linux-kernel - Re: [PATCH -next] pwm: pca9685: Remove set but not used variable 'pwm'

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Date:   Sat, 1 Jun 2019 18:04:59 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Sven Van Asbroeck <thesven73@...il.com>
Cc:     YueHaibing <yuehaibing@...wei.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-pwm@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] pwm: pca9685: Remove set but not used variable
 'pwm'

Hello Sven,

On Sat, Jun 01, 2019 at 09:03:09AM -0400, Sven Van Asbroeck wrote:
> Hi YueHaibing,
> 
> On Fri, May 31, 2019 at 11:49 PM YueHaibing <yuehaibing@...wei.com> wrote:
> >
> >         mutex_lock(&pca->lock);
> > -       pwm = &pca->chip.pwms[offset];
> >         mutex_unlock(&pca->lock);
> 
> Thanks for noticing this issue. However it should be fixed differently.
> 
> This was introduced by Uwe's clean-up patch:
> commit e926b12c611c2095c79 ("pwm: Clear chip_data in pwm_put()")
> 
> But Uwe did not realize that in this case, the pwm chip_data is used as a
> synchronization mechanism between pwm and gpio. Moving the chip_data
> clear out of the mutex breaks this mechanism.

> 
> I think the following would restore the mechanism:
> 
> >         mutex_lock(&pca->lock);
> >        pwm = &pca->chip.pwms[offset];
> > +     pwm_set_chip_data(pwm, NULL);
> >         mutex_unlock(&pca->lock);

I didn't look into the driver to try to understand that, but the
definitely needs a comment to explain for the next person to think they
can do a cleanup here.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |