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

struct rnoop_data {
    struct list_head queue;
};

static int rnoop_allow_merge(struct request_queue *q, struct request *rq,
                 struct bio *bio)
{
    /* Always disallow merges */
    return 0;
}

static void rnoop_merged_requests(struct request_queue *q, struct request *rq,
                 struct request *next)
{
    list_del_init(&next->queuelist);
}

static int rnoop_dispatch(struct request_queue *q, int force)
{
    struct rnoop_data *nd = q->elevator->elevator_data;

    if (!list_empty(&nd->queue)) {
        struct request *rq;
        rq = list_entry(nd->queue.next, struct request, queuelist);
        list_del_init(&rq->queuelist);
        /* Do not sort requests inside dispatch list */
        elv_dispatch_add_tail(q, rq);
        return 1;
    }
    return 0;
}

static void rnoop_add_request(struct request_queue *q, struct request *rq)
{
    struct rnoop_data *nd = q->elevator->elevator_data;

    list_add_tail(&rq->queuelist, &nd->queue);
}

static struct request *
rnoop_former_request(struct request_queue *q, struct request *rq)
{
    struct rnoop_data *nd = q->elevator->elevator_data;

    if (rq->queuelist.prev == &nd->queue)
        return NULL;
    return list_entry(rq->queuelist.prev, struct request, queuelist);
}

static struct request *
rnoop_latter_request(struct request_queue *q, struct request *rq)
{
    struct rnoop_data *nd = q->elevator->elevator_data;

    if (rq->queuelist.next == &nd->queue)
        return NULL;
    return list_entry(rq->queuelist.next, struct request, queuelist);
}

static int rnoop_init_queue(struct request_queue *q)
{
    struct rnoop_data *nd;

    nd = kmalloc_node(sizeof(*nd), GFP_KERNEL, q->node);
    if (!nd)
        return -ENOMEM;

    INIT_LIST_HEAD(&nd->queue);
    q->elevator->elevator_data = nd;
    return 0;
}

static void rnoop_exit_queue(struct elevator_queue *e)
{
    struct rnoop_data *nd = e->elevator_data;

    BUG_ON(!list_empty(&nd->queue));
    kfree(nd);
}

static struct elevator_type elevator_rnoop = {
    .ops = {
        .elevator_allow_merge_fn    = rnoop_allow_merge,
        .elevator_merge_req_fn      = rnoop_merged_requests,
        .elevator_dispatch_fn       = rnoop_dispatch,
        .elevator_add_req_fn        = rnoop_add_request,
        .elevator_former_req_fn     = rnoop_former_request,
        .elevator_latter_req_fn     = rnoop_latter_request,
        .elevator_init_fn       = rnoop_init_queue,
        .elevator_exit_fn       = rnoop_exit_queue,
    },
    .elevator_name = "rnoop",
    .elevator_owner = THIS_MODULE,
};

static int __init rnoop_init(void)
{
    return elv_register(&elevator_rnoop);
}

static void __exit rnoop_exit(void)
{
    elv_unregister(&elevator_rnoop);
}

module_init(rnoop_init);
module_exit(rnoop_exit);


MODULE_AUTHOR("Jens Axboe");
MODULE_AUTHOR("Arianna Avanzini");
MODULE_AUTHOR("Riccardo Berto");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Really No-op IO scheduler");