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

[Tony Lindgren]

Hi,

* Uwe Kleine-König <u.kleine-koenig at pengutronix.de> [700101 02:00]:
> From: Steffen Trumtrar <s.trumtrar at pengutronix.de>
> 
> 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 at pengutronix.de>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
> ---
> Hello,
> 
> a few other drivers have such an early exit already (imx, stm32-usart).
> I wonder if it applies to all UART drivers that there is nothing to do
> in .start_tx() if the circ buffer is empty and there is no x_char to
> send. In this case it would be more sensible to ensure in serial_core
> that .start_tx() is only ever called if there is something to do.

This patch seems to cause a runtime PM regression in v5.18-rc1 where
8250 is never idled for omaps.

Looks like the return added here is not paired with put for the
serial8250_rpm_get_tx() call above?

Regards,

Tony

#regzbot ^introduced 932d596378b0

>  drivers/tty/serial/8250/8250_port.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 3b12bfc1ed67..5d4668f12f71 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1657,6 +1657,9 @@ 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;
> 
> base-commit: 754e0b0e35608ed5206d6a67a791563c631cec07
> -- 
> 2.34.1
>