serial:serial_core: Allow use of CTS for PPS line discipline

Add a "pps_4wire" file to serial ports in sysfs in case the kernel is
configured with CONFIG_PPS_CLIENT_LDISC. Writing 1 to the file enables
the use of CTS instead of DCD for PPS signal input. This is necessary
in case a serial port is not completely wired.
Though this affects PPS processing the patch is against the serial core
as the source of the serial port PPS event dispatching has to be
modified. Furthermore it should be possible to modify the source of
serial port PPS event dispatching before changing the line discipline.

Signed-off-by: Andreas Steinmetz <ast@domdv.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Tested-by: Steve Sakoman <steve@sakoman.com>
Tested-by: Eric Gallimore <egallimore@ucsd.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/ABI/testing/sysfs-tty b/Documentation/ABI/testing/sysfs-tty
index 9eb3c2b6b0406eeec340f31a47f837ea4f542258..441105a75d1f6b8cc955a15c345a2b8561d47c68 100644 (file)
--- a/Documentation/ABI/testing/sysfs-tty
+++ b/Documentation/ABI/testing/sysfs-tty
@@ -154,3 +154,12 @@ Description:
                 device specification. For example, when user sets 7bytes on
                 16550A, which has 1/4/8/14 bytes trigger, the RX trigger is
                 automatically changed to 4 bytes.
+
+What:          /sys/class/tty/ttyS0/pps_4wire
+Date:          September 2018
+Contact:       Steve Sakoman <steve@sakoman.com>
+Description:
+                Shows/sets "4 wire" mode for the PPS input to the serial driver.
+                For fully implemented serial ports PPS is normally provided
+                on the DCD line. For partial "4 wire" implementations CTS is
+                used instead of DCD.

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 80bb56facfb684423e415e702e334fb97be152b1..ed0133395cc7881f0c8cef725a0d93272a8bee43 100644 (file)
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2664,6 +2664,57 @@ static ssize_t uart_get_attr_iomem_reg_shift(struct device *dev,
        return snprintf(buf, PAGE_SIZE, "%d\n", tmp.iomem_reg_shift);
 }
 
+static ssize_t pps_4wire_show(struct device *dev,
+       struct device_attribute *attr, char *buf)
+{
+       struct tty_port *port = dev_get_drvdata(dev);
+       struct uart_state *state = container_of(port, struct uart_state, port);
+       struct uart_port *uport;
+       int mode = 0;
+
+       mutex_lock(&port->mutex);
+       uport = uart_port_check(state);
+       if (!uport)
+               goto out;
+
+       mode = uport->pps_4wire;
+
+out:
+       mutex_unlock(&port->mutex);
+       return sprintf(buf, mode ? "yes\n" : "no\n");
+}
+
+static ssize_t pps_4wire_store(struct device *dev,
+       struct device_attribute *attr, const char *buf, size_t count)
+{
+       struct tty_port *port = dev_get_drvdata(dev);
+       struct uart_state *state = container_of(port, struct uart_state, port);
+       struct uart_port *uport;
+       bool mode;
+       int ret;
+
+       if (!count)
+               return -EINVAL;
+
+       ret = kstrtobool(buf, &mode);
+       if (ret < 0)
+               return ret;
+
+       mutex_lock(&port->mutex);
+       uport = uart_port_check(state);
+       if (!uport)
+               goto out;
+
+       spin_lock_irq(&uport->lock);
+       uport->pps_4wire = mode;
+       spin_unlock_irq(&uport->lock);
+
+out:
+       mutex_unlock(&port->mutex);
+       return count;
+}
+static DEVICE_ATTR_RW(pps_4wire);
+
 static DEVICE_ATTR(type, S_IRUSR | S_IRGRP, uart_get_attr_type, NULL);
 static DEVICE_ATTR(line, S_IRUSR | S_IRGRP, uart_get_attr_line, NULL);
 static DEVICE_ATTR(port, S_IRUSR | S_IRGRP, uart_get_attr_port, NULL);
@@ -2692,6 +2743,7 @@ static struct attribute *tty_dev_attrs[] = {
        &dev_attr_io_type.attr,
        &dev_attr_iomem_base.attr,
        &dev_attr_iomem_reg_shift.attr,
+       &dev_attr_pps_4wire.attr,
        NULL,
        };
 
@@ -2748,6 +2800,9 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
                goto out;
        }
 
+       /* assert that pps handling is done via DCD as default */
+       uport->pps_4wire = 0;
+
        /*
         * If this port is a console, then the spinlock is already
         * initialised.
@@ -2923,7 +2978,7 @@ void uart_handle_dcd_change(struct uart_port *uport, unsigned int status)
 
        lockdep_assert_held_once(&uport->lock);
 
-       if (tty) {
+       if (tty && !uport->pps_4wire) {
                ld = tty_ldisc_ref(tty);
                if (ld) {
                        if (ld->ops->dcd_change)
@@ -2952,8 +3007,21 @@ EXPORT_SYMBOL_GPL(uart_handle_dcd_change);
  */
 void uart_handle_cts_change(struct uart_port *uport, unsigned int status)
 {
+       struct tty_port *port = &uport->state->port;
+       struct tty_struct *tty = port->tty;
+       struct tty_ldisc *ld;
+
        lockdep_assert_held_once(&uport->lock);
 
+       if (tty && uport->pps_4wire) {
+               ld = tty_ldisc_ref(tty);
+               if (ld) {
+                       if (ld->ops->dcd_change)
+                               ld->ops->dcd_change(tty, status);
+                       tty_ldisc_deref(ld);
+               }
+       }
+
        uport->icount.cts++;
 
        if (uart_softcts_mode(uport)) {

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 406edae44ca3070151c36ed04a72c7dda2e24305..079793e5d3fa7f323cd2a13f224c8b32585fe37a 100644 (file)
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -255,7 +255,8 @@ struct uart_port {
        struct device           *dev;                   /* parent device */
        unsigned char           hub6;                   /* this should be in the 8250 driver */
        unsigned char           suspended;
-       unsigned char           unused[2];
+       unsigned char           pps_4wire;              /* CTS instead of DCD */
+       unsigned char           unused;
        const char              *name;                  /* port name */
        struct attribute_group  *attr_group;            /* port specific attributes */
        const struct attribute_group **tty_groups;      /* all attributes (serial core use only) */