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

[Alex Deucher]

On Sun, Mar 21, 2021 at 8:12 PM Evan Benn <evanbenn at chromium.org> wrote:
>
> On Sat, Mar 20, 2021 at 8:36 AM Alex Deucher <alexdeucher at gmail.com> wrote:
> >
> > On Fri, Mar 19, 2021 at 5:31 PM Evan Benn <evanbenn at gmail.com> wrote:
> > >
> > > On Sat, 20 Mar 2021 at 02:10, Harry Wentland <harry.wentland at amd.com> wrote:
> > > > On 2021-03-19 10:22 a.m., Alex Deucher wrote:
> > > > > On Fri, Mar 19, 2021 at 3:23 AM Evan Benn <evanbenn at chromium.org> 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 at chromium.org>
> > > > >>
> > > > >> ---
> > > > >> 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.
> > >
> >
> > Maybe the panel string from the EDID?  Either that or something from
> > dmi data?  Harry would probably have a better idea.
>
> One problem with keying from panel EDID is that for example the grunt chromebook
> platform has more than 100 different panels already shipped. Add to that that
> repair centers or people repairing their own device will use 'compatible'
> panels. I'm sure the AMD windows laptops have even more variety!
>

Do all of those "compatible" panels work with the min backlight level
of 0?  If so, maybe something platform specific like a DMI string
would make more sense.

Alex


> >
> > Alex
> >
> > > >
> > > > 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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> > > >