Our implementation of
+skel_write_bulk_callback merely reports if the urb was completed
+successfully or not and then returns.
+
+The read function works a bit differently from the write function in
+that we do not use an urb to transfer data from the device to the
+driver. Instead we call the usb_bulk_msg function, which can be used
+to send or receive data from a device without having to create urbs and
+handle urb completion callback functions. We call the usb_bulk_msg
+function, giving it a buffer into which to place any data received from
+the device and a timeout value. If the timeout period expires without
+receiving any data from the device, the function will fail and return an
+error message. This can be shown with the following code:
+
+::
+
+ /* do an immediate bulk read to get data from the device */
+ retval = usb_bulk_msg (skel->dev,
+ usb_rcvbulkpipe (skel->dev,
+ skel->bulk_in_endpointAddr),
+ skel->bulk_in_buffer,
+ skel->bulk_in_size,
+ &count, HZ*10);
+ /* if the read was successful, copy the data to user space */
+ if (!retval) {
+ if (copy_to_user (buffer, skel->bulk_in_buffer, count))
+ retval = -EFAULT;
+ else
+ retval = count;
+ }
+
+
+The usb_bulk_msg function can be very useful for doing single reads or
+writes to a device; however, if you need to read or write constantly to
+a device, it is recommended to set up your own urbs and submit them to
+the USB subsystem.
+
+When the user program releases the file handle that it has been using to
+talk to the device, the release function in the driver is called. In
+this function we decrement our private usage count and wait for possible
+pending writes:
+
+::
+
+ /* decrement our usage count for the device */
+ --skel->open_count;
+
+
+One of the more difficult problems that USB drivers must be able to
+handle smoothly is the fact that the USB device may be removed from the
+system at any point in time, even if a program is currently talking to
+it. It needs to be able to shut down any current reads and writes and
+notify the user-space programs that the device is no longer there. The
+following code (function :c:func:`skel_delete()`) is an example of
+how to do this:
+
+::
+
+ static inline void skel_delete (struct usb_skel *dev)
+ {
+ kfree (dev->bulk_in_buffer);
+ if (dev->bulk_out_buffer != NULL)
+ usb_free_coherent (dev->udev, dev->bulk_out_size,
+ dev->bulk_out_buffer,
+ dev->write_urb->transfer_dma);
+ usb_free_urb (dev->write_urb);
+ kfree (dev);
+ }
+
+
+If a program currently has an open handle to the device, we reset the
+flag ``device_present``. For every read, write, release and other
+functions that expect a device to be present, the driver first checks
+this flag to see if the device is still present. If not, it releases
+that the device has disappeared, and a -ENODEV error is returned to the
+user-space program. When the release function is eventually called, it
+determines if there is no device and if not, it does the cleanup that
+the skel_disconnect function normally does if there are no open files
+on the device (see Listing 5).
+
+Isochronous Data
+================
+
+This usb-skeleton driver does not have any examples of interrupt or
+isochronous data being sent to or from the device. Interrupt data is
+sent almost exactly as bulk data is, with a few minor exceptions.
+Isochronous data works differently with continuous streams of data being
+sent to or from the device. The audio and video camera drivers are very
+good examples of drivers that handle isochronous data and will be useful
+if you also need to do this.
+
+Conclusion
+==========
+
+Writing Linux USB device drivers is not a difficult task as the
+usb-skeleton driver shows. This driver, combined with the other current
+USB drivers, should provide enough examples to help a beginning author
+create a working driver in a minimal amount of time. The linux-usb-devel
+mailing list archives also contain a lot of helpful information.
+
+Resources
+=========
+
+The Linux USB Project:
+`http://www.linux-usb.org/ <http://www.linux-usb.org>`__
+
+Linux Hotplug Project:
+`http://linux-hotplug.sourceforge.net/ <http://linux-hotplug.sourceforge.net>`__
+
+Linux USB Working Devices List:
+`http://www.qbik.ch/usb/devices/ <http://www.qbik.ch/usb/devices>`__
+
+linux-usb-devel Mailing List Archives:
+http://marc.theaimsgroup.com/?l=linux-usb-devel
+
+Programming Guide for Linux USB Device Drivers:
+http://usb.cs.tum.edu/usbdoc
+
+USB Home Page: http://www.usb.org
projects / openwrt / staging / blogic.git / commitdiff