drm/amd/display: Detect dpcd_rev when hotplug mst monitor

[Why]
Once mst topology is constructed, later on new connected monitors
are reported to source by CSN message. Within CSN, there is no
carried info of DPCD_REV comparing to LINK_ADDRESS reply. As the
result, we might leave some ports connected to DP but without DPCD
revision number which will affect us determining the capability of
the DP Rx.

[How]
Send out remote DPCD read when the port's dpcd_rev is 0x0 in
detect_ctx(). Firstly, read out the value from DPCD 0x2200. If the
return value is 0x0, it's likely the DP1.2 DP Rx then we reques
revision from DPCD 0x0 again.
Reviewed-by: Hersen Wu <hersenwu@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@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>

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

@@ -344,12 +344,48 @@ dm_dp_mst_detect(struct drm_connector *connector,
 {
 	struct amdgpu_dm_connector *aconnector = to_amdgpu_dm_connector(connector);
 	struct amdgpu_dm_connector *master = aconnector->mst_port;
+	struct drm_dp_mst_port *port = aconnector->port;
+	int connection_status;
 
 	if (drm_connector_is_unregistered(connector))
 		return connector_status_disconnected;
 
-	return drm_dp_mst_detect_port(connector, ctx, &master->mst_mgr,
+	connection_status = drm_dp_mst_detect_port(connector, ctx, &master->mst_mgr,
 				      aconnector->port);
+
+	if (port->pdt != DP_PEER_DEVICE_NONE && !port->dpcd_rev) {
+		uint8_t dpcd_rev;
+		int ret;
+
+		ret = drm_dp_dpcd_readb(&port->aux, DP_DP13_DPCD_REV, &dpcd_rev);
+
+		if (ret == 1) {
+			port->dpcd_rev = dpcd_rev;
+
+			/* Could be DP1.2 DP Rx case*/
+			if (!dpcd_rev) {
+				ret = drm_dp_dpcd_readb(&port->aux, DP_DPCD_REV, &dpcd_rev);
+
+				if (ret == 1)
+					port->dpcd_rev = dpcd_rev;
+			}
+
+			if (!dpcd_rev)
+				DRM_DEBUG_KMS("Can't decide DPCD revision number!");
+		}
+
+		/*
+		 * Could be legacy sink, logical port etc on DP1.2.
+		 * Will get Nack under these cases when issue remote
+		 * DPCD read.
+		 */
+		if (ret != 1)
+			DRM_DEBUG_KMS("Can't access DPCD");
+	} else if (port->pdt == DP_PEER_DEVICE_NONE) {
+		port->dpcd_rev = 0;
+	}
+
+	return connection_status;
 }
 
 static int dm_dp_mst_atomic_check(struct drm_connector *connector,