[RFC] block: avoid the unnecessary blk_bio_discard_split()

[Tom Yan <tom.ty89@gmail.com>]

It doesn't seem necessary to have the redundant layer of splitting.
The request size will even be more consistent / aligned to the cap.

Signed-off-by: Tom Yan <tom.ty89@gmail.com>
---
 block/blk-lib.c   | 5 ++++-
 block/blk-merge.c | 2 +-
 block/blk.h       | 8 ++++++--
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/block/blk-lib.c b/block/blk-lib.c
index e90614fd8d6a..f606184a9050 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -85,9 +85,12 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 		 *   is split in device drive, the split ones are very probably
 		 *   to be aligned to discard_granularity of the device's queue.
 		 */
-		if (granularity_aligned_lba == sector_mapped)
+		if (granularity_aligned_lba == sector_mapped) {
 			req_sects = min_t(sector_t, nr_sects,
 					  bio_aligned_discard_max_sectors(q));
+			if (!req_sects)
+				return -EOPNOTSUPP;
+		}
 		else
 			req_sects = min_t(sector_t, nr_sects,
 					  granularity_aligned_lba - sector_mapped);
diff --git a/block/blk-merge.c b/block/blk-merge.c
index bcf5e4580603..2439216585d9 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -59,6 +59,7 @@ static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
 	return bio_will_gap(req->q, NULL, bio, req->bio);
 }
 
+/* deprecated */
 static struct bio *blk_bio_discard_split(struct request_queue *q,
 					 struct bio *bio,
 					 struct bio_set *bs,
@@ -303,7 +304,6 @@ void __blk_queue_split(struct bio **bio, unsigned int *nr_segs)
 	switch (bio_op(*bio)) {
 	case REQ_OP_DISCARD:
 	case REQ_OP_SECURE_ERASE:
-		split = blk_bio_discard_split(q, *bio, &q->bio_split, nr_segs);
 		break;
 	case REQ_OP_WRITE_ZEROES:
 		split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split,
diff --git a/block/blk.h b/block/blk.h
index dfab98465db9..e7e31a8c4930 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -281,8 +281,12 @@ static inline unsigned int bio_allowed_max_sectors(struct request_queue *q)
 static inline unsigned int bio_aligned_discard_max_sectors(
 					struct request_queue *q)
 {
-	return round_down(UINT_MAX, q->limits.discard_granularity) >>
-			SECTOR_SHIFT;
+	unsigned int discard_max_sectors, granularity;
+	discard_max_sectors = min(q->limits.max_discard_sectors,
+				  bio_allowed_max_sectors(q));
+	/* Zero-sector (unknown) and one-sector granularities are the same.  */
+	granularity = max(q->limits.discard_granularity >> SECTOR_SHIFT, 1U)
+	return round_down(max, granularity);
 }
 
 /*
-- 
2.29.2