[PATCH 1/3] TTY: add support for "tty slave" devices.

[NeilBrown <neilb@suse.de>]

A "tty slave" is a device connected via UART.
It may need a driver to, for example, power the device on
when the tty is opened, and power it off when the tty
is released.

A "tty slave" is a platform device which is declared as a
child of the uart in device-tree:

&uart1 {
	bluetooth {
		compatible = "tty,regulator";
		vdd-supply = <&vaux4>;
	};
};

runtime power management is used to power-up the device
on tty_open() and  power-down on tty_release().

Signed-off-by: NeilBrown <neilb@suse.de>
---
 .../devicetree/bindings/serial/of-serial.txt       |    4 ++++
 drivers/tty/tty_io.c                               |   22 ++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/Documentation/devicetree/bindings/serial/of-serial.txt b/Documentation/devicetree/bindings/serial/of-serial.txt
index 8c4fd0332028..b59501ee2f21 100644
--- a/Documentation/devicetree/bindings/serial/of-serial.txt
+++ b/Documentation/devicetree/bindings/serial/of-serial.txt
@@ -39,6 +39,10 @@ Optional properties:
   driver is allowed to detect support for the capability even without this
   property.
 
+Optional child node:
+- a platform device listed as a child node will be probed and
+  powered-on whenever the tty is in use (open).
+
 Example:
 
 	uart@80230000 {
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 0508a1d8e4cd..7acdc6f093f4 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -95,6 +95,8 @@
 #include <linux/seq_file.h>
 #include <linux/serial.h>
 #include <linux/ratelimit.h>
+#include <linux/pm_runtime.h>
+#include <linux/of_platform.h>
 
 #include <linux/uaccess.h>
 
@@ -1683,6 +1685,17 @@ static int tty_release_checks(struct tty_struct *tty, struct tty_struct *o_tty,
 	return 0;
 }
 
+static int open_child(struct device *dev, void *data)
+{
+	pm_runtime_get_sync(dev);
+	return 0;
+}
+static int release_child(struct device *dev, void *data)
+{
+	pm_runtime_put_autosuspend(dev);
+	return 0;
+}
+
 /**
  *	tty_release		-	vfs callback for close
  *	@inode: inode of tty
@@ -1712,6 +1725,8 @@ int tty_release(struct inode *inode, struct file *filp)
 	long	timeout = 0;
 	int	once = 1;
 
+	if (tty->dev)
+		device_for_each_child(tty->dev, NULL, release_child);
 	if (tty_paranoia_check(tty, inode, __func__))
 		return 0;
 
@@ -2118,6 +2133,8 @@ retry_open:
 		__proc_set_tty(current, tty);
 	spin_unlock_irq(&current->sighand->siglock);
 	tty_unlock(tty);
+	if (tty->dev)
+		device_for_each_child(tty->dev, NULL, open_child);
 	mutex_unlock(&tty_mutex);
 	return 0;
 err_unlock:
@@ -3207,6 +3224,11 @@ struct device *tty_register_device_attr(struct tty_driver *driver,
 	retval = device_register(dev);
 	if (retval)
 		goto error;
+	if (device && device->of_node)
+		/* Children are platform devices and will be
+		 * runtime_pm managed by this tty.
+		 */
+		of_platform_populate(device->of_node, NULL, NULL, dev);
 
 	return dev;