linux-kernel - [PATCH v2] drm/amd/display: Initialise backlight level values from hw

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Message-Id: <20260112152856.2616532-1-vivek@collabora.com>
Date: Mon, 12 Jan 2026 15:28:56 +0000
From: Vivek Das Mohapatra <vivek@...labora.com>
To: superm1@...nel.org
Cc: airlied@...il.com,
	alexander.deucher@....com,
	amd-gfx@...ts.freedesktop.org,
	christian.koenig@....com,
	dri-devel@...ts.freedesktop.org,
	harry.wentland@....com,
	kernel@...labora.com,
	linux-kernel@...r.kernel.org,
	simona@...ll.ch,
	siqueira@...lia.com,
	sunpeng.li@....com,
	vivek@...labora.com
Subject: [PATCH v2] drm/amd/display: Initialise backlight level values from hw

Internal backlight levels are initialised from ACPI but the values
are sometimes out of sync with the levels in effect until there has
been a read from hardware (eg triggered by reading from sysfs).

This means that the first drm_commit can cause the levels to be set
to a different value than the actual starting one, which results in
a sudden change in brightness.

This path shows the problem (when the values are out of sync):

   amdgpu_dm_atomic_commit_tail()
   -> amdgpu_dm_commit_streams()
   -> amdgpu_dm_backlight_set_level(..., dm->brightness[n])

This patch calls the backlight ops get_brightness explicitly
at the end of backlight registration to make sure dm->brightness[n]
is in sync with the actual hardware levels.

Signed-off-by: Vivek Das Mohapatra <vivek@...labora.com>
---
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c  | 18 +++++++++++++++++-
 1 file changed, 17 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 354e359c4507..9e8cbfeee915 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -5258,6 +5258,8 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	struct amdgpu_dm_backlight_caps *caps;
 	char bl_name[16];
 	int min, max;
+	int real_brightness;
+	int init_brightness;
 
 	if (aconnector->bl_idx == -1)
 		return;
@@ -5282,6 +5284,8 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	} else
 		props.brightness = props.max_brightness = MAX_BACKLIGHT_LEVEL;
 
+	init_brightness = props.brightness;
+
 	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;
@@ -5300,8 +5304,20 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	if (IS_ERR(dm->backlight_dev[aconnector->bl_idx])) {
 		drm_err(drm, "DM: Backlight registration failed!\n");
 		dm->backlight_dev[aconnector->bl_idx] = NULL;
-	} else
+	} else {
+		/*
+		 * dm->brightness[x] can be inconsistent just after startup until
+		 * ops.get_brightness is called.
+		 */
+		real_brightness =
+			amdgpu_dm_backlight_ops.get_brightness(dm->backlight_dev[aconnector->bl_idx]);
+
+		if (real_brightness != init_brightness) {
+			dm->actual_brightness[aconnector->bl_idx] = real_brightness;
+			dm->brightness[aconnector->bl_idx] = real_brightness;
+		}
 		drm_dbg_driver(drm, "DM: Registered Backlight device: %s\n", bl_name);
+	}
 }
 
 static int initialize_plane(struct amdgpu_display_manager *dm,
-- 
2.39.5