linux-kernel - [PATCH] serial: stm32: do not deassert RS485 RTS GPIO prematurely

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Message-ID: <20250320152540.709091-1-cheick.traore@foss.st.com>
Date: Thu, 20 Mar 2025 16:25:40 +0100
From: Cheick Traore <cheick.traore@...s.st.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby
	<jirislaby@...nel.org>, Marek Vasut <marex@...x.de>
CC: Cheick Traore <cheick.traore@...s.st.com>, <linux-kernel@...r.kernel.org>,
        <linux-serial@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: [PATCH] serial: stm32: do not deassert RS485 RTS GPIO prematurely

If stm32_usart_start_tx is called with an empty xmit buffer, RTS GPIO
could be deasserted prematurely, as bytes in TX FIFO are still
transmitting.
So this patch remove rts disable when xmit buffer is empty.

Fixes: d7c76716169d ("serial: stm32: Use TC interrupt to deassert GPIO RTS in RS485 mode")
Signed-off-by: Cheick Traore <cheick.traore@...s.st.com>
---
 drivers/tty/serial/stm32-usart.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 4c97965ec43b..ad06b760cfca 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -965,10 +965,8 @@ static void stm32_usart_start_tx(struct uart_port *port)
 {
 	struct tty_port *tport = &port->state->port;
 
-	if (kfifo_is_empty(&tport->xmit_fifo) && !port->x_char) {
-		stm32_usart_rs485_rts_disable(port);
+	if (kfifo_is_empty(&tport->xmit_fifo) && !port->x_char)
 		return;
-	}
 
 	stm32_usart_rs485_rts_enable(port);
 
-- 
2.34.1