[PATCH V10 16/19] block: document usage of bio iterator helpers

[Ming Lei]

Now multi-page bvec is supported, some helpers may return page by
page, meantime some may return segment by segment, this patch
documents the usage.

Cc: Dave Chinner <dchinner@xxxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: dm-devel@xxxxxxxxxx
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: Shaohua Li <shli@xxxxxxxxxx>
Cc: linux-raid@xxxxxxxxxxxxxxx
Cc: linux-erofs@xxxxxxxxxxxxxxxx
Cc: David Sterba <dsterba@xxxxxxxx>
Cc: linux-btrfs@xxxxxxxxxxxxxxx
Cc: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Cc: linux-xfs@xxxxxxxxxxxxxxx
Cc: Gao Xiang <gaoxiang25@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Cc: linux-ext4@xxxxxxxxxxxxxxx
Cc: Coly Li <colyli@xxxxxxx>
Cc: linux-bcache@xxxxxxxxxxxxxxx
Cc: Boaz Harrosh <ooo@xxxxxxxxxxxxxxx>
Cc: Bob Peterson <rpeterso@xxxxxxxxxx>
Cc: cluster-devel@xxxxxxxxxx
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
 Documentation/block/biovecs.txt | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt
index 25689584e6e0..bfafb70d0d9e 100644
--- a/Documentation/block/biovecs.txt
+++ b/Documentation/block/biovecs.txt
@@ -117,3 +117,29 @@ Other implications:
    size limitations and the limitations of the underlying devices. Thus
    there's no need to define ->merge_bvec_fn() callbacks for individual block
    drivers.
+
+Usage of helpers:
+=================
+
+* The following helpers whose names have the suffix of "_all" can only be used
+on non-BIO_CLONED bio, and usually they are used by filesystem code, and driver
+shouldn't use them because bio may have been split before they got to the driver:
+
+	bio_for_each_segment_all()
+	bio_first_bvec_all()
+	bio_first_page_all()
+	bio_last_bvec_all()
+
+* The following helpers iterate over single-page bvec, and the local
+variable of 'struct bio_vec' or the reference records single-page IO
+vector during the itearation:
+
+	bio_for_each_segment()
+	bio_for_each_segment_all()
+
+* The following helper iterates over multi-page bvec, and each bvec may
+include multiple physically contiguous pages, and the local variable of
+'struct bio_vec' or the reference records multi-page IO vector during the
+itearation:
+
+	bio_for_each_bvec()
-- 
2.9.5