diff --git a/external/mdds/UnpackedTarball_mdds.mk b/external/mdds/UnpackedTarball_mdds.mk
index 225e5d8938d3..3c664336430b 100644
--- a/external/mdds/UnpackedTarball_mdds.mk
+++ b/external/mdds/UnpackedTarball_mdds.mk
@@ -13,6 +13,9 @@ $(eval $(call gb_UnpackedTarball_set_tarball,mdds,$(MDDS_TARBALL)))
 
 $(eval $(call gb_UnpackedTarball_set_patchlevel,mdds,0))
 
+# * external/mdds/Wunused-but-set-variable.patch upstream at
+#   <https://gitlab.com/mdds/mdds/-/merge_requests/29> "-Werror,-Wunused-but-set-variable (Clang 13
+#   trunk)":
 $(eval $(call gb_UnpackedTarball_add_patches,mdds,\
     external/mdds/Wunused-but-set-variable.patch \
 ))