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

[Jani Nikula <jani.nikula@linux.intel.com>]

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