block: Add blk_bio_map_sg() helper

Add a helper to map a bio to a scatterlist, modelled after
blk_rq_map_sg.

This helper is useful for any driver that wants to create
a scatterlist from its ->make_request_fn method.

Changes in v2:
 - Use __blk_segment_map_sg to avoid duplicated code
 - Add cocbook style function comment

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: Shaohua Li <shli@kernel.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

2 files changed, 39 insertions, 0 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 576b68e79248..e76279e41162 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -209,6 +209,43 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 }
 EXPORT_SYMBOL(blk_rq_map_sg);
 
+/**
+ * blk_bio_map_sg - map a bio to a scatterlist
+ * @q: request_queue in question
+ * @bio: bio being mapped
+ * @sglist: scatterlist being mapped
+ *
+ * Note:
+ *    Caller must make sure sg can hold bio->bi_phys_segments entries
+ *
+ * Will return the number of sg entries setup
+ */
+int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
+                   struct scatterlist *sglist)
+{
+        struct bio_vec *bvec, *bvprv;
+        struct scatterlist *sg;
+        int nsegs, cluster;
+        unsigned long i;
+
+        nsegs = 0;
+        cluster = blk_queue_cluster(q);
+
+        bvprv = NULL;
+        sg = NULL;
+        bio_for_each_segment(bvec, bio, i) {
+                __blk_segment_map_sg(q, bvec, sglist, &bvprv, &sg,
+                                     &nsegs, &cluster);
+        } /* segments in bio */
+
+        if (sg)
+                sg_mark_end(sg);
+
+        BUG_ON(bio->bi_phys_segments && nsegs > bio->bi_phys_segments);
+        return nsegs;
+}
+EXPORT_SYMBOL(blk_bio_map_sg);
+
 static inline int ll_new_hw_segment(struct request_queue *q,
                                     struct request *req,
                                     struct bio *bio)
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 281516ae8b4e..dc632975d54f 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -894,6 +894,8 @@ extern void blk_queue_flush_queueable(struct request_queue *q, bool queueable);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
 
 extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
+extern int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
+                          struct scatterlist *sglist);
 extern void blk_dump_rq_flags(struct request *, char *);
 extern long nr_blockdev_pages(void);