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 10:23]:
> On Mon, Apr 11, 2022 at 01:10:34PM +0300, Tony Lindgren wrote:
> > * 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 :)
> 
> 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.

Hmm I'm pretty sure rs485 has the runtime PM usage count is currently
unbalanced. To me it seems em485->start_tx_timer calls start_tx()
again from serial8250_em485_handle_start_tx().

Anyways, let's deal with the regression patch first, this can wait a bit.

Regards,

Tony