diff --git a/src/serial.c b/src/serial.c
index 8ddd38e..1b5a4cf 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -5,6 +5,8 @@
  *
  * All Rights Reserved
  *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation; either version 2 of the License, or