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

[Christian König]

Am 27.07.21 um 01:38 schrieb Rob Clark:
> From: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>
> ---
>   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);