Flaky brightness on Renoir

[Harry Wentland]

We've seen similar problems internally.

Michael, does this "fix" your issue?

Anthony, looks like smooth_brightness is problematic on (some) renoir
systems. Thoughts?

Thanks,
Harry

On 2020-05-20 5:47 p.m., Alex Deucher wrote:
> Adding some display people.
> 
> On Wed, May 20, 2020 at 5:46 PM Alexander Monakov <amonakov at ispras.ru> wrote:
>>
>> Hello,
>>
>> I have a laptop with the recent Renoir SoC. Screen brightness is
>> controlled via the amdgpu driver. Unfortunately it doesn't work
>> properly: brightness doesn't go below a certain threshold. In one
>> experiment I've found the threshold to be about 95 (of 255), which is
>> quite high.
>>
>> There's a bugreport on the tracker from a person with the same laptop
>> model as mine that says that the threshold may vary from boot to boot.
>>
>> So far I was able to find a workaround: avoiding
>> dmcu_set_backlight_level like in the patch below gives more reliable
>> backlight control (but at the expense of breaking "actual_brightness"
>> sysfs file, because it reads from DMCU registers).
>>
>> What might be the problem and can I help investigate this further? Would
>> really like to see this work properly.
>>
>> Alexander
>>
>> diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_abm.c b/drivers/gpu/drm/amd/display/dc/dce/dce_abm.c
>> index b8a3fc505c9b..3274b0d15893 100644
>> --- a/drivers/gpu/drm/amd/display/dc/dce/dce_abm.c
>> +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_abm.c
>> @@ -428,7 +428,7 @@ static bool dce_abm_set_backlight_level_pwm(
>>                         backlight_pwm_u16_16, backlight_pwm_u16_16);
>>
>>         /* If DMCU is in reset state, DMCU is uninitialized */
>> -       if (use_smooth_brightness)
>> +       if (0 && use_smooth_brightness)
>>                 dmcu_set_backlight_level(abm_dce,
>>                                 backlight_pwm_u16_16,
>>                                 frame_ramp,
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