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 <redacted>
Tested-by: Yegor Yefremov <redacted>
---
 Documentation/serial/driver            |   25 ++++++
 drivers/tty/serial/Kconfig             |    3 +
 drivers/tty/serial/Makefile            |    3 +
 drivers/tty/serial/serial_mctrl_gpio.c |  143 ++++++++++++++++++++++++++++++++
 drivers/tty/serial/serial_mctrl_gpio.h |  110 ++++++++++++++++++++++++
 5 files changed, 284 insertions(+)
 create mode 100644 drivers/tty/serial/serial_mctrl_gpio.c
 create mode 100644 drivers/tty/serial/serial_mctrl_gpio.h

diff --git a/Documentation/serial/driver b/Documentation/serial/driver
index c3a7689..3bba1ae 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.

diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index 5d9b01a..95afd31 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig

@@ -1510,4 +1510,7 @@ config SERIAL_ST_ASC_CONSOLE
 
 endmenu
 
+config SERIAL_MCTRL_GPIO
+	tristate

+
 endif # TTY

diff --git a/drivers/tty/serial/Makefile b/drivers/tty/serial/Makefile
index 3680854..bcf31da 100644
--- a/drivers/tty/serial/Makefile
+++ b/drivers/tty/serial/Makefile

@@ -87,3 +87,6 @@ obj-$(CONFIG_SERIAL_EFM32_UART) += efm32-uart.o
 obj-$(CONFIG_SERIAL_ARC)	+= arc_uart.o
 obj-$(CONFIG_SERIAL_RP2)	+= rp2.o
 obj-$(CONFIG_SERIAL_FSL_LPUART)	+= fsl_lpuart.o
+
+# GPIOLIB helpers for modem control lines
+obj-$(CONFIG_SERIAL_MCTRL_GPIO) += serial_mctrl_gpio.o