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

On Wed, Jul 28, 2021 at 4:37 AM Christian König ckoenig.leichtzumerken@gmail.com wrote:

Am 28.07.21 um 09:03 schrieb Christian König:

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Am 27.07.21 um 16:25 schrieb Rob Clark:

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On Tue, Jul 27, 2021 at 12:11 AM Christian König ckoenig.leichtzumerken@gmail.com wrote:

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Am 27.07.21 um 01:38 schrieb Rob Clark:

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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.

Right, I ended up keeping track of the nearest deadline in patch 5/4 which added drm/msm support:

https://patchwork.freedesktop.org/patch/447138/

But if we do have the ktime_t in dma_fence in dma_fence, we can add some checks and avoid calling back to the driver if a later deadline is set on a fence that already has an earlier deadline. OTOH I suppose I can push all that back to the driver to start, and we can revisit once we have more drivers implementing deadline support.

Thinking more about it we could probably kill the spinlock pointer and make the flags 32bit if we absolutely need that here.

If we had a 'struct dma_fence_context' we could push the spinlock, ops pointer, and u64 context into that and replace with a single dma_fence_context ptr, fwiw

BR, -R

But I still don't see the need for that, especially since most drivers probably won't implement it.

Regards, Christian.

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Regards, Christian.

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BR, -R

...

Regards, Christian.

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   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);