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