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

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Date:   Tue, 29 Aug 2017 16:05:34 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Derek Basehore <dbasehore@...omium.org>
Cc:     linux-kernel@...r.kernel.org, jingoohan1@...il.com,
        lee.jones@...aro.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH] pwm_bl: Fix overflow condition

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.

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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