[v2] pwm_bl: Fix overflow condition

Message ID 20170829203434.6826-1-dbasehore@chromium.org
State New
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Series
  • [v2] pwm_bl: Fix overflow condition
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Commit Message

dbasehore . Aug. 29, 2017, 8:34 p.m.
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(-)

Comments

Brian Norris Aug. 29, 2017, 8:46 p.m. | #1
On Tue, Aug 29, 2017 at 01:34:34PM -0700, Derek Basehore wrote:
> 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>
> ---

I believe this is the correct v2 patch, and I agree that Derek is
confused by his mail client :)

Reviewed-by: Brian Norris <briannorris@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
> 
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Thierry Reding Aug. 30, 2017, 2:27 p.m. | #2
On Tue, Aug 29, 2017 at 01:34:34PM -0700, Derek Basehore wrote:
> 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(-)

I'm still confused by the brightness-levels-scale comment in the other
thread, but this looks like a good fix for a potential overflow so:

Acked-by: Thierry Reding <thierry.reding@gmail.com>
Brian Norris Aug. 30, 2017, 4:26 p.m. | #3
On Wed, Aug 30, 2017 at 04:27:41PM +0200, Thierry Reding wrote:
> I'm still confused by the brightness-levels-scale comment in the other
> thread,

I believe Derek was accidentally referring to a change that's only in
our downstream tree.

> but this looks like a good fix for a potential overflow so:

Right, it's still an applicable overflow without our downstream changes.

Brian

> Acked-by: Thierry Reding <thierry.reding@gmail.com>
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Lee Jones Sept. 4, 2017, 3:16 p.m. | #4
On Tue, 29 Aug 2017, Derek Basehore wrote:

> 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(-)

Applied for v4.15, thanks.

Patch

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)