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

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

On Fri, 27 May 2016 09:35:33 -0700
Brian Norris <briannorris@chromium.org> wrote:

> 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@chromium.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@chromium.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.

->period and ->duty_cycle are unsigned now ;).

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com