[PATCHv2] serial: 8250: Fix runtime PM for start_tx() for empty buffer

[Tony Lindgren <tony@atomide.com>]

Commit 932d596378b0 ("serial: 8250: Return early in .start_tx() if there
are no chars to send") caused a regression where the drivers implementing
runtime PM stopped idling. This is because serial8250_rpm_put_tx() is now
unbalanced on early return, it normally gets called at __stop_tx().

Fixes: 932d596378b0 ("serial: 8250: Return early in .start_tx() if there are no chars to send")
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>
---

Changes since v1:

- Call serial8250_rpm_get_tx() only after buffer empty test as suggested
  by Uwe and Johan

- Drop RS885 related patch for now, it needs to be investigated further

---
 drivers/tty/serial/8250/8250_port.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1675,11 +1675,11 @@ static void serial8250_start_tx(struct uart_port *port)
 	struct uart_8250_port *up = up_to_u8250p(port);
 	struct uart_8250_em485 *em485 = up->em485;
 
-	serial8250_rpm_get_tx(up);
-
 	if (!port->x_char && uart_circ_empty(&port->state->xmit))
 		return;
 
+	serial8250_rpm_get_tx(up);
+
 	if (em485 &&
 	    em485->active_timer == &em485->start_tx_timer)
 		return;
-- 
2.35.1