diff
-void drm_client_register(struct drm_client_dev *client)
+int drm_client_register(struct drm_client_dev *client)
 {
+    if (!client || !client->dev)
+        return -EINVAL;
     struct drm_device *dev = client->dev;
     int ret;
 
     mutex_lock(&dev->clientlist_mutex);
     // ... rest of the function
+    return 0;
 }
