[RFC 1/3] dma-fence: Add boost fence op

[Christian König]

Am 20.05.21 um 16:07 schrieb Rob Clark:
> On Wed, May 19, 2021 at 11:47 PM Christian König
> <christian.koenig at amd.com> wrote:
>> Uff, that looks very hardware specific to me.
> Howso?  I'm not sure I agree.. and even if it was not useful for some
> hw, it should be useful for enough drivers (and harm no drivers), so I
> still think it is a good idea
>
> The fallback plan is to go the i915 route and stop using atomic
> helpers and do the same thing inside the driver, but that doesn't help
> any of the cases where you have a separate kms and gpu driver.

Yeah, that's certainly not something we want.

>> As far as I can see you can also implement completely inside the backend
>> by starting a timer on enable_signaling, don't you?
> Not really.. I mean, the fact that something waited on a fence could
> be a useful input signal to gpu freq governor, but it is entirely
> insufficient..
>
> If the cpu is spending a lot of time waiting on a fence, cpufreq will
> clock down so you spend less time waiting.  And no problem has been
> solved.  You absolutely need the concept of a missed deadline, and a
> timer doesn't give you that.

Ok then I probably don't understand the use case here.

What exactly do you try to solve?

Thanks,
Christian.

>
> BR,
> -R
>
>> Christian.
>>
>> Am 19.05.21 um 20:38 schrieb Rob Clark:
>>> From: Rob Clark <robdclark at chromium.org>
>>>
>>> Add a way to hint to the fence signaler that a fence waiter has missed a
>>> deadline waiting on the fence.
>>>
>>> In some cases, missing a vblank can result in lower gpu utilization,
>>> when really we want to go in the opposite direction and boost gpu freq.
>>> The boost callback gives some feedback to the fence signaler that we
>>> are missing deadlines, so it can take this into account in it's freq/
>>> utilization calculations.
>>>
>>> Signed-off-by: Rob Clark <robdclark at chromium.org>
>>> ---
>>>    include/linux/dma-fence.h | 26 ++++++++++++++++++++++++++
>>>    1 file changed, 26 insertions(+)
>>>
>>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>>> index 9f12efaaa93a..172702521acc 100644
>>> --- a/include/linux/dma-fence.h
>>> +++ b/include/linux/dma-fence.h
>>> @@ -231,6 +231,17 @@ struct dma_fence_ops {
>>>        signed long (*wait)(struct dma_fence *fence,
>>>                            bool intr, signed long timeout);
>>>
>>> +     /**
>>> +      * @boost:
>>> +      *
>>> +      * Optional callback, to indicate that a fence waiter missed a deadline.
>>> +      * This can serve as a signal that (if possible) whatever signals the
>>> +      * fence should boost it's clocks.
>>> +      *
>>> +      * This can be called in any context that can call dma_fence_wait().
>>> +      */
>>> +     void (*boost)(struct dma_fence *fence);
>>> +
>>>        /**
>>>         * @release:
>>>         *
>>> @@ -586,6 +597,21 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
>>>        return ret < 0 ? ret : 0;
>>>    }
>>>
>>> +/**
>>> + * dma_fence_boost - hint from waiter that it missed a deadline
>>> + *
>>> + * @fence: the fence that caused the missed deadline
>>> + *
>>> + * This function gives a hint from a fence waiter that a deadline was
>>> + * missed, so that the fence signaler can factor this in to device
>>> + * power state decisions
>>> + */
>>> +static inline void dma_fence_boost(struct dma_fence *fence)
>>> +{
>>> +     if (fence->ops->boost)
>>> +             fence->ops->boost(fence);
>>> +}
>>> +
>>>    struct dma_fence *dma_fence_get_stub(void);
>>>    u64 dma_fence_context_alloc(unsigned num);
>>>