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