[PATCH] block: Document the bio splitting functions

[Bart Van Assche <bvanassche@acm.org>]

Since what the bio splitting functions do is nontrivial, document these
functions.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-merge.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 1ea00da12ca3..038eaee4438a 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -192,6 +192,23 @@ static bool bvec_split_segs(const struct request_queue *q,
 	return !!len;
 }
 
+/**
+ * blk_bio_segment_split - split a bio in two bios
+ * @q:    [in] request queue pointer
+ * @bio:  [in] bio to be split
+ * @bs:	  [in] bio set to allocate the clone from
+ * @segs: [out] number of segments in the bio with the first half of the sectors
+ *
+ * Clones @bio, updates the bi_iter of the clone to represent the first sectors
+ * of @bio and updates @bio->bi_iter to represent the remaining sectors. The
+ * following is guaranteed for the cloned bio:
+ * - That it has at most get_max_io_size(@q, @bio) sectors.
+ * - That it has at most queue_max_segments(@q) segments.
+ *
+ * Except for discard requests the cloned bio will point at the bi_io_vec of
+ * the original bio. It is the responsibility of the caller to ensure that the
+ * original bio is not freed before the cloned bio.
+ */
 static struct bio *blk_bio_segment_split(struct request_queue *q,
 					 struct bio *bio,
 					 struct bio_set *bs,
@@ -248,6 +265,16 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 	return bio_split(bio, sectors, GFP_NOIO, bs);
 }
 
+/**
+ * __blk_queue_split - split a bio and submit the second half
+ * @q:       [in] request queue pointer
+ * @bio:     [in, out] bio to be split
+ * @nr_segs: [out] number of segments in the first bio
+ *
+ * Splits a bio into two bios, chains the two bios, submits the second half
+ * and stores a pointer to the first half in *@bio. If the second bio is still
+ * too big it will be split by a recursive call to this function.
+ */
 void __blk_queue_split(struct request_queue *q, struct bio **bio,
 		unsigned int *nr_segs)
 {
@@ -292,6 +319,14 @@ void __blk_queue_split(struct request_queue *q, struct bio **bio,
 	}
 }
 
+/**
+ * blk_queue_split - split a bio and submit the second half
+ * @q:   [in] request queue pointer
+ * @bio: [in, out] bio to be split
+ *
+ * Splits a bio into two bios, chains the two bios, submits the second half
+ * and stores a pointer to the first half in *@bio.
+ */
 void blk_queue_split(struct request_queue *q, struct bio **bio)
 {
 	unsigned int nr_segs;
-- 
2.22.0.410.gd8fdbe21b5-goog