diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index 5a3be16ca93d..d431ea8198e4 100644
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -413,6 +413,16 @@ fail:
 	return ret;
 }
 
+/*
+ * btrfs_csum_one_bio - Calculates checksums of the data contained inside a bio
+ * @inode:	 Owner of the data inside the bio
+ * @bio:	 Contains the data to be checksummed
+ * @file_start:  offset in file this bio begins to describe
+ * @contig:	 Boolean. If true/1 means all bio vecs in this bio are
+ *		 contiguous and they begin at @file_start in the file. False/0
+ *		 means this bio can contains potentially discontigous bio vecs
+ *		 so the logical offset of each should be calculated separately.
+ */
 blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
 		       u64 file_start, int contig)
 {