[PATCH v2 2/5] drm: Add Reusable task barrier.

[William Lewis]

Some typographical error nitpicks inline.

On 12/13/19 10:54 AM, Andrey Grodzovsky wrote:
> It is used to synchronize N threads at a rendevouz point before execution
rendezvous
> of critical code that has to be started by all the threads at approximatly
approximately
> the same time.
>
> v2: Remove mention of reset use case, improve doc.
>
> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
> ---
>   include/drm/task_barrier.h | 107 +++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 107 insertions(+)
>   create mode 100644 include/drm/task_barrier.h
>
> diff --git a/include/drm/task_barrier.h b/include/drm/task_barrier.h
> new file mode 100644
> index 0000000..087e3f6
> --- /dev/null
> +++ b/include/drm/task_barrier.h
> @@ -0,0 +1,107 @@
> +/*
> + * Copyright 2019 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.
> + *
> + */
> +#include <linux/semaphore.h>
> +#include <linux/atomic.h>
> +
> +/*
> + * Reusable 2 PHASE task barrier (randevouz point) implementation for N tasks.
rendezvous
> + * Based on the Little book of sempahores - http://greenteapress.com/semaphores/LittleBookOfSemaphores.pdf
semaphores
> + */
> +
> +
> +
> +#ifndef DRM_TASK_BARRIER_H_
> +#define DRM_TASK_BARRIER_H_
> +
> +/*
> + * Represents an instance of a task barrier.
> + */
> +struct task_barrier {
> +	unsigned int n;
> +	atomic_t count;
> +	struct semaphore enter_turnstile;
> +	struct semaphore exit_turnstile;
> +};
> +
> +static inline void task_barrier_signal_turnstile(struct semaphore *turnstile,
> +						 unsigned int n)
> +{
> +	int i;
> +
> +	for (i = 0 ; i < n; i++)
> +		up(turnstile);
> +}
> +
> +static inline void task_barrier_init(struct task_barrier *tb)
> +{
> +	tb->n = 0;
> +	atomic_set(&tb->count, 0);
> +	sema_init(&tb->enter_turnstile, 0);
> +	sema_init(&tb->exit_turnstile, 0);
> +}
> +
> +static inline void task_barrier_add_task(struct task_barrier *tb)
> +{
> +	tb->n++;
> +}
> +
> +static inline void task_barrier_rem_task(struct task_barrier *tb)
> +{
> +	tb->n--;
> +}
> +
> +/*
> + * Lines up all the threads BEFORE the critical point.
> + *
> + * When all thread passed this code the entry barrier is back to locked state.
threads pass
> + */
> +static inline void task_barrier_enter(struct task_barrier *tb)
> +{
> +	if (atomic_inc_return(&tb->count) == tb->n)
> +		task_barrier_signal_turnstile(&tb->enter_turnstile, tb->n);
> +
> +	down(&tb->enter_turnstile);
> +}
> +
> +/*
> + * Lines up all the threads AFTER the critical point.
> + *
> + * This function is used to avoid any one thread running ahead if the barrier is
> + *  used repeatedly .
> + */
> +static inline void task_barrier_exit(struct task_barrier *tb)
> +{
> +	if (atomic_dec_return(&tb->count) == 0)
> +		task_barrier_signal_turnstile(&tb->exit_turnstile, tb->n);
> +
> +	down(&tb->exit_turnstile);
> +}
> +
> +/* Convinieince function when nothing to be done in between entry and exit */
Convenience
> +static inline void task_barrier_full(struct task_barrier *tb)
> +{
> +	task_barrier_enter(tb);
> +	task_barrier_exit(tb);
> +}
> +
> +#endif