* [PATCH] pwm_bl: Fix overflow condition
@ 2017-08-28 20:00 Derek Basehore
2017-08-29 14:05 ` Thierry Reding
0 siblings, 1 reply; 6+ messages in thread
From: Derek Basehore @ 2017-08-28 20:00 UTC (permalink / raw)
To: linux-kernel
Cc: thierry.reding, jingoohan1, lee.jones, linux-pwm, Derek Basehore
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;
}
static int pwm_backlight_update_status(struct backlight_device *bl)
--
2.14.1.342.g6490525c54-goog
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH] pwm_bl: Fix overflow condition
2017-08-28 20:00 [PATCH] pwm_bl: Fix overflow condition Derek Basehore
@ 2017-08-29 14:05 ` Thierry Reding
2017-08-29 18:45 ` dbasehore .
0 siblings, 1 reply; 6+ messages in thread
From: Thierry Reding @ 2017-08-29 14:05 UTC (permalink / raw)
To: Derek Basehore; +Cc: linux-kernel, jingoohan1, lee.jones, linux-pwm
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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
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] pwm_bl: Fix overflow condition
2017-08-29 14:05 ` Thierry Reding
@ 2017-08-29 18:45 ` dbasehore .
2017-08-30 14:13 ` Thierry Reding
0 siblings, 1 reply; 6+ messages in thread
From: dbasehore . @ 2017-08-29 18:45 UTC (permalink / raw)
To: Thierry Reding; +Cc: linux-kernel, jingoohan1, Lee Jones, linux-pwm
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
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] pwm_bl: Fix overflow condition
2017-08-29 18:45 ` dbasehore .
@ 2017-08-30 14:13 ` Thierry Reding
0 siblings, 0 replies; 6+ messages in thread
From: Thierry Reding @ 2017-08-30 14:13 UTC (permalink / raw)
To: dbasehore .; +Cc: linux-kernel, jingoohan1, Lee Jones, linux-pwm
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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
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^ permalink raw reply [flat|nested] 6+ messages in thread
* [PATCH] pwm_bl: Fix overflow condition
@ 2017-08-29 20:27 Derek Basehore
2017-08-29 20:34 ` dbasehore .
0 siblings, 1 reply; 6+ messages in thread
From: Derek Basehore @ 2017-08-29 20:27 UTC (permalink / raw)
To: linux-kernel
Cc: thierry.reding, jingoohan1, lee.jones, briannorris, linux-pwm,
Derek Basehore
This fixes an overflow condition that can happen with high max
brightness and period values in compute_duty_cycle. This fixes it by
using a 64 bit variable for computing the duty cycle.
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..5ccc8e34b7a6 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;
+ u64 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)
--
2.14.1.342.g6490525c54-goog
^ permalink raw reply related [flat|nested] 6+ messages in thread
end of thread, other threads:[~2017-08-30 14:13 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2017-08-28 20:00 [PATCH] pwm_bl: Fix overflow condition Derek Basehore
2017-08-29 14:05 ` Thierry Reding
2017-08-29 18:45 ` dbasehore .
2017-08-30 14:13 ` Thierry Reding
2017-08-29 20:27 Derek Basehore
2017-08-29 20:34 ` dbasehore .
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