drm/amd/display: Add a backlight module option

There seem devices that don't work with the aux channel backlight
control.  For allowing such users to test with the other backlight
control method, provide a new module option, aux_backlight, to specify
enabling or disabling the aux backport support explicitly.  As
default, the aux support is detected by the hardware capability.

v2: make the backlight option generic in case we add future
backlight types (Alex)

BugLink: https://bugzilla.opensuse.org/show_bug.cgi?id=1180749
BugLink: https://gitlab.freedesktop.org/drm/amd/-/issues/1438Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -181,6 +181,7 @@ extern uint amdgpu_dc_feature_mask;
 extern uint amdgpu_freesync_vid_mode;
 extern uint amdgpu_dc_debug_mask;
 extern uint amdgpu_dm_abm_level;
+extern int amdgpu_backlight;
 extern struct amdgpu_mgpu_info mgpu_info;
 extern int amdgpu_ras_enable;
 extern uint amdgpu_ras_mask;

drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c

@@ -783,6 +783,10 @@ uint amdgpu_dm_abm_level;
 MODULE_PARM_DESC(abmlevel, "ABM level (0 = off (default), 1-4 = backlight reduction level) ");
 module_param_named(abmlevel, amdgpu_dm_abm_level, uint, 0444);
 
+int amdgpu_backlight = -1;
+MODULE_PARM_DESC(backlight, "Backlight control (0 = pwm, 1 = aux, -1 auto (default))");
+module_param_named(backlight, amdgpu_backlight, bint, 0444);
+
 /**
  * DOC: tmz (int)
  * Trusted Memory Zone (TMZ) is a method to protect data being written

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

@@ -2324,6 +2324,11 @@ static void update_connector_ext_caps(struct amdgpu_dm_connector *aconnector)
 	    caps->ext_caps->bits.hdr_aux_backlight_control == 1)
 		caps->aux_support = true;
 
+	if (amdgpu_backlight == 0)
+		caps->aux_support = false;
+	else if (amdgpu_backlight == 1)
+		caps->aux_support = true;
+
 	/* From the specification (CTA-861-G), for calculating the maximum
 	 * luminance we need to use:
 	 *	Luminance = 50*2**(CV/32)