tty: serial: Add modem control gpio support for STM32 UART

STM32 UART controllers have the built in modem control support using
dedicated gpios which can be enabled using 'st,hw-flow-ctrl' flag in DT.
But there might be cases where the board design need to use different
gpios for modem control.

For supporting such cases, this commit adds modem control gpio support
to STM32 UART controller using mctrl_gpio driver.

Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Manivannan Sadhasivam <[e-mail]>
Acked-by: Fabrice Gasnier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/Kconfig

···
 	tristate "STMicroelectronics STM32 serial port support"
 	select SERIAL_CORE
 	depends on ARCH_STM32 || COMPILE_TEST
+	select SERIAL_MCTRL_GPIO if GPIOLIB
 	help
 	  This driver is for the on-chip Serial Controller on
 	  STMicroelectronics STM32 MCUs.

drivers/tty/serial/stm32-usart.c

···
 #include <linux/tty_flip.h>
 #include <linux/tty.h>
 
+#include "serial_mctrl_gpio.h"
 #include "stm32-usart.h"
 
 static void stm32_stop_tx(struct uart_port *port);
···
 		stm32_set_bits(port, ofs->cr3, USART_CR3_RTSE);
 	else
 		stm32_clr_bits(port, ofs->cr3, USART_CR3_RTSE);
+
+	mctrl_gpio_set(stm32_port->gpios, mctrl);
 }
 
 static unsigned int stm32_get_mctrl(struct uart_port *port)
 {
+	struct stm32_port *stm32_port = to_stm32_port(port);
+	unsigned int ret;
+
 	/* This routine is used to get signals of: DCD, DSR, RI, and CTS */
-	return TIOCM_CAR | TIOCM_DSR | TIOCM_CTS;
+	ret = TIOCM_CAR | TIOCM_DSR | TIOCM_CTS;
+
+	return mctrl_gpio_get(stm32_port->gpios, &ret);
+}
+
+static void stm32_enable_ms(struct uart_port *port)
+{
+	mctrl_gpio_enable_ms(to_stm32_port(port)->gpios);
+}
+
+static void stm32_disable_ms(struct uart_port *port)
+{
+	mctrl_gpio_disable_ms(to_stm32_port(port)->gpios);
 }
 
 /* Transmit stop */
···
 	struct stm32_usart_config *cfg = &stm32_port->info->cfg;
 	u32 val, isr;
 	int ret;
+
+	/* Disable modem control interrupts */
+	stm32_disable_ms(port);
 
 	val = USART_CR1_TXEIE | USART_CR1_TE;
 	val |= stm32_port->cr1_irq | USART_CR1_RE;
···
 		cr3 |= USART_CR3_CTSE | USART_CR3_RTSE;
 	}
 
+	/* Handle modem control interrupts */
+	if (UART_ENABLE_MS(port, termios->c_cflag))
+		stm32_enable_ms(port);
+	else
+		stm32_disable_ms(port);
+
 	usartdiv = DIV_ROUND_CLOSEST(port->uartclk, baud);
 
 	/*
···
 	.throttle	= stm32_throttle,
 	.unthrottle	= stm32_unthrottle,
 	.stop_rx	= stm32_stop_rx,
+	.enable_ms	= stm32_enable_ms,
 	.break_ctl	= stm32_break_ctl,
 	.startup	= stm32_startup,
 	.shutdown	= stm32_shutdown,
···
 
 	stm32port->port.uartclk = clk_get_rate(stm32port->clk);
 	if (!stm32port->port.uartclk) {
-		clk_disable_unprepare(stm32port->clk);
 		ret = -EINVAL;
+		goto err_clk;
 	}
+
+	stm32port->gpios = mctrl_gpio_init(&stm32port->port, 0);
+	if (IS_ERR(stm32port->gpios)) {
+		ret = PTR_ERR(stm32port->gpios);
+		goto err_clk;
+	}
+
+	/* Both CTS/RTS gpios and "st,hw-flow-ctrl" should not be specified */
+	if (stm32port->hw_flow_control) {
+		if (mctrl_gpio_to_gpiod(stm32port->gpios, UART_GPIO_CTS) ||
+		    mctrl_gpio_to_gpiod(stm32port->gpios, UART_GPIO_RTS)) {
+			dev_err(&pdev->dev, "Conflicting RTS/CTS config\n");
+			ret = -EINVAL;
+			goto err_clk;
+		}
+	}
+
+	return ret;
+
+err_clk:
+	clk_disable_unprepare(stm32port->clk);
 
 	return ret;
 }

drivers/tty/serial/stm32-usart.h

···
 	bool fifoen;
 	int wakeirq;
 	int rdr_mask;		/* receive data register mask */
+	struct mctrl_gpios *gpios; /* modem control gpios */
 };
 
 static struct stm32_port stm32_ports[STM32_MAX_PORTS];