Re: [PATCH v2] pwm: improve args checking in pwm_apply_state()

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Wed, 22 Jun 2016 12:16:59 -0700
Brian Norris <briannorris@chromium.org> wrote:

> On Wed, Jun 22, 2016 at 10:04:22AM +0200, Boris Brezillon wrote:
> > From 0610f7e24976e176054bce20445ff42d8aea9513 Mon Sep 17 00:00:00 2001
> > From: Boris Brezillon <boris.brezillon@free-electrons.com>
> > Date: Wed, 22 Jun 2016 09:25:14 +0200
> > Subject: [PATCH] pwm: Fix pwm_apply_args()
> > 
> > Commit 5ec803edcb70 ("pwm: Add core infrastructure to allow atomic
> > updates"), implemented pwm_disable() as a wrapper around
> > pwm_apply_state(), and then, commit ef2bf4997f7d ("pwm: Improve args
> > checking in pwm_apply_state()") added missing checks on the ->period
> > value in pwm_apply_state() to ensure we were not passing inappropriate
> > values to the ->config() or ->apply() methods.
> > 
> > The conjunction of these 2 commits led to a case where pwm_disable()
> > was no longer succeeding, thus preventing the polarity setting done
> > in pwm_apply_args().
> > 
> > Set a valid period in pwm_apply_args() to ensure polarity setting
> > won't be rejected.
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> > Suggested-by: Brian Norris <briannorris@chromium.org>
> > Fixes: 5ec803edcb70 ("pwm: Add core infrastructure to allow atomic updates")
> > ---
> >  include/linux/pwm.h | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> > 
> > diff --git a/include/linux/pwm.h b/include/linux/pwm.h
> > index 908b67c847cd..c038ae36b10e 100644
> > --- a/include/linux/pwm.h
> > +++ b/include/linux/pwm.h
> > @@ -464,6 +464,8 @@ static inline bool pwm_can_sleep(struct pwm_device *pwm)
> >  
> >  static inline void pwm_apply_args(struct pwm_device *pwm)
> >  {
> > +	struct pwm_state state = { };
> > +
> >  	/*
> >  	 * PWM users calling pwm_apply_args() expect to have a fresh config
> >  	 * where the polarity and period are set according to pwm_args info.
> > @@ -476,18 +478,20 @@ static inline void pwm_apply_args(struct pwm_device *pwm)
> >  	 * at startup (even if they are actually enabled), thus authorizing
> >  	 * polarity setting.
> >  	 *
> > -	 * Instead of setting ->enabled to false, we call pwm_disable()
> > -	 * before pwm_set_polarity() to ensure that everything is configured
> > -	 * as expected, and the PWM is really disabled when the user request
> > -	 * it.  
> 
> I was confused by this original text when reading it the first time. I
> like the replacement text and implementation, as it seems to make more
> sense.

Well, it should have been done this way from the beginning, but
pwm_apply_args() was introduced before the commit introducing the atomic
APIs, and I forgot to update it when moving to the atomic approach :-/.

> 
> > +	 * To fulfill this requirement, we apply a new state which disables
> > +	 * the PWM device and set the reference period and polarity config.
> >  	 *
> >  	 * Note that PWM users requiring a smooth handover between the
> >  	 * bootloader and the kernel (like critical regulators controlled by
> >  	 * PWM devices) will have to switch to the atomic API and avoid calling
> >  	 * pwm_apply_args().
> >  	 */
> > -	pwm_disable(pwm);
> > -	pwm_set_polarity(pwm, pwm->args.polarity);  
> 
> Notably, you're dropping the 'if (!pwm) { }' safety checks that are part
> of pwm_disable() and pwm_set_polarity(). But I don't think there should
> be any users relying on that.

Indeed. I can add it back here if you prefer, but honestly, PWM users
that are not checking the value returned by pwm_get() should be
considered buggy IMHO, and a NULL pointer exception is a good way to
make people realize they are not properly using the API :).

> 
> > +
> > +	state.enabled = false;
> > +	state.polarity = pwm->args.polarity;
> > +	state.period = pwm->args.period;
> > +
> > +	pwm_apply_state(pwm, &state);
> >  }
> >  
> >  struct pwm_lookup {  
> 
> Reviewed-by: Brian Norris <briannorris@chromium.org>