[PATCH V6 30/30] block: document usage of bio iterator helpers

[Ming Lei <ming.lei@redhat.com>]

Now multipage bvec is supported, and some helpers may return page by
page, and some may return segment by segment, this patch documents the
usage for helping us use them correctly.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 Documentation/block/biovecs.txt | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt
index 25689584e6e0..3ab72566141f 100644
--- a/Documentation/block/biovecs.txt
+++ b/Documentation/block/biovecs.txt
@@ -117,3 +117,33 @@ 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 "_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_chunk_segment_all()
+	bio_for_each_chunk_all()
+	bio_pages_all()
+	bio_first_bvec_all()
+	bio_first_page_all()
+	bio_last_bvec_all()
+
+* The following helpers iterate bio page by page, and the local variable of
+'struct bio_vec' or the reference records single page io vector during the
+iteration:
+
+	bio_for_each_segment()
+	bio_for_each_segment_all()
+
+* The following helpers iterate bio chunk by chunk, and each chunk may
+include multiple physically contiguous pages, and the local variable of
+'struct bio_vec' or the reference records multi page io vector during the
+iteration:
+
+	bio_for_each_chunk()
+	bio_for_each_chunk_all()
-- 
2.9.5