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

[Johan Hovold <johan@kernel.org>]

On Mon, Apr 11, 2022 at 02:16:57PM +0300, Tony Lindgren wrote:
> 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;

Reviewed-by: Johan Hovold <johan@kernel.org>