[PATCH V3] serial: stm32: Add RS485 RTS GPIO control again

[Marek Vasut <marex@xxxxxxx>]

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.

Fixes: 7df5081cbf5e ("serial: stm32: Add RS485 RTS GPIO control")
Signed-off-by: Marek Vasut <marex@xxxxxxx>
Cc: Alexandre Torgue <alexandre.torgue@xxxxxx>
Cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
Cc: Manivannan Sadhasivam <mani@xxxxxxxxxx>
Cc: Fabrice Gasnier <fabrice.gasnier@xxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: linux-stm32@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
V2: Use mctrl_gpio_set() instead of stm32_set_mctrl()
V3: - Actually toggle the RTS line before and after TX
    - Undo 7df5081cbf5e ("serial: stm32: Add RS485 RTS GPIO control")
      which was previous version of this patch ; I messed up.
---
 drivers/tty/serial/stm32-usart.c | 33 ++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 143300a80090..23f7453441ae 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -129,13 +129,9 @@ 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);
@@ -541,17 +537,42 @@ static void stm32_disable_ms(struct uart_port *port)
 /* Transmit stop */
 static void stm32_stop_tx(struct uart_port *port)
 {
+	struct stm32_port *stm32_port = to_stm32_port(port);
+	struct serial_rs485 *rs485conf = &port->rs485;
+
 	stm32_tx_interrupt_disable(port);
+
+	if (rs485conf->flags & SER_RS485_ENABLED) {
+		if (rs485conf->flags & SER_RS485_RTS_ON_SEND) {
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl & ~TIOCM_RTS);
+		} else {
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl | TIOCM_RTS);
+		}
+	}
 }
 
 /* There are probably characters waiting to be transmitted. */
 static void stm32_start_tx(struct uart_port *port)
 {
+	struct stm32_port *stm32_port = to_stm32_port(port);
+	struct serial_rs485 *rs485conf = &port->rs485;
 	struct circ_buf *xmit = &port->state->xmit;
 
 	if (uart_circ_empty(xmit))
 		return;
 
+	if (rs485conf->flags & SER_RS485_ENABLED) {
+		if (rs485conf->flags & SER_RS485_RTS_ON_SEND) {
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl | TIOCM_RTS);
+		} else {
+			mctrl_gpio_set(stm32_port->gpios,
+					stm32_port->port.mctrl & ~TIOCM_RTS);
+		}
+	}
+
 	stm32_transmit_chars(port);
 }
 
@@ -851,13 +872,9 @@ 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 {
-- 
2.28.0