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

From: Clemens Gruber <clemens.gruber@pqgruber.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: "Andy Shevchenko" <andy.shevchenko@gmail.com>,
	"Matthias Schiffer" <matthias.schiffer@ew.tq-group.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	linux-pwm@vger.kernel.org,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/4] pwm: pca9685: remove unused duty_cycle struct element
Date: Mon, 30 Mar 2020 18:10:23 +0200	[thread overview]
Message-ID: <20200330161023.GB777@workstation.tuxnet> (raw)
In-Reply-To: <20200330160238.GD2817345@ulmo>

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