Re: [PATCH 1/2] serial: 8250: Fix runtime PM for start_tx() for RS485

[Tony Lindgren <tony@atomide.com>]

* Johan Hovold <johan@kernel.org> [220411 09:54]:
> On Mon, Apr 11, 2022 at 12:48:04PM +0300, Tony Lindgren wrote:
> > 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
> > @@ -1681,8 +1681,10 @@ static void serial8250_start_tx(struct uart_port *port)
> >  		return;
> >  
> >  	if (em485 &&
> > -	    em485->active_timer == &em485->start_tx_timer)
> > +	    em485->active_timer == &em485->start_tx_timer) {
> > +		serial8250_rpm_put_tx(up);
> >  		return;
> > +	}
> 
> I was just taking a quick look at your report about this and also
> noticed this return statement.
> 
> The runtime PM implementation is a bit of mess as we've discussed
> elsewhere, but the change you propose here doesn't look right.

Frankly "a bit of mess" applies "a bit more" than just the serial runtime
PM :)

> start_tx() can be deferred in the rs485 case, but that doesn't mean you
> should suspend the device here. In fact, that look like it would just
> break runtime PM (the parts that may work to some extent).

AFAIK there's currently nothing paired with the serial8250_rpm_get_tx(up)
call at the beginning of serial8250_start_tx() for the early exit cases
if start_tx_rs485() or __start_tx() won't get called.

Care to clarify a bit more what you have in mind?

Regards,

Tony