[PATCH 2/2] serial: stm32: Re-assert RTS/DE GPIO in RS485 mode only if more data are transmitted

[Marek Vasut]

The stm32_usart_transmit_chars() may be called with empty or stopped
transmit queue, and no XON/OFF character pending. This can happen at
the end of transmission, where this last call is used to either handle
the XON/XOFF x_char, or disable TX interrupt if queue is empty or
stopped.

If that occurs, do not assert the RS485 RTS/DE GPIO anymore, as the
GPIO would remain asserted past the end of transmission and that would
block the RS485 bus after the transmission.

Only assert the RS485 RTS/DE GPIO if there is either pending XON/XOFF
x_char, or at least one character in running transmit queue.

Fixes: d7c76716169d ("serial: stm32: Use TC interrupt to deassert GPIO RTS in RS485 mode")
Signed-off-by: Marek Vasut <marex at denx.de>
---
Cc: Alexandre Torgue <alexandre.torgue at foss.st.com>
Cc: Erwan Le Ray <erwan.leray at foss.st.com>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: Jean Philippe Romain <jean-philippe.romain at foss.st.com>
Cc: Jiri Slaby <jirislaby at kernel.org>
Cc: Johan Hovold <johan at kernel.org>
Cc: Lukas Wunner <lukas at wunner.de>
Cc: Maxime Coquelin <mcoquelin.stm32 at gmail.com>
Cc: Valentin Caron <valentin.caron at foss.st.com>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-serial at vger.kernel.org
Cc: linux-stm32 at st-md-mailman.stormreply.com
---
 drivers/tty/serial/stm32-usart.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index bf51e2152dd5a..1e38fc9b10c11 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -693,8 +693,9 @@ static void stm32_usart_transmit_chars(struct uart_port *port)
 	int ret;
 
 	if (!stm32_port->hw_flow_control &&
-	    port->rs485.flags & SER_RS485_ENABLED) {
-		stm32_port->txdone = false;
+	    port->rs485.flags & SER_RS485_ENABLED &&
+	    (port->x_char ||
+	     !(uart_circ_empty(xmit) || uart_tx_stopped(port)))) {
 		stm32_usart_tc_interrupt_disable(port);
 		stm32_usart_rs485_rts_enable(port);
 	}
-- 
2.39.1