serial: 8250: Optimize irq enable after console write

Commit 7f98030 ("serial: 8250: Support console on software emulated
rs485 ports") amended serial8250_console_write() with rs485 support, but
positioned the invocation of ->rs485_stop_tx() after re-enablement of
interrupts.  The irq handler and ->console_write() are serialized with
the port spinlock, so no problem there, but due to the rs485 delay, the
irq handler may unnecessarily spin for a while.  Avoid that by moving
->rs485_stop_tx() before re-enablement of interrupts, which also mirrors
the order at the beginning of serial8250_console_write().

Signed-off-by: Lukas Wunner <[email protected]>
Link: https://lore.kernel.org/r/019839cb1f61b01210b6ff9ac9f9079ca77f8411.1585319447.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: l1k

drivers/tty/serial/8250/8250_port.c

@@ -3268,14 +3268,15 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 	 *	and restore the IER
 	 */
 	wait_for_xmitr(up, BOTH_EMPTY);
-	serial_port_out(port, UART_IER, ier);
 
 	if (em485) {
 		mdelay(port->rs485.delay_rts_after_send);
 		if (em485->tx_stopped)
 			up->rs485_stop_tx(up);
 	}
 
+	serial_port_out(port, UART_IER, ier);
+
 	/*
 	 *	The receive handling will happen properly because the
 	 *	receive ready bit will still be set; it is not cleared