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

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Date:   Tue, 29 Aug 2017 11:45:43 -0700
From:   "dbasehore ." <dbasehore@...omium.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, jingoohan1@...il.com,
        Lee Jones <lee.jones@...aro.org>, linux-pwm@...r.kernel.org
Subject: Re: [PATCH] pwm_bl: Fix overflow condition

On Tue, Aug 29, 2017 at 7:05 AM, Thierry Reding
<thierry.reding@...il.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@...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.

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