[PATCH AUTOSEL 6.17] drm/amd/display: Indicate when custom brightness curves are in use

From: Mario Limonciello <superm1@kernel.org>

[ Upstream commit 68f3c044f37d9f50d67417fa8018d9cf16423458 ]

[Why]
There is a `scale` sysfs attribute that can be used to indicate when
non-linear brightness scaling is in use.  As Custom brightness curves
work by linear interpolation of points the scale is no longer linear.

[How]
Indicate non-linear scaling when custom brightness curves in use and
linear scaling otherwise.

Reviewed-by: Alex Hung <redacted>
Signed-off-by: Mario Limonciello <superm1@kernel.org>
Signed-off-by: Wayne Lin <redacted>
Tested-by: Dan Wheeler <redacted>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

YES

- What it changes
  - Sets `backlight_properties.scale` during backlight registration to
    reflect actual brightness scaling:
    - Marks non-linear when custom brightness curves are used:
      drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:5071
    - Marks linear otherwise:
      drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:5073
  - Gate is identical to where custom curves are actually applied
    (`caps->data_points` present and debug mask not set):
    drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:5069

- Why it matters (user-visible bug)
  - The backlight class exposes a sysfs `scale` attribute that reports
    the brightness scale type from `bd->props.scale`:
    drivers/video/backlight/backlight.c:264
  - Without this patch, AMDGPU leaves `props.scale` at its zero-
    initialized default (unknown) due to `props = { 0 }`:
    drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:5041
  - When custom brightness curves are in use, AMDGPU actually performs a
    piecewise linear interpolation of luminance vs. input signal (i.e.,
    non-linear to the user’s scale), see the interpolation path and the
    same debug-mask gate:
    drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:4826
  - Reporting “unknown” is inaccurate and prevents userspace from
    correctly indicating or adapting to non-linear scaling.

- Scope and risk
  - Small, localized change in a single function at device registration
    time; no architectural changes.
  - Does not alter brightness programming, only improves sysfs
    reporting. The backlight core only reads `props.scale` for the
    `scale` sysfs attribute (no behavioral dependency):
    drivers/video/backlight/backlight.c:264
  - Uses established backlight scale enums: `BACKLIGHT_SCALE_LINEAR` and
    `BACKLIGHT_SCALE_NON_LINEAR`: include/linux/backlight.h:83,
    include/linux/backlight.h:91
  - Matches existing pattern in other backlight drivers that already set
    `props.scale`.

- Stable backport criteria
  - Fixes a user-visible correctness issue (sysfs attribute previously
    “unknown” despite known scaling behavior).
  - Minimal risk of regression; confined to AMDGPU backlight
    registration.
  - No new features or ABI additions—just accurate population of an
    existing, stable attribute.

- Note on applicability
  - Targets stable trees that already have `backlight_properties.scale`
    and the `scale` sysfs attribute. For trees lacking these, mechanical
    backporting would need adaptation, but for kernels with the field
    present, this is straightforward and safe.

 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 5 ++++-
 1 file changed, 4 insertions(+), 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 afe3a8279c3a9..8eb2fc4133487 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5032,8 +5032,11 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	} else
 		props.brightness = props.max_brightness = MAX_BACKLIGHT_LEVEL;
 
-	if (caps->data_points && !(amdgpu_dc_debug_mask & DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE))
+	if (caps->data_points && !(amdgpu_dc_debug_mask & DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE)) {
 		drm_info(drm, "Using custom brightness curve\n");
+		props.scale = BACKLIGHT_SCALE_NON_LINEAR;
+	} else
+		props.scale = BACKLIGHT_SCALE_LINEAR;
 	props.type = BACKLIGHT_RAW;
 
 	snprintf(bl_name, sizeof(bl_name), "amdgpu_bl%d",

-- 
2.51.0