[PATCH v21 01/12] block: Support block devices that preserve the order of write requests

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

Some storage controllers preserve the request order per hardware queue.
Some but not all device mapper drivers preserve the bio order. Introduce
the feature flag BLK_FEAT_ORDERED_HWQ to allow block drivers and stacked
drivers to indicate that the order of write commands is preserved per
hardware queue and hence that serialization of writes per zone is not
required if all pending writes are submitted to the same hardware queue.

Cc: Damien Le Moal <dlemoal@kernel.org>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Nitesh Shetty <nj.shetty@samsung.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-settings.c   | 2 ++
 include/linux/blkdev.h | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index a000daafbfb4..45ab1d644720 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -698,6 +698,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 		t->features &= ~BLK_FEAT_NOWAIT;
 	if (!(b->features & BLK_FEAT_POLL))
 		t->features &= ~BLK_FEAT_POLL;
+	if (!(b->features & BLK_FEAT_ORDERED_HWQ))
+		t->features &= ~BLK_FEAT_ORDERED_HWQ;
 
 	t->flags |= (b->flags & BLK_FLAG_MISALIGNED);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 5f14c20c8bc0..3ea6c77746c5 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -337,6 +337,12 @@ typedef unsigned int __bitwise blk_features_t;
 /* skip this queue in blk_mq_(un)quiesce_tagset */
 #define BLK_FEAT_SKIP_TAGSET_QUIESCE	((__force blk_features_t)(1u << 13))
 
+/*
+ * The request order is preserved per hardware queue by the block driver and by
+ * the block device.
+ */
+#define BLK_FEAT_ORDERED_HWQ		((__force blk_features_t)(1u << 14))
+
 /* undocumented magic for bcache */
 #define BLK_FEAT_RAID_PARTIAL_STRIPES_EXPENSIVE \
 	((__force blk_features_t)(1u << 15))