[PATCHv2] nvme: only use power of two io boundaries

[Keith Busch]

The kernel requires a power of two for boundaries because that's the
only way it can efficiently split commands that cross them. A
controller, however, may report a non-power of two boundary.

The driver had been rounding the controller's value to one the kernel
can use, but splitting on the wrong boundary provides no benefit on the
device side, and actually incurs additional submission overhead from
non-optimal splits. Don't provide any boundary hint if the controller's
value can't be used and log a warning when initially observing a disk's
unreported io boundary.

Signed-off-by: Keith Busch <kbusch at kernel.org>
---
v1->v2:

  Warn for the namespace with unusable io boundary.

 drivers/nvme/host/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 154942fc64eb..9bbbd600621f 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -2097,8 +2097,12 @@ static int __nvme_revalidate_disk(struct gendisk *disk, struct nvme_id_ns *id)
 		}
 	}
 
-	if (iob && !blk_queue_is_zoned(ns->queue))
-		blk_queue_chunk_sectors(ns->queue, rounddown_pow_of_two(iob));
+	if (is_power_of_2(iob) && !blk_queue_is_zoned(ns->queue))
+		blk_queue_chunk_sectors(ns->queue, iob);
+	else if (iob && !(disk->flags & GENHD_FL_UP))
+		dev_warn(ctrl->device, "namespace:%u has unused io boundary:%u\n",
+			ns->head->ns_id, iob);
+
 	nvme_update_disk_info(disk, ns, id);
 #ifdef CONFIG_NVME_MULTIPATH
 	if (ns->head->disk) {
-- 
2.24.1