linux_kernel/drivers/block/noop-iosched.c

/*
 * elevator noop
 */
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/bio.h>
#include <linux/module.h>
#include <linux/init.h>

/*
 * See if we can find a request that this buffer can be coalesced with.
 */
static int elevator_noop_merge(request_queue_t *q, struct request **req,
			       struct bio *bio)
{
	struct list_head *entry = &q->queue_head;
	struct request *__rq;
	int ret;

	if ((ret = elv_try_last_merge(q, bio))) {
		*req = q->last_merge;
		return ret;
	}

	while ((entry = entry->prev) != &q->queue_head) {
		__rq = list_entry_rq(entry);

		if (__rq->flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER))
			break;
		else if (__rq->flags & REQ_STARTED)
			break;

		if (!blk_fs_request(__rq))
			continue;

		if ((ret = elv_try_merge(__rq, bio))) {
			*req = __rq;
			q->last_merge = __rq;
			return ret;
		}
	}

	return ELEVATOR_NO_MERGE;
}

static void elevator_noop_merge_requests(request_queue_t *q, struct request *req,
					 struct request *next)
{
	list_del_init(&next->queuelist);
}

static void elevator_noop_add_request(request_queue_t *q, struct request *rq,
				      int where)
{
	if (where == ELEVATOR_INSERT_FRONT)
		list_add(&rq->queuelist, &q->queue_head);
	else
		list_add_tail(&rq->queuelist, &q->queue_head);

	/*
	 * new merges must not precede this barrier
	 */
	if (rq->flags & REQ_HARDBARRIER)
		q->last_merge = NULL;
	else if (!q->last_merge)
		q->last_merge = rq;
}

static struct request *elevator_noop_next_request(request_queue_t *q)
{
	if (!list_empty(&q->queue_head))
		return list_entry_rq(q->queue_head.next);

	return NULL;
}

static struct elevator_type elevator_noop = {
	.ops = {
		.elevator_merge_fn		= elevator_noop_merge,
		.elevator_merge_req_fn		= elevator_noop_merge_requests,
		.elevator_next_req_fn		= elevator_noop_next_request,
		.elevator_add_req_fn		= elevator_noop_add_request,
	},
	.elevator_name = "noop",
	.elevator_owner = THIS_MODULE,
};

static int __init noop_init(void)
{
	return elv_register(&elevator_noop);
}

static void __exit noop_exit(void)
{
	elv_unregister(&elevator_noop);
}

module_init(noop_init);
module_exit(noop_exit);


MODULE_AUTHOR("Jens Axboe");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("No-op IO scheduler");