tty/serial: Add GPIOLIB helpers for controlling modem lines

This patch add some helpers to control modem lines (CTS/RTS/DSR...) via
GPIO.
This will be useful for many boards which have a serial controller that
only handle CTS/RTS pins (or even just RX/TX).

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/Documentation/serial/driver b/Documentation/serial/driver
index c3a7689a90e6..3bba1aeb799c 100644
--- a/Documentation/serial/driver
+++ b/Documentation/serial/driver
@@ -429,3 +429,28 @@ thus:
                 struct uart_port        port;
                 int                     my_stuff;
         };
+
+Modem control lines via GPIO
+----------------------------
+
+Some helpers are provided in order to set/get modem control lines via GPIO.
+
+mctrl_gpio_init(dev, idx):
+        This will get the {cts,rts,...}-gpios from device tree if they are
+        present and request them, set direction etc, and return an
+        allocated structure. devm_* functions are used, so there's no need
+        to call mctrl_gpio_free().
+
+mctrl_gpio_free(dev, gpios):
+        This will free the requested gpios in mctrl_gpio_init().
+        As devm_* function are used, there's generally no need to call
+        this function.
+
+mctrl_gpio_to_gpiod(gpios, gidx)
+        This returns the gpio structure associated to the modem line index.
+
+mctrl_gpio_set(gpios, mctrl):
+        This will sets the gpios according to the mctrl state.
+
+mctrl_gpio_get(gpios, mctrl):
+        This will update mctrl with the gpios values.