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

…are transmitted

[ Upstream commit c47527c ]

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: d7c7671 ("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>

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);
 	}