linux-kernel - [PATCH v1] drm/amd/display: Initialise backlight level values from hw

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Message-Id: <20260112122140.2609679-1-vivek@collabora.com>
Date: Mon, 12 Jan 2026 12:20:46 +0000
From: Vivek Das Mohapatra <vivek@...labora.com>
To: Harry Wentland <harry.wentland@....com>,
	Leo Li <sunpeng.li@....com>,
	Rodrigo Siqueira <siqueira@...lia.com>,
	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>
Cc: kernel@...labora.com,
	amd-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v1] 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    | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

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..50f0547ed63c 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;
@@ -5297,6 +5301,18 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 					  &amdgpu_dm_backlight_ops, &props);
 	dm->brightness[aconnector->bl_idx] = props.brightness;
 
+	/*
+	 * 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;
+	}
+
 	if (IS_ERR(dm->backlight_dev[aconnector->bl_idx])) {
 		drm_err(drm, "DM: Backlight registration failed!\n");
 		dm->backlight_dev[aconnector->bl_idx] = NULL;
-- 
2.39.5