diff --git a/drivers/s390/cio/idset.h b/drivers/s390/cio/idset.h
index ca1398a..7543da4 100644
--- a/drivers/s390/cio/idset.h
+++ b/drivers/s390/cio/idset.h
@@ -22,5 +22,6 @@
 int idset_sch_contains(struct idset *set, struct subchannel_id id);
 int idset_sch_get_first(struct idset *set, struct subchannel_id *id);
 int idset_is_empty(struct idset *set);
+void idset_add_set(struct idset *to, struct idset *from);
 
 #endif /* S390_IDSET_H */