/usr/src/sys/dev/gpib/mt.c:626:7: error: this statement may fall through [-Werror=implicit-fallthrough=]
    if ((sc->sc_stat1 & SR1_BOT) ||
       ^
/usr/src/sys/dev/gpib/mt.c:631:7: note: here
       case -2:
       ^~~~
/usr/src/sys/dev/gpib/mt.c:637:7: error: this statement may fall through [-Werror=implicit-fallthrough=]
    if (sc->sc_flags & (MTF_DSJTIMEO | MTF_STATTIMEO)) {
       ^
/usr/src/sys/dev/gpib/mt.c:642:7: note: here
       case 2:
       ^~~~
cc1: all warnings being treated as errors

Index: sys/dev/gpib/mt.c
===================================================================
RCS file: /cvsroot/src/sys/dev/gpib/mt.c,v
retrieving revision 1.31
diff -u -r1.31 mt.c
--- sys/dev/gpib/mt.c	28 Oct 2017 04:53:56 -0000	1.31
+++ sys/dev/gpib/mt.c	23 Feb 2019 11:47:00 -0000
@@ -628,6 +628,7 @@
 				sc->sc_flags &= ~MTF_REW;
 				break;
 			}
+			/* FALLTHROUGH */
 		    case -2:
 			/*
 			 * -2 means "timeout" reading DSJ, which is probably
@@ -639,6 +640,7 @@
 				    mtstart_callout, sc);
 				return;
 			}
+			/* FALLTHROUGH */
 		    case 2:
 			if (bp->b_cmd != MTNOP || !(bp->b_flags & B_CMD)) {
 				bp->b_error = EBUSY;