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 > >> > >> _______________________________________________ > >> dri-devel mailing list > >> dri-devel@xxxxxxxxxxxxxxxxxxxxx > >> https://lists.freedesktop.org/mailman/listinfo/dri-devel>> _______________________________________________ > > dri-devel mailing list > > dri-devel@xxxxxxxxxxxxxxxxxxxxx > > https://lists.freedesktop.org/mailman/listinfo/dri-devel>> > _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
