LXR linux/include/drm/spsc

   1/*
   2 * Copyright 2017 Advanced Micro Devices, Inc.
   3 *
   4 * Permission is hereby granted, free of charge, to any person obtaining a
   5 * copy of this software and associated documentation files (the "Software"),
   6 * to deal in the Software without restriction, including without limitation
   7 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8 * and/or sell copies of the Software, and to permit persons to whom the
   9 * Software is furnished to do so, subject to the following conditions:
  10 *
  11 * The above copyright notice and this permission notice shall be included in
  12 * all copies or substantial portions of the Software.
  13 *
  14 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  15 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  16 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  17 * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
  18 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
  19 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
  20 * OTHER DEALINGS IN THE SOFTWARE.
  21 *
  22 */
  23
  24#ifndef DRM_SCHEDULER_SPSC_QUEUE_H_
  25#define DRM_SCHEDULER_SPSC_QUEUE_H_
  26
  27#include <linux/atomic.h>
  28#include <linux/preempt.h>
  29
  30/** SPSC lockless queue */
  31
  32struct spsc_node {
  33
  34        /* Stores spsc_node* */
  35        struct spsc_node *next;
  36};
  37
  38struct spsc_queue {
  39
  40         struct spsc_node *head;
  41
  42        /* atomic pointer to struct spsc_node* */
  43        atomic_long_t tail;
  44
  45        atomic_t job_count;
  46};
  47
  48static inline void spsc_queue_init(struct spsc_queue *queue)
  49{
  50        queue->head = NULL;
  51        atomic_long_set(&queue->tail, (long)&queue->head);
  52        atomic_set(&queue->job_count, 0);
  53}
  54
  55static inline struct spsc_node *spsc_queue_peek(struct spsc_queue *queue)
  56{
  57        return queue->head;
  58}
  59
  60static inline int spsc_queue_count(struct spsc_queue *queue)
  61{
  62        return atomic_read(&queue->job_count);
  63}
  64
  65static inline bool spsc_queue_push(struct spsc_queue *queue, struct spsc_node *node)
  66{
  67        struct spsc_node **tail;
  68
  69        node->next = NULL;
  70
  71        preempt_disable();
  72
  73        tail = (struct spsc_node **)atomic_long_xchg(&queue->tail, (long)&node->next);
  74        WRITE_ONCE(*tail, node);
  75        atomic_inc(&queue->job_count);
  76
  77        /*
  78         * In case of first element verify new node will be visible to the consumer
  79         * thread when we ping the kernel thread that there is new work to do.
  80         */
  81        smp_wmb();
  82
  83        preempt_enable();
  84
  85        return tail == &queue->head;
  86}
  87
  88
  89static inline struct spsc_node *spsc_queue_pop(struct spsc_queue *queue)
  90{
  91        struct spsc_node *next, *node;
  92
  93        /* Verify reading from memory and not the cache */
  94        smp_rmb();
  95
  96        node = READ_ONCE(queue->head);
  97
  98        if (!node)
  99                return NULL;
 100
 101        next = READ_ONCE(node->next);
 102        WRITE_ONCE(queue->head, next);
 103
 104        if (unlikely(!next)) {
 105                /* slowpath for the last element in the queue */
 106
 107                if (atomic_long_cmpxchg(&queue->tail,
 108                                (long)&node->next, (long) &queue->head) != (long)&node->next) {
 109                        /* Updating tail failed wait for new next to appear */
 110                        do {
 111                                smp_rmb();
 112                        } while (unlikely(!(queue->head = READ_ONCE(node->next))));
 113                }
 114        }
 115
 116        atomic_dec(&queue->job_count);
 117        return node;
 118}
 119
 120
 121
 122#endif /* DRM_SCHEDULER_SPSC_QUEUE_H_ */
 123