diff -pNaru5 driver/2.6.27.orig/dgrp_tty.c driver/2.6.27/dgrp_tty.c
--- driver/2.6.27.orig/dgrp_tty.c	2020-01-28 12:01:10.000000000 -0500
+++ driver/2.6.27/dgrp_tty.c	2021-08-10 14:00:21.627782755 -0400
@@ -3246,10 +3246,13 @@ static int dgrp_tty_digisetedelay(struct
 	drp_param(ch);
 
 	return 0;
 }
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,13,0)
+int tty_check_change(struct tty_struct *tty);
+#endif
 
 /*
  *	The usual assortment of ioctl's
  *
  *	note:  use tty_check_change to make sure that we are not