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




Hi YueHaibing,

On Fri, May 31, 2019 at 11:49 PM YueHaibing <yuehaibing@xxxxxxxxxx> 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);

This would of course clear the pwm chip_data twice, once in the driver and
once in the core, but that's not a problem.

I'd like to hear Mika Westerberg's opinion, because he introduced this
synchronization mechanism back in 2016.

[Adding Mika]

Sven