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serial: 8250: Return early in .start_tx() if there are no chars to send

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Don't start the whole chain for TX if there is no data to send. This is
mostly relevant for rs485 mode as there might be rts-before-send and
rts-after-send delays involved.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220217211839.443039-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Steffen Trumtrar 2022-02-17 22:18:39 +01:00 committed by Greg Kroah-Hartman
parent 52fa786609
commit 932d596378
1 changed files with 3 additions and 0 deletions
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3
drivers/tty/serial/8250/8250_port.c
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@ -1665,6 +1665,9 @@ static void serial8250_start_tx(struct uart_port *port)
serial8250_rpm_get_tx(up);
if (!port->x_char && uart_circ_empty(&port->state->xmit))
return;
if (em485 &&
em485->active_timer == &em485->start_tx_timer)
return;