Re: [PATCH 1/4] pwm: pca9685: remove unused duty_cycle struct element

Clemens Gruber
2020-03-30T16:10:27Z

On Mon, Mar 30, 2020 at 06:02:38PM +0200, Thierry Reding wrote:
> On Mon, Mar 30, 2020 at 04:18:22PM +0300, Andy Shevchenko wrote:
> > On Mon, Mar 30, 2020 at 4:09 PM Thierry Reding <thierry.reding@gmail.com> wrote:
> > >
> > > On Wed, Feb 26, 2020 at 02:52:26PM +0100, Matthias Schiffer wrote:
> > > > duty_cycle was only set, never read.
> > > >
> > > > Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> > > > ---
> > > > drivers/pwm/pwm-pca9685.c | 4 ----
> > > > 1 file changed, 4 deletions(-)
> > >
> > > Applied, thanks.
> >
> > I'm not sure this patch is correct.
>
> What makes you say that? If you look at the code, the driver sets this
> field to either 0 or some duty cycle value but ends up never using it.
> Why would it be wrong to remove that code?
>
> > We already have broken GPIO in this driver. Do we need more breakage?
>
> My understanding is that nobody was able to pinpoint exactly when this
> regressed, or if this only worked by accident to begin with. It sounds
> like Clemens has a way of testing this driver, so perhaps we can solve
> that GPIO issue while we're at it.
>
> The last discussion on this seems to have been around the time when you
> posted a fix for it:
>
> https://patchwork.ozlabs.org/patch/1156012/
>
> But then Sven had concerns that that also wasn't guaranteed to work:
>
> https://lkml.org/lkml/2019/6/2/73
>
> So I think we could either apply your patch to restore the old behaviour
> which I assume you tested, so at least it seems to work in practice,
> even if there's still a potential race that Sven pointed out in the
> above link.
>
> I'd prefer something alternative because it's obviously confusing and
> completely undocumented. Mika had already proposed something that's a
> little bit better, though still somewhat confusing.
>
> Oh... actually reading further through those threads there seems to be a
> patch from Sven that was reviewed by Mika but then nothing happened:
>
> https://lkml.org/lkml/2019/6/4/1039
>
> with the corresponding patchwork URL:
>
> https://patchwork.ozlabs.org/patch/1110083/
>
> Andy, Clemens, do you have a way of testing the GPIO functionality of
> this driver? If so, it'd be great if you could check the above patch
> from Sven to fix PWM/GPIO interop.

Yes. I'll have a look and report back in a few days.

Clemens