drm/amd/display: Only read ACPI backlight caps once

JIRA: https://issues.redhat.com/browse/RHEL-75958

commit cd711c87c2862be5e71eee79901f94e1c943f9fc
Author: Mario Limonciello <mario.limonciello@amd.com>
Date:   Thu May 29 11:33:44 2025 -0500

    drm/amd/display: Only read ACPI backlight caps once

    commit ffcaed1 upstream.

    [WHY]
    Backlight caps are read already in amdgpu_dm_update_backlight_caps().
    They may be updated by update_connector_ext_caps(). Reading again when
    registering backlight device may cause wrong values to be used.

    [HOW]
    Use backlight caps already registered to the dm.

    Cc: Mario Limonciello <mario.limonciello@amd.com>
    Cc: Alex Deucher <alexander.deucher@amd.com>
    Reviewed-by: Roman Li <roman.li@amd.com>
    Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
    Signed-off-by: Alex Hung <alex.hung@amd.com>
    Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    (cherry picked from commit 148144f)
    Cc: stable@vger.kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: José Expósito <jexposit@redhat.com>

Author: jexposit

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

@@ -4832,7 +4832,7 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	struct drm_device *drm = aconnector->base.dev;
 	struct amdgpu_display_manager *dm = &drm_to_adev(drm)->dm;
 	struct backlight_properties props = { 0 };
-	struct amdgpu_dm_backlight_caps caps = { 0 };
+	struct amdgpu_dm_backlight_caps *caps;
 	char bl_name[16];
 	int min, max;
 
@@ -4846,20 +4846,20 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 		return;
 	}
 
-	amdgpu_acpi_get_backlight_caps(&caps);
-	if (caps.caps_valid && get_brightness_range(&caps, &min, &max)) {
+	caps = &dm->backlight_caps[aconnector->bl_idx];
+	if (get_brightness_range(caps, &min, &max)) {
 		if (power_supply_is_system_supplied() > 0)
-			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps.ac_level, 100);
+			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->ac_level, 100);
 		else
-			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps.dc_level, 100);
+			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->dc_level, 100);
 		/* min is zero, so max needs to be adjusted */
 		props.max_brightness = max - min;
 		drm_dbg(drm, "Backlight caps: min: %d, max: %d, ac %d, dc %d\n", min, max,
-			caps.ac_level, caps.dc_level);
+			caps->ac_level, caps->dc_level);
 	} else
 		props.brightness = AMDGPU_MAX_BL_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.max_brightness = AMDGPU_MAX_BL_LEVEL;
 	props.type = BACKLIGHT_RAW;