linux-kernel - Re: [PATCH 1/4] pwm: pca9685: remove unused duty

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Message-ID: <20200401163640.GA91358@workstation.tuxnet>
Date:   Wed, 1 Apr 2020 18:36:40 +0200
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-pwm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sven Van Asbroeck <TheSven73@...il.com>
Subject: Re: [PATCH 1/4] pwm: pca9685: remove unused duty_cycle struct element

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@...il.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@...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.

Looks good. Tested it today and I can no longer reproduce the issues
when switching between PWM and GPIO modes.
It did not apply cleanly on the current mainline or for-next branch, so
I'll send a fixed up version of the patch with my Tested-by tag shortly.

I noticed an unrelated issue when disabling and enabling the channel
though, for which I will either send a patch or maybe try to convert the
driver to the atomic API first and then look if it is still a problem.
(Issue is that if you disable the channel, the LED_OFF counter is
cleared, which means you have to reconfigure the duty cycle after
reenabling. It's probably better if only the FULL_OFF bit is toggled in
enable/disable as it has precedence over the others anyway and then the
previous state would not be changed..?)

Clemens