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.

Fixes: 2fe87f5 ("drm/amd/display: Set default brightness according to ACPI")
Signed-off-by: Vivek Das Mohapatra <vivek@collabora.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Vivek Das Mohapatra

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5279,6 +5279,8 @@ 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;
@@ -5303,6 +5305,8 @@ 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;
@@ -5321,8 +5325,20 @@ 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,