[PATCH 2/5] drm: Add Reusable task barrier.
Grodzovsky, Andrey
Andrey.Grodzovsky at amd.com
Thu Dec 12 14:50:25 UTC 2019
[AMD Official Use Only - Internal Distribution Only]
______________________________________
From: Christian König <ckoenig.leichtzumerken at gmail.com>
Sent: 12 December 2019 03:31
To: Alex Deucher; Grodzovsky, Andrey
Cc: Deucher, Alexander; Ma, Le; Quan, Evan; amd-gfx list; Zhang, Hawking
Subject: Re: [PATCH 2/5] drm: Add Reusable task barrier.
Am 12.12.19 um 09:24 schrieb Christian König:
> Am 11.12.19 um 21:19 schrieb Alex Deucher:
>> On Wed, Dec 11, 2019 at 3:07 PM Andrey Grodzovsky
>> <andrey.grodzovsky at amd.com> wrote:
>>> It is used to synchronize N threads at a rendevouz point before
>>> execution
>>> of critical code that has to be started by all the threads at
>>> approximatly
>>> the same time.
>>>
>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>> You should resend to dri-devel since this task barrier is being added
>> to common code.
>
> Additional to that this whole thing has the potential to raise lockdep
> warnings and if I'm not completely mistaken doesn't even work correctly.
Can you give me a potential lockdep scenario ?
Andrey
>
> See Linux kernel semaphores don't allow negative values (the count
> field in struct semaphore is unsigned).
Ok, forget what I've wrote. That indeed seems to be supported, some
other drivers are already using semaphores the same way.
Regards,
Christian.
>
> Regards,
> Christian.
>
>>
>> Alex
>>
>>> ---
>>> include/drm/task_barrier.h | 106
>>> +++++++++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 106 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..81fb0f7
>>> --- /dev/null
>>> +++ b/include/drm/task_barrier.h
>>> @@ -0,0 +1,106 @@
>>> +/*
>>> + * 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.
>>> + * Based on the Little book of sempahores -
>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgreenteapress.com%2Fwp%2Fsemaphores%2F&data=02%7C01%7Candrey.grodzovsky%40amd.com%7C96f8c28ae4bd43f2922208d77eddb60f%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637117363029062995&sdata=6xXZXyDIKmQM8ET5hPIZ%2FbJrOMQqK4oYw8dGYS1rxcY%3D&reserved=0
>>> + */
>>> +
>>> +
>>> +
>>> +#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.
>>> + */
>>> +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 of
>>> the reset if
>>> + * the barrier is used in a loop (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);
>>> +}
>>> +
>>> +static inline void task_barrier_full(struct task_barrier *tb)
>>> +{
>>> + task_barrier_enter(tb);
>>> + task_barrier_exit(tb);
>>> +}
>>> +
>>> +#endif
>>> --
>>> 2.7.4
>>>
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