diff --git a/include/block/block.h b/include/block/block.h
index cdec3639a3..397b5e8d44 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -205,6 +205,9 @@ enum {
      * This permission (which is weaker than BLK_PERM_WRITE) is both enough and
      * required for writes to the block node when the caller promises that
      * the visible disk content doesn't change.
+     *
+     * As the BLK_PERM_WRITE permission is strictly stronger, either is
+     * sufficient to perform an unchanging write.
      */
     BLK_PERM_WRITE_UNCHANGED    = 0x04,