serial: stm32: fix software flow control transfer

x_char is ignored by stm32_usart_start_tx() when xmit buffer is empty.

Fix start_tx condition to allow x_char to be sent.

Fixes: 48a6092 ("serial: stm32-usart: Add STM32 USART Driver")
Cc: stable <[email protected]>
Signed-off-by: Erwan Le Ray <[email protected]>
Signed-off-by: Valentin Caron <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: VCASTM

drivers/tty/serial/stm32-usart.c

@@ -742,7 +742,7 @@ static void stm32_usart_start_tx(struct uart_port *port)
 	struct serial_rs485 *rs485conf = &port->rs485;
 	struct circ_buf *xmit = &port->state->xmit;
 
-	if (uart_circ_empty(xmit))
+	if (uart_circ_empty(xmit) && !port->x_char)
 		return;
 
 	if (rs485conf->flags & SER_RS485_ENABLED) {