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 <[email protected]> > --- > 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. Also, the semantics of do_div() are that it takes an unsigned dividend, so your duty_cycle should be a u64. Thierry
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