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

[Johan Hovold]

On Mon, Apr 11, 2022 at 01:10:34PM +0300, Tony Lindgren wrote:
> * Johan Hovold <johan@xxxxxxxxxx> [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 :)

Heh. I'm afraid that's all too true. :)

> > 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?

The problem is that that serial8250_rpm_put_tx() you're adding may
suspend the device unconditionally (i.e. regardless of any previous
calls to serial8250_rpm_get_tx()).

If rs485 tx is just being deferred you mustn't suspend the device before
it has had a chance to start transmitting.

Johan