drm/dp/mst: make sure mst_primary mstb is valid in work function

commit 9254ec49 upstream.

This validates the mst_primary under the lock, and then calls
into the check and send function. This makes the code a lot
easier to understand the locking rules in.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -1185,7 +1185,7 @@ static void drm_dp_check_and_send_link_address(struct drm_dp_mst_topology_mgr *m
 					       struct drm_dp_mst_branch *mstb)
 {
 	struct drm_dp_mst_port *port;
-
+	struct drm_dp_mst_branch *mstb_child;
 	if (!mstb->link_address_sent) {
 		drm_dp_send_link_address(mgr, mstb);
 		mstb->link_address_sent = true;
@@ -1200,17 +1200,31 @@ static void drm_dp_check_and_send_link_address(struct drm_dp_mst_topology_mgr *m
 		if (!port->available_pbn)
 			drm_dp_send_enum_path_resources(mgr, mstb, port);
 
-		if (port->mstb)
-			drm_dp_check_and_send_link_address(mgr, port->mstb);
+		if (port->mstb) {
+			mstb_child = drm_dp_get_validated_mstb_ref(mgr, port->mstb);
+			if (mstb_child) {
+				drm_dp_check_and_send_link_address(mgr, mstb_child);
+				drm_dp_put_mst_branch_device(mstb_child);
+			}
+		}
 	}
 }
 
 static void drm_dp_mst_link_probe_work(struct work_struct *work)
 {
 	struct drm_dp_mst_topology_mgr *mgr = container_of(work, struct drm_dp_mst_topology_mgr, work);
+	struct drm_dp_mst_branch *mstb;
 
-	drm_dp_check_and_send_link_address(mgr, mgr->mst_primary);
-
+	mutex_lock(&mgr->lock);
+	mstb = mgr->mst_primary;
+	if (mstb) {
+		kref_get(&mstb->kref);
+	}
+	mutex_unlock(&mgr->lock);
+	if (mstb) {
+		drm_dp_check_and_send_link_address(mgr, mstb);
+		drm_dp_put_mst_branch_device(mstb);
+	}
 }
 
 static bool drm_dp_validate_guid(struct drm_dp_mst_topology_mgr *mgr,