serial: stm32: Add RS485 RTS GPIO control

While the STM32 does support RS485 drive-enable control within the
UART IP itself, some systems have the drive-enable line connected
to a pin which cannot be pinmuxed as RTS. Add support for toggling
the RTS GPIO line using the modem control GPIOs to provide at least
some sort of emulation.
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Manivannan Sadhasivam <mani@kernel.org>
Cc: Fabrice Gasnier <fabrice.gasnier@st.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-stm32@st-md-mailman.stormreply.com
Link: https://lore.kernel.org/r/20200725144947.537007-1-marex@denx.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -129,9 +129,13 @@ static int stm32_config_rs485(struct uart_port *port,
 		if (rs485conf->flags & SER_RS485_RTS_ON_SEND) {
 			cr3 &= ~USART_CR3_DEP;
 			rs485conf->flags &= ~SER_RS485_RTS_AFTER_SEND;
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl & ~TIOCM_RTS);
 		} else {
 			cr3 |= USART_CR3_DEP;
 			rs485conf->flags |= SER_RS485_RTS_AFTER_SEND;
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl | TIOCM_RTS);
 		}
 
 		writel_relaxed(cr3, port->membase + ofs->cr3);
@@ -847,9 +851,13 @@ static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
 		if (rs485conf->flags & SER_RS485_RTS_ON_SEND) {
 			cr3 &= ~USART_CR3_DEP;
 			rs485conf->flags &= ~SER_RS485_RTS_AFTER_SEND;
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl & ~TIOCM_RTS);
 		} else {
 			cr3 |= USART_CR3_DEP;
 			rs485conf->flags |= SER_RS485_RTS_AFTER_SEND;
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl | TIOCM_RTS);
 		}
 
 	} else {