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

[Joe Konno <joe.konno@linux.intel.com>]

Many thanks, Chris. I'll incorporate your feedback into v2.

On 09/23/2014 11:04 PM, Chris Wilson wrote:
> On Tue, Sep 23, 2014 at 03:50:25PM -0700, Joe Konno wrote:
>> From: Joe Konno <joe.konno@intel.com>
>>
>> Improper integer division-- truncated rather than rounded-- in the
> That's confusing. It reads that you are converting to using truncated,
> rather than to rounding.
>
>> 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
>>
>> Signed-off-by: Joe Konno <joe.konno@intel.com>
>> ---
>>  drivers/gpu/drm/i915/intel_panel.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
>> index f17ada3..c40f837 100644
>> --- a/drivers/gpu/drm/i915/intel_panel.c
>> +++ b/drivers/gpu/drm/i915/intel_panel.c
>> @@ -421,7 +421,8 @@ 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 = (target_val + ((source_max - source_min) / 2)) /
>> +		(source_max - source_min);
> target_val = DIV_ROUND_CLOSEST(target_val, source_max - source_min);
>
>>  	target_val += target_min;
>>  
>>  	return target_val;