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

[Christian König]

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.
>
> 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://greenteapress.com/wp/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.
>>> + */
>>> +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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