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

[Evan Benn]

On Sat, 20 Mar 2021 at 02:10, Harry Wentland <harry.wentland@xxxxxxx> wrote:
> 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
> >> ATIF_FUNCTION_QUERY_BRIGHTNESS_TRANSFER_CHARACTERISTICS
> >> 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: https://bugzilla.kernel.org/show_bug.cgi?id=203439>>>
> >> 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.

Thanks Harry + Alex,

I agree this solution is not the best.

>
> I agree a whitelist approach might be best.

Do you have any idea what an allowlist could be keyed on?
Is the flickering you observed here a function of the panel or the gpu
or some other component?
Maybe we could move the minimum level into the logic for that hardware.

>
> 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.

All the chromebooks I have worked with LCD + LED backlight have been
fine with a backlight set to 0.
We do have OLED panels too, but I'm not aware of what they do.

> Harry
>
> > Alex
> >
> >
> >>
> >> Also reviewed here: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2748377>>>
> >>   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;
> >>   }
> >>
> >> -#define AMDGPU_DM_DEFAULT_MIN_BACKLIGHT 12
> >> +#define AMDGPU_DM_DEFAULT_MIN_BACKLIGHT 0
> >>   #define AMDGPU_DM_DEFAULT_MAX_BACKLIGHT 255
> >>   #define AUX_BL_DEFAULT_TRANSITION_TIME_MS 50
> >>
> >> --
> >> 2.31.0.291.g576ba9dcdaf-goog
> >>
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