linux-kernel - [PATCH] drm/amd/display: Set AMDGPU_DM_DEFAULT_MIN

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Date:   Fri, 19 Mar 2021 16:44:33 +1100
From:   Evan Benn <evanbenn@...omium.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Anand <amistry@...omium.org>, Evan Benn <evanbenn@...omium.org>,
        Alex Deucher <alexander.deucher@....com>,
        Aurabindo Pillai <aurabindo.pillai@....com>,
        Bas Nieuwenhuizen <bas@...nieuwenhuizen.nl>,
        Bhawanpreet Lakha <Bhawanpreet.Lakha@....com>,
        Christian König <christian.koenig@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>, Eryk Brol <eryk.brol@....com>,
        Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Nicholas Kazlauskas <nicholas.kazlauskas@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        Simon Ser <contact@...rsion.fr>,
        Stylon Wang <stylon.wang@....com>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: [PATCH] drm/amd/display: Set AMDGPU_DM_DEFAULT_MIN_BACKLIGHT to 0

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@...omium.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.

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