| Message ID | 20170828200033.40673-1-dbasehore@chromium.org |
|---|---|
| State | Superseded |
| Headers | show |
| Series | pwm_bl: Fix overflow condition | expand |
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@chromium.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. Also, the semantics of do_div() are that it takes an unsigned dividend, so your duty_cycle should be a u64. Thierry
On Tue, Aug 29, 2017 at 7:05 AM, Thierry Reding <thierry.reding@gmail.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@chromium.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. If there aren't levels defined in a DTS, duty_cycle can be up to this value. I'll change the CM to describe what's happening based on the variable names from the function instead. > > Also, the semantics of do_div() are that it takes an unsigned dividend, > so your duty_cycle should be a u64. I'll change it. > > Thierry -- To unsubscribe from this list: send the line "unsubscribe linux-pwm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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@gmail.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@chromium.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
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; } static int pwm_backlight_update_status(struct backlight_device *bl)
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@chromium.org> --- drivers/video/backlight/pwm_bl.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)