Re: [PATCH] drm/amd/display: Set AMDGPU_DM_DEFAULT_MIN_BACKLIGHT to 0

From: Harry Wentland
Date: Fri Mar 19 2021 - 11:08:44 EST

On 2021-03-19 10:22 a.m., Alex Deucher wrote:
On Fri, Mar 19, 2021 at 3:23 AM Evan Benn <evanbenn@xxxxxxxxxxxx> wrote:

AMDGPU_DM_DEFAULT_MIN_BACKLIGHT was set to the value of 12
to ensure no display backlight will flicker at low user brightness
settings. However this value is quite bright, so for devices that do not
implement the ACPI ATIF
functionality the user cannot set the brightness to a low level even if
the display would support such a low PWM.

This ATIF feature is not implemented on for example AMD grunt chromebooks.

Signed-off-by: Evan Benn <evanbenn@xxxxxxxxxxxx>

I could not find a justification for the reason for the value. It has
caused some noticable regression for users:>>>
Maybe this can be either user controlled or userspace configured, but
preventing users from turning their backlight dim seems wrong.

My understanding is that some panels flicker if you set the min to a
value too low. This was a safe minimum if the platform didn't specify
it's own safe minimum. I think we'd just be trading one bug for
another (flickering vs not dim enough). Maybe a whitelist or
blacklist would be a better solution?

Yeah, this is a NACK from me as-is for the reasons Alex described.

I agree a whitelist approach might be best.

Is this fix perhaps for OLED panels? If so we could use a different min-value for OLED panels that don't do PWM, but use 12 for everything else.



Also reviewed here:>>>
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 573cf17262da..0129bd69b94e 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -3151,7 +3151,7 @@ static int amdgpu_dm_mode_config_init(struct amdgpu_device *adev)
return 0;



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