linux-kernel - Re: [PATCH] pwm: don't allow duty cycle higher than period

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Message-ID: <20160527163533.GA80118@google.com>
Date:	Fri, 27 May 2016 09:35:33 -0700
From:	Brian Norris <briannorris@...omium.org>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Brian Norris <computersforpeace@...il.com>,
	Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH] pwm: don't allow duty cycle higher than period

Hi Boris,

On Fri, May 27, 2016 at 09:34:39AM +0200, Boris Brezillon wrote:
> On Thu, 26 May 2016 14:05:30 -0700
> Brian Norris <briannorris@...omium.org> wrote:
> 
> > It doesn't make sense to allow the duty cycle to be larger than the
> > period. I can see this behavior by, e.g.:
> > 
> >   # echo 1 > /sys/class/pwm/pwmchip0/export
> >   # cat /sys/class/pwm/pwmchip0/pwm1/period
> >   100
> >   # echo 101 > /sys/class/pwm/pwmchip0/pwm1/duty_cycle
> >   [... driver may or may not reject the value, or trigger some logic bug ...]
> > 
> > It's better to see:
> > 
> >   # echo 1 > /sys/class/pwm/pwmchip0/export
> >   # cat /sys/class/pwm/pwmchip0/pwm1/period
> >   100
> >   # echo 101 > /sys/class/pwm/pwmchip0/pwm1/duty_cycle
> >   -bash: echo: write error: Invalid argument
> > 
> > Signed-off-by: Brian Norris <briannorris@...omium.org>
> > ---
> >  drivers/pwm/core.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > index dba3843c53b8..9246b60f894a 100644
> > --- a/drivers/pwm/core.c
> > +++ b/drivers/pwm/core.c
> > @@ -463,6 +463,9 @@ int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state)
> >  	if (!memcmp(state, &pwm->state, sizeof(*state)))
> >  		return 0;
> >  
> > +	if (state->duty_cycle > state->period)
> > +		return -EINVAL;
> > +
> 
> Argh, I forgot to move the pwm_config() checks [1] into
> pwm_apply_state() :-/.

Oh, I didn't actually notice this was a regression.

> I think we should check all the corner cases (see this diff [2]),

Now that you mention it, I think you've also dropped some signed
(negative value) checking in pwm_config(). I'll squash in your diff +
some pwm_config() fixes.

> once done you can add my
> 
> Acked-by: Boris Brezillon <boris.brezillon@...e-electrons.com>

I'll send v2 without your ack, since I'm going to add a tiny bit extra.
That'll give you a chance to ack the final (?) version.

> Thierry, can you include that in your material for 4.7-rc1?

That sounds like it would be a good idea, IMO. Thanks for noticing this
was a regression! :)

Regards,
Brian

> Thanks,
> 
> Boris
> 
> [1]http://lxr.free-electrons.com/source/drivers/pwm/core.c#L443
> [2]http://code.bulix.org/wtqja4-99473
> -- 
> Boris Brezillon, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com