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SubjectRe: [RFC 1/4] dma-fence: Add deadline awareness
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Am 27.07.21 um 16:25 schrieb Rob Clark:
> On Tue, Jul 27, 2021 at 12:11 AM Christian König
> <ckoenig.leichtzumerken@gmail.com> wrote:
>> Am 27.07.21 um 01:38 schrieb Rob Clark:
>>> From: Rob Clark <robdclark@chromium.org>
>>>
>>> Add a way to hint to the fence signaler of an upcoming deadline, such as
>>> vblank, which the fence waiter would prefer not to miss. This is to aid
>>> the fence signaler in making power management decisions, like boosting
>>> frequency as the deadline approaches and awareness of missing deadlines
>>> so that can be factored in to the frequency scaling.
>>>
>>> Signed-off-by: Rob Clark <robdclark@chromium.org>
>>> ---
>>> drivers/dma-buf/dma-fence.c | 39 +++++++++++++++++++++++++++++++++++++
>>> include/linux/dma-fence.h | 17 ++++++++++++++++
>>> 2 files changed, 56 insertions(+)
>>>
>>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
>>> index ce0f5eff575d..2e0d25ab457e 100644
>>> --- a/drivers/dma-buf/dma-fence.c
>>> +++ b/drivers/dma-buf/dma-fence.c
>>> @@ -910,6 +910,45 @@ dma_fence_wait_any_timeout(struct dma_fence **fences, uint32_t count,
>>> }
>>> EXPORT_SYMBOL(dma_fence_wait_any_timeout);
>>>
>>> +
>>> +/**
>>> + * dma_fence_set_deadline - set desired fence-wait deadline
>>> + * @fence: the fence that is to be waited on
>>> + * @deadline: the time by which the waiter hopes for the fence to be
>>> + * signaled
>>> + *
>>> + * Inform the fence signaler of an upcoming deadline, such as vblank, by
>>> + * which point the waiter would prefer the fence to be signaled by. This
>>> + * is intended to give feedback to the fence signaler to aid in power
>>> + * management decisions, such as boosting GPU frequency if a periodic
>>> + * vblank deadline is approaching.
>>> + */
>>> +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline)
>>> +{
>>> + unsigned long flags;
>>> +
>>> + if (dma_fence_is_signaled(fence))
>>> + return;
>>> +
>>> + spin_lock_irqsave(fence->lock, flags);
>>> +
>>> + /* If we already have an earlier deadline, keep it: */
>>> + if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags) &&
>>> + ktime_before(fence->deadline, deadline)) {
>>> + spin_unlock_irqrestore(fence->lock, flags);
>>> + return;
>>> + }
>>> +
>>> + fence->deadline = deadline;
>>> + set_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags);
>>> +
>>> + spin_unlock_irqrestore(fence->lock, flags);
>>> +
>>> + if (fence->ops->set_deadline)
>>> + fence->ops->set_deadline(fence, deadline);
>>> +}
>>> +EXPORT_SYMBOL(dma_fence_set_deadline);
>>> +
>>> /**
>>> * dma_fence_init - Initialize a custom fence.
>>> * @fence: the fence to initialize
>>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>>> index 6ffb4b2c6371..4e6cfe4e6fbc 100644
>>> --- a/include/linux/dma-fence.h
>>> +++ b/include/linux/dma-fence.h
>>> @@ -88,6 +88,7 @@ struct dma_fence {
>>> /* @timestamp replaced by @rcu on dma_fence_release() */
>>> struct rcu_head rcu;
>>> };
>>> + ktime_t deadline;
>> Mhm, adding the flag sounds ok to me but I'm a bit hesitating adding the
>> deadline as extra field here.
>>
>> We tuned the dma_fence structure intentionally so that it is only 64 bytes.
> Hmm, then I guess you wouldn't be a fan of also adding an hrtimer?
>
> We could push the ktime_t (and timer) down into the derived fence
> class, but I think there is going to need to be some extra storage
> *somewhere*.. maybe the fence signaler could get away with just
> storing the nearest upcoming deadline per fence-context instead?

I would just push that into the driver instead.

You most likely don't want the deadline per fence anyway in complex
scenarios, but rather per frame. And a frame is usually composed from
multiple fences.

Regards,
Christian.

>
> BR,
> -R
>
>> Regards,
>> Christian.
>>
>>> u64 context;
>>> u64 seqno;
>>> unsigned long flags;
>>> @@ -99,6 +100,7 @@ enum dma_fence_flag_bits {
>>> DMA_FENCE_FLAG_SIGNALED_BIT,
>>> DMA_FENCE_FLAG_TIMESTAMP_BIT,
>>> DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
>>> + DMA_FENCE_FLAG_HAS_DEADLINE_BIT,
>>> DMA_FENCE_FLAG_USER_BITS, /* must always be last member */
>>> };
>>>
>>> @@ -261,6 +263,19 @@ struct dma_fence_ops {
>>> */
>>> void (*timeline_value_str)(struct dma_fence *fence,
>>> char *str, int size);
>>> +
>>> + /**
>>> + * @set_deadline:
>>> + *
>>> + * Callback to allow a fence waiter to inform the fence signaler of an
>>> + * upcoming deadline, such as vblank, by which point the waiter would
>>> + * prefer the fence to be signaled by. This is intended to give feedback
>>> + * to the fence signaler to aid in power management decisions, such as
>>> + * boosting GPU frequency.
>>> + *
>>> + * This callback is optional.
>>> + */
>>> + void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
>>> };
>>>
>>> void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
>>> @@ -586,6 +601,8 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
>>> return ret < 0 ? ret : 0;
>>> }
>>>
>>> +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline);
>>> +
>>> struct dma_fence *dma_fence_get_stub(void);
>>> struct dma_fence *dma_fence_allocate_private_stub(void);
>>> u64 dma_fence_context_alloc(unsigned num);

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