diff --git a/tools/mkimage.h b/tools/mkimage.h
index 950e190..ecb3032 100644
--- a/tools/mkimage.h
+++ b/tools/mkimage.h
@@ -132,7 +132,10 @@
 	/*
 	 * This callback function will be executed for variable size record
 	 * It is expected to build this header in memory and return its length
-	 * and a pointer to it
+	 * and a pointer to it by using image_type_params.header_size and
+	 * image_type_params.hdr. The return value shall indicate if an
+	 * additional padding should be used when copying the data image
+	 * by returning the padding length.
 	 */
 	int (*vrec_header) (struct mkimage_params *,
 		struct image_type_params *);