linux-kernel - Re: [PATCH 2/2] serial: 8250: Fix runtime PM for start

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Message-ID: <YlP++Eygz2aFKd1u@atomide.com>
Date:   Mon, 11 Apr 2022 13:12:08 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Johan Hovold <johan@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-serial@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Steffen Trumtrar <s.trumtrar@...gutronix.de>,
        "Matwey V. Kornilov" <matwey@....msu.ru>
Subject: Re: [PATCH 2/2] serial: 8250: Fix runtime PM for start_tx() for
 empty buffer

* Uwe Kleine-König <u.kleine-koenig@...gutronix.de> [220411 09:59]:
> 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@...gutronix.de>
> > Cc: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> > Signed-off-by: Tony Lindgren <tony@...mide.com>
> > ---
> >  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;
> > +	}
> 
> Assuming you don't need serial8250_rpm_get_tx() to check the condition,
> it would be easier to move the early return before the call to
> serial8250_rpm_get_tx().

Yeah good suggestion, that should work.

Regards,

Tony