Flaky brightness on Renoir

[Alexander Monakov]

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,