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

[Harry Wentland]

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.

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.

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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