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 11:53]:
> On Mon, Apr 11, 2022 at 01:32:58PM +0300, Tony Lindgren wrote:
> > * 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;
> > > > > > +	}
> 
> > > 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().
> 
> It appears to call __start_tx(), but note that the only call to
> serial8250_rpm_get_tx() is in serial8250_start_tx() which this patch
> would have cancelled out.

OK

> Also note that the serial8250_rpm_get/set_tx() calls aren't supposed to
> be balanced. get() can be called multiple times and will only increment
> the PM usage counter once, while put() will decrement the counter
> whenever get() has been called once (and hence potentially suspend the
> device immediately).
> 
> Messy indeed.

Yeah that is not nice.

I'll send a patch to prepare things for runtime PM that will hopefully
make things a bit easier as discussed earlier.

Regards,

Tony