diff --git a/include/linux/input.h b/include/linux/input.h
index a81671453575..2740d080ec6b 100644
--- a/include/linux/input.h
+++ b/include/linux/input.h
@@ -116,6 +116,7 @@ struct input_keymap_entry {
 
 /**
  * EVIOCGMTSLOTS(len) - get MT slot values
+ * @len: size of the data buffer in bytes
  *
  * The ioctl buffer argument should be binary equivalent to
  *