linux-kernel - Re: [PATCH] pwm_bl: Fix overflow condition

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Message-ID: <20170830141309.GB1361@ulmo>
Date:   Wed, 30 Aug 2017 16:13:09 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     "dbasehore ." <dbasehore@...omium.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, jingoohan1@...il.com,
        Lee Jones <lee.jones@...aro.org>, linux-pwm@...r.kernel.org
Subject: Re: [PATCH] pwm_bl: Fix overflow condition

On Tue, Aug 29, 2017 at 11:45:43AM -0700, dbasehore . wrote:
> On Tue, Aug 29, 2017 at 7:05 AM, Thierry Reding
> <thierry.reding@...il.com> wrote:
> > On Mon, Aug 28, 2017 at 01:00:33PM -0700, Derek Basehore wrote:
> >> This fixes and overflow condition that happens with a high value of
> >> brightness-levels-scale by using a 64-bit variable. The issue would
> >> prevent a range of higher brightness levels from being set.
> >>
> >> Signed-off-by: Derek Basehore <dbasehore@...omium.org>
> >> ---
> >>  drivers/video/backlight/pwm_bl.c | 7 +++++--
> >>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> >> index 76311ec5e400..e7ffd2108acf 100644
> >> --- a/drivers/video/backlight/pwm_bl.c
> >> +++ b/drivers/video/backlight/pwm_bl.c
> >> @@ -88,14 +88,17 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)
> >>  static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness)
> >>  {
> >>       unsigned int lth = pb->lth_brightness;
> >> -     int duty_cycle;
> >> +     s64 duty_cycle;
> >>
> >>       if (pb->levels)
> >>               duty_cycle = pb->levels[brightness];
> >>       else
> >>               duty_cycle = brightness;
> >>
> >> -     return (duty_cycle * (pb->period - lth) / pb->scale) + lth;
> >> +     duty_cycle *= pb->period - lth;
> >> +     do_div(duty_cycle, pb->scale);
> >> +
> >> +     return duty_cycle + lth;
> >>  }
> >
> > I don't think your commit message accurately describes the change here.
> > The overflow that you're preventing might happen with a large value of
> > pb->period (or rather, in combination with a large value of duty_cycle)
> > but it's unrelated to pb->scale.
> 
> I'm referring to the of property brightness-levels-scale.

I don't see a brightness-levels-scale property in either the DT bindings
or the pwm-backlight driver. Do you have any pointers?

Thierry

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