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
32 struct spsc_node {
33
34 /* Stores spsc_node* */
35 struct spsc_node *next;
36 };
37
38 struct 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
48 static 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
55 static inline struct spsc_node *spsc_queue_peek(struct spsc_queue *queue)
56 {
57 return queue->head;
58 }
59
60 static inline int spsc_queue_count(struct spsc_queue *queue)
61 {
62 return atomic_read(&queue->job_count);
63 }
64
65 static 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
89 static 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_ */