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SubjectRe: [RFC 1/4] dma-fence: Add deadline awareness
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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.

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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