drm/amd/display: Add debugging message for brightness caps

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

commit 66089fa8c9ed162744037ab0375e38cc74c7f7ed
Author: Mario Limonciello <mario.limonciello@amd.com>
Date:   Wed May 14 16:00:43 2025 -0500

    drm/amd/display: Add debugging message for brightness caps

    commit 4b61b8a upstream.

    [Why]
    Default BIOS brightness caps are buried in ACPI.

    [How]
    Add extra dynamic debug that can show default brightness caps.

    Reviewed-by: Alex Hung <alex.hung@amd.com>
    Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
    Signed-off-by: Wayne Lin <wayne.lin@amd.com>
    Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    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

@@ -4834,6 +4834,7 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	struct backlight_properties props = { 0 };
 	struct amdgpu_dm_backlight_caps caps = { 0 };
 	char bl_name[16];
+	int min, max;
 
 	if (aconnector->bl_idx == -1)
 		return;
@@ -4846,11 +4847,15 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	}
 
 	amdgpu_acpi_get_backlight_caps(&caps);
-	if (caps.caps_valid) {
+	if (caps.caps_valid && get_brightness_range(&caps, &min, &max)) {
 		if (power_supply_is_system_supplied() > 0)
 			props.brightness = caps.ac_level;
 		else
 			props.brightness = caps.dc_level;
+		/* 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);
 	} else
 		props.brightness = AMDGPU_MAX_BL_LEVEL;