Re: [PATCH 2/2] serial: 8250: Fix runtime PM for start_tx() for empty buffer

[Johan Hovold]

On Mon, Apr 11, 2022 at 12:48:05PM +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.
> 
> We need to call serial8250_rpm_put_tx() on early exit, it normally gets
> called later on at __stop_tx().
> 
> Fixes: 932d596378b0 ("serial: 8250: Return early in .start_tx() if there are no chars to send")
> Cc: Steffen Trumtrar <s.trumtrar@xxxxxxxxxxxxxx>
> Cc: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
> ---
>  drivers/tty/serial/8250/8250_port.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> 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
> @@ -1677,8 +1677,10 @@ static void serial8250_start_tx(struct uart_port *port)
>  
>  	serial8250_rpm_get_tx(up);
>  
> -	if (!port->x_char && uart_circ_empty(&port->state->xmit))
> +	if (!port->x_char && uart_circ_empty(&port->state->xmit)) {
> +		serial8250_rpm_put_tx(up);
>  		return;
> +	}

Move this before the runtime pm get instead?

>  
>  	if (em485 &&
>  	    em485->active_timer == &em485->start_tx_timer) {

Johan