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_ */ 123