md: set rotational bit

if all disks in an array are non-rotational, set the array
non-rotational.

This only works for array with all disks populated at startup. Support
for disk hotadd/hotremove could be added later if necessary.
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -5297,6 +5297,21 @@ int md_run(struct mddev *mddev)
 		return err;
 	}
 	if (mddev->queue) {
+		bool nonrot = true;
+
+		rdev_for_each(rdev, mddev) {
+			if (rdev->raid_disk >= 0 &&
+			    !blk_queue_nonrot(bdev_get_queue(rdev->bdev))) {
+				nonrot = false;
+				break;
+			}
+		}
+		if (mddev->degraded)
+			nonrot = false;
+		if (nonrot)
+			queue_flag_set_unlocked(QUEUE_FLAG_NONROT, mddev->queue);
+		else
+			queue_flag_clear_unlocked(QUEUE_FLAG_NONROT, mddev->queue);
 		mddev->queue->backing_dev_info.congested_data = mddev;
 		mddev->queue->backing_dev_info.congested_fn = md_congested;
 	}