drm/amd/display: Fix DSC not Enabled on Direct MST Sink

[WHY & HOW]
For the scenario when a dsc capable MST sink device is directly
connected, it needs to use max dsc compression as the link bw constraint.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Roman Li <roman.li@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Fangzhi Zuo

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

@@ -1604,31 +1604,31 @@ enum dc_status dm_dp_mst_is_port_support_mode(
 	unsigned int upper_link_bw_in_kbps = 0, down_link_bw_in_kbps = 0;
 	unsigned int max_compressed_bw_in_kbps = 0;
 	struct dc_dsc_bw_range bw_range = {0};
-	struct drm_dp_mst_topology_mgr *mst_mgr;
+	uint16_t full_pbn = aconnector->mst_output_port->full_pbn;
 
 	/*
-	 * check if the mode could be supported if DSC pass-through is supported
-	 * AND check if there enough bandwidth available to support the mode
-	 * with DSC enabled.
+	 * Consider the case with the depth of the mst topology tree is equal or less than 2
+	 * A. When dsc bitstream can be transmitted along the entire path
+	 *    1. dsc is possible between source and branch/leaf device (common dsc params is possible), AND
+	 *    2. dsc passthrough supported at MST branch, or
+	 *    3. dsc decoding supported at leaf MST device
+	 *    Use maximum dsc compression as bw constraint
+	 * B. When dsc bitstream cannot be transmitted along the entire path
+	 *    Use native bw as bw constraint
 	 */
 	if (is_dsc_common_config_possible(stream, &bw_range) &&
-	    aconnector->mst_output_port->passthrough_aux) {
-		mst_mgr = aconnector->mst_output_port->mgr;
-		mutex_lock(&mst_mgr->lock);
-
+	   (aconnector->mst_output_port->passthrough_aux ||
+	    aconnector->dsc_aux == &aconnector->mst_output_port->aux)) {
 		cur_link_settings = stream->link->verified_link_cap;
 
 		upper_link_bw_in_kbps = dc_link_bandwidth_kbps(aconnector->dc_link,
-							       &cur_link_settings
-							       );
-		down_link_bw_in_kbps = kbps_from_pbn(aconnector->mst_output_port->full_pbn);
+							       &cur_link_settings);
+		down_link_bw_in_kbps = kbps_from_pbn(full_pbn);
 
 		/* pick the bottleneck */
 		end_to_end_bw_in_kbps = min(upper_link_bw_in_kbps,
 					    down_link_bw_in_kbps);
 
-		mutex_unlock(&mst_mgr->lock);
-
 		/*
 		 * use the maximum dsc compression bandwidth as the required
 		 * bandwidth for the mode
@@ -1643,8 +1643,7 @@ enum dc_status dm_dp_mst_is_port_support_mode(
 		/* check if mode could be supported within full_pbn */
 		bpp = convert_dc_color_depth_into_bpc(stream->timing.display_color_depth) * 3;
 		pbn = drm_dp_calc_pbn_mode(stream->timing.pix_clk_100hz / 10, bpp, false);
-
-		if (pbn > aconnector->mst_output_port->full_pbn)
+		if (pbn > full_pbn)
 			return DC_FAIL_BANDWIDTH_VALIDATE;
 	}