Flaky brightness on Renoir

[Alex Deucher]

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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