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
