diff --git a/include/linux/usb.h b/include/linux/usb.h
index 7454865..512ab16 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1264,6 +1264,8 @@
  * @sg: scatter gather buffer list, the buffer size of each element in
  * 	the list (except the last) must be divisible by the endpoint's
  * 	max packet size if no_sg_constraint isn't set in 'struct usb_bus'
+ * 	(FIXME: scatter-gather under xHCI is broken for periodic transfers.
+ * 	Do not use urb->sg for interrupt endpoints for now, only bulk.)
  * @num_mapped_sgs: (internal) number of mapped sg entries
  * @num_sgs: number of entries in the sg list
  * @transfer_buffer_length: How big is transfer_buffer.  The transfer may