Re: [PATCH] drm/i915: intel_backlight scale() math WA v2

Jani Nikula
2014-09-24T17:08:20Z

On Wed, 24 Sep 2014, Hans de Goede <hdegoede@redhat.com> wrote:
> Hi,
>
> On 09/24/2014 05:54 PM, Joe Konno wrote:
>> From: Joe Konno <joe.konno@intel.com>
>>
>> Improper truncated integer division in the scale() function causes
>> actual_brightness != brightness. This (partial) work-around should be
>> sufficient for a majority of use-cases, but it is by no means a complete
>> solution.
>>
>> TODO: Determine how best to scale "user" values to "hw" values, and
>> vice-versa, when the ranges are of different sizes. That would be a
>> buggy scenario even with this work-around.
>>
>> The issue was introduced in the following (v3.17-rc1) commit:
>>
>> 6dda730 drm/i915: respect the VBT minimum backlight brightness
>>
>> v2: (thanks to Chris Wilson) clarify commit message, use rounded division
>> macro
>
> I wonder why do scaling at all, why not simply shift hw_min - hw_max range
> to 0 - (hw_max - hw_min) range and set max_brightness as seen by userspace
> to (hw_max - hw_min) ?

Mostly in preparation for a future where we expose an arbitrary range,
say 0..100 or 0..255 to the userspace.

BR,
Jani.

>
> Regards,
>
> Hans
>
>
>>
>> Signed-off-by: Joe Konno <joe.konno@intel.com>
>> ---
>> drivers/gpu/drm/i915/intel_panel.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
>> index f17ada3..dcdfbb3 100644
>> --- a/drivers/gpu/drm/i915/intel_panel.c
>> +++ b/drivers/gpu/drm/i915/intel_panel.c
>> @@ -421,7 +421,7 @@ static uint32_t scale(uint32_t source_val,
>> /* avoid overflows */
>> target_val = (uint64_t)(source_val - source_min) *
>> (target_max - target_min);
>> - do_div(target_val, source_max - source_min);
>> + target_val = DIV_ROUND_CLOSEST(target_val, source_max - source_min);
>> target_val += target_min;
>>
>> return target_val;
>>
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--
Jani Nikula, Intel Open Source Technology Center