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

[Christian König]

Am 20.05.21 um 16:54 schrieb Rob Clark:
> On Thu, May 20, 2021 at 7:11 AM Christian König
> <christian.koenig at amd.com> wrote:
>>
>>
>> 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?
> Basically situations where you are ping-ponging between GPU and CPU..
> for example if you are double buffering instead of triple buffering,
> and doing vblank sync'd pageflips.  The GPU, without any extra signal,
> could get stuck at 30fps and a low gpu freq, because it ends up idle
> while waiting for an extra vblank cycle for the next back-buffer to
> become available.  Whereas if it boosted up to a higher freq and
> stopped missing a vblank deadline, it would be less idle due to
> getting the next back-buffer sooner (due to not missing a vblank
> deadline).

Ok the is the why, but what about the how?

How does it help to have this boost callback and not just start a time 
on enable signaling and stop it when the signal arrives?

Regards,
Christian.

>
> BR,
> -R
>
>> 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);
>>>>>
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