/*
 * Copyright 2017 Advanced Micro Devices, Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 *
 */

#ifndef DRM_SCHEDULER_SPSC_QUEUE_H_
#define DRM_SCHEDULER_SPSC_QUEUE_H_

#include <linux/atomic.h>
#include <linux/preempt.h>

/** SPSC lockless queue */

struct spsc_node {

        /* Stores spsc_node* */
        struct spsc_node *next;
};

struct spsc_queue {

         struct spsc_node *head;

        /* atomic pointer to struct spsc_node* */
        atomic_long_t tail;

        atomic_t job_count;
};

static inline void spsc_queue_init(struct spsc_queue *queue)
{
        queue->head = NULL;
        atomic_long_set(&queue->tail, (long)&queue->head);
        atomic_set(&queue->job_count, 0);
}

static inline struct spsc_node *spsc_queue_peek(struct spsc_queue *queue)
{
        return queue->head;
}

static inline int spsc_queue_count(struct spsc_queue *queue)
{
        return atomic_read(&queue->job_count);
}

static inline bool spsc_queue_push(struct spsc_queue *queue, struct spsc_node *node)
{
        struct spsc_node **tail;

        node->next = NULL;

        preempt_disable();

        atomic_inc(&queue->job_count);
        smp_mb__after_atomic();

        tail = (struct spsc_node **)atomic_long_xchg(&queue->tail, (long)&node->next);
        WRITE_ONCE(*tail, node);

        /*
         * In case of first element verify new node will be visible to the consumer
         * thread when we ping the kernel thread that there is new work to do.
         */
        smp_wmb();

        preempt_enable();

        return tail == &queue->head;
}


static inline struct spsc_node *spsc_queue_pop(struct spsc_queue *queue)
{
        struct spsc_node *next, *node;

        /* Verify reading from memory and not the cache */
        smp_rmb();

        node = READ_ONCE(queue->head);

        if (!node)
                return NULL;

        next = READ_ONCE(node->next);
        WRITE_ONCE(queue->head, next);

        if (unlikely(!next)) {
                /* slowpath for the last element in the queue */

                if (atomic_long_cmpxchg(&queue->tail,
                                (long)&node->next, (long) &queue->head) != (long)&node->next) {
                        /* Updating tail failed wait for new next to appear */
                        do {
                                smp_rmb();
                        } while (unlikely(!(queue->head = READ_ONCE(node->next))));
                }
        }

        atomic_dec(&queue->job_count);
        return node;
}



#endif /* DRM_SCHEDULER_SPSC_QUEUE_H_ */