Basic ``sysfs`` Interfaces for System Suspend and Hibernation
=============================================================
-The following files located in the :file:`/sys/power/` directory can be used by
-user space for sleep states control.
+The power management subsystem provides userspace with a unified ``sysfs``
+interface for system sleep regardless of the underlying system architecture or
+platform. That interface is located in the :file:`/sys/power/` directory
+(assuming that ``sysfs`` is mounted at :file:`/sys`) and it consists of the
+following attributes (files):
``state``
This file contains a list of strings representing sleep states supported
to start a transition of the system into the sleep state represented by
that string.
- In particular, the strings "disk", "freeze" and "standby" represent the
+ In particular, the "disk", "freeze" and "standby" strings represent the
:ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and
- :ref:`standby <standby>` sleep states, respectively. The string "mem"
+ :ref:`standby <standby>` sleep states, respectively. The "mem" string
is interpreted in accordance with the contents of the ``mem_sleep`` file
described below.
associated with the "mem" string in the ``state`` file described above.
The strings that may be present in this file are "s2idle", "shallow"
- and "deep". The string "s2idle" always represents :ref:`suspend-to-idle
+ and "deep". The "s2idle" string always represents :ref:`suspend-to-idle
<s2idle>` and, by convention, "shallow" and "deep" represent
:ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`,
respectively.
Writing one of the listed strings into this file causes the system
suspend variant represented by it to be associated with the "mem" string
in the ``state`` file. The string representing the suspend variant
- currently associated with the "mem" string in the ``state`` file
- is listed in square brackets.
+ currently associated with the "mem" string in the ``state`` file is
+ shown in square brackets.
If the kernel does not support system suspend, this file is not present.
``disk``
- This file contains a list of strings representing different operations
- that can be carried out after the hibernation image has been saved. The
- possible options are as follows:
+ This file controls the operating mode of hibernation (Suspend-to-Disk).
+ Specifically, it tells the kernel what to do after creating a
+ hibernation image.
+
+ Reading from it returns a list of supported options encoded as:
``platform``
Put the system into a special low-power state (e.g. ACPI S4) to
platform firmware to take a simplified initialization path after
wakeup.
+ It is only available if the platform provides a special
+ mechanism to put the system to sleep after creating a
+ hibernation image (platforms with ACPI do that as a rule, for
+ example).
+
``shutdown``
Power off the system.
the hibernation image and continue. Otherwise, use the image
to restore the previous state of the system.
+ It is available if system suspend is supported.
+
``test_resume``
Diagnostic operation. Load the image as though the system had
just woken up from hibernation and the currently running kernel
instance was a restore kernel and follow up with full system
resume.
- Writing one of the listed strings into this file causes the option
+ Writing one of the strings listed above into this file causes the option
represented by it to be selected.
- The currently selected option is shown in square brackets which means
+ The currently selected option is shown in square brackets, which means
that the operation represented by it will be carried out after creating
- and saving the image next time hibernation is triggered by writing
- ``disk`` to :file:`/sys/power/state`.
+ and saving the image when hibernation is triggered by writing ``disk``
+ to :file:`/sys/power/state`.
If the kernel does not support hibernation, this file is not present.
+``image_size``
+ This file controls the size of hibernation images.
+
+ It can be written a string representing a non-negative integer that will
+ be used as a best-effort upper limit of the image size, in bytes. The
+ hibernation core will do its best to ensure that the image size will not
+ exceed that number, but if that turns out to be impossible to achieve, a
+ hibernation image will still be created and its size will be as small as
+ possible. In particular, writing '0' to this file causes the size of
+ hibernation images to be minimum.
+
+ Reading from it returns the current image size limit, which is set to
+ around 2/5 of the available RAM size by default.
+
+``pm_trace``
+ This file controls the "PM trace" mechanism saving the last suspend
+ or resume event point in the RTC memory across reboots. It helps to
+ debug hard lockups or reboots due to device driver failures that occur
+ during system suspend or resume (which is more common) more effectively.
+
+ If it contains "1", the fingerprint of each suspend/resume event point
+ in turn will be stored in the RTC memory (overwriting the actual RTC
+ information), so it will survive a system crash if one occurs right
+ after storing it and it can be used later to identify the driver that
+ caused the crash to happen.
+
+ It contains "0" by default, which may be changed to "1" by writing a
+ string representing a nonzero integer into it.
+
According to the above, there are two ways to make the system go into the
:ref:`suspend-to-idle <s2idle>` state. The first one is to write "freeze"
directly to :file:`/sys/power/state`. The second one is to write "s2idle" to
The default suspend variant (ie. the one to be used without writing anything
into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems
supporting :ref:`suspend-to-RAM <s2ram>`) or "s2idle", but it can be overridden
-by the value of the "mem_sleep_default" parameter in the kernel command line.
-On some ACPI-based systems, depending on the information in the ACPI tables, the
-default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported.
+by the value of the ``mem_sleep_default`` parameter in the kernel command line.
+On some systems with ACPI, depending on the information in the ACPI tables, the
+default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported in
+principle.
+++ /dev/null
-===========================================
-Power Management Interface for System Sleep
-===========================================
-
-Copyright (c) 2016 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
-
-The power management subsystem provides userspace with a unified sysfs interface
-for system sleep regardless of the underlying system architecture or platform.
-The interface is located in the /sys/power/ directory (assuming that sysfs is
-mounted at /sys).
-
-/sys/power/state is the system sleep state control file.
-
-Reading from it returns a list of supported sleep states, encoded as:
-
-- 'freeze' (Suspend-to-Idle)
-- 'standby' (Power-On Suspend)
-- 'mem' (Suspend-to-RAM)
-- 'disk' (Suspend-to-Disk)
-
-Suspend-to-Idle is always supported. Suspend-to-Disk is always supported
-too as long the kernel has been configured to support hibernation at all
-(ie. CONFIG_HIBERNATION is set in the kernel configuration file). Support
-for Suspend-to-RAM and Power-On Suspend depends on the capabilities of the
-platform.
-
-If one of the strings listed in /sys/power/state is written to it, the system
-will attempt to transition into the corresponding sleep state. Refer to
-Documentation/admin-guide/pm/sleep-states.rst for a description of each of
-those states.
-
-/sys/power/disk controls the operating mode of hibernation (Suspend-to-Disk).
-Specifically, it tells the kernel what to do after creating a hibernation image.
-
-Reading from it returns a list of supported options encoded as:
-
-- 'platform' (put the system into sleep using a platform-provided method)
-- 'shutdown' (shut the system down)
-- 'reboot' (reboot the system)
-- 'suspend' (trigger a Suspend-to-RAM transition)
-- 'test_resume' (resume-after-hibernation test mode)
-
-The currently selected option is printed in square brackets.
-
-The 'platform' option is only available if the platform provides a special
-mechanism to put the system to sleep after creating a hibernation image (ACPI
-does that, for example). The 'suspend' option is available if Suspend-to-RAM
-is supported. Refer to Documentation/power/basic-pm-debugging.rst for the
-description of the 'test_resume' option.
-
-To select an option, write the string representing it to /sys/power/disk.
-
-/sys/power/image_size controls the size of hibernation images.
-
-It can be written a string representing a non-negative integer that will be
-used as a best-effort upper limit of the image size, in bytes. The hibernation
-core will do its best to ensure that the image size will not exceed that number.
-However, if that turns out to be impossible to achieve, a hibernation image will
-still be created and its size will be as small as possible. In particular,
-writing '0' to this file will enforce hibernation images to be as small as
-possible.
-
-Reading from this file returns the current image size limit, which is set to
-around 2/5 of available RAM by default.
-
-/sys/power/pm_trace controls the PM trace mechanism saving the last suspend
-or resume event point in the RTC across reboots.
-
-It helps to debug hard lockups or reboots due to device driver failures that
-occur during system suspend or resume (which is more common) more effectively.
-
-If /sys/power/pm_trace contains '1', the fingerprint of each suspend/resume
-event point in turn will be stored in the RTC memory (overwriting the actual
-RTC information), so it will survive a system crash if one occurs right after
-storing it and it can be used later to identify the driver that caused the crash
-to happen (see Documentation/power/s2ram.rst for more information).
-
-Initially it contains '0' which may be changed to '1' by writing a string
-representing a nonzero integer into it.
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