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

[ Upstream commit c47527cbcc3c50800f34b8c684f29721f75de246 ]

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@denx.de>
Link: https://lore.kernel.org/r/20230223042252.95480-2-marex@denx.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 0e6ef24..28edbaf 100644 (file)
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -690,8 +690,9 @@ 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);
        }