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From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Fri, 19 Mar 2021 10:22:12 -0400
Message-ID: <CADnq5_OguuMsqT7MVC=ieNZm9mqyVUsGpQDHr59BWtBJJUvFoA@mail.gmail.com>
Subject: Re: [PATCH] drm/amd/display: Set AMDGPU_DM_DEFAULT_MIN_BACKLIGHT to 0
To:     Evan Benn <evanbenn@chromium.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Stylon Wang <stylon.wang@amd.com>,
        Eryk Brol <eryk.brol@amd.com>, David Airlie <airlied@linux.ie>,
        Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>,
        Leo Li <sunpeng.li@amd.com>,
        Aurabindo Pillai <aurabindo.pillai@amd.com>,
        Maling list - DRI developers 
        <dri-devel@lists.freedesktop.org>,
        Alex Deucher <alexander.deucher@amd.com>,
        Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        Anand <amistry@chromium.org>
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On Fri, Mar 19, 2021 at 3:23 AM Evan Benn <evanbenn@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@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?

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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