diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
index 50988b21a21b09b0108e65b479f495ba01b3190b..842814bc0e4f9c4b0bf1b0521be16ffad040a923 100644
--- a/drivers/usb/gadget/udc/core.c
+++ b/drivers/usb/gadget/udc/core.c
@@ -238,6 +238,9 @@ EXPORT_SYMBOL_GPL(usb_ep_free_request);
  * arranges to poll once per interval, and the gadget driver usually will
  * have queued some data to transfer at that time.
  *
+ * Note that @req's ->complete() callback must never be called from
+ * within usb_ep_queue() as that can create deadlock situations.
+ *
  * Returns zero, or a negative error code.  Endpoints that are not enabled
  * report errors; errors will also be
  * reported when the usb peripheral is disconnected.