linux-kernel - Re: [PATCH v4 01/14] dma-buf/dma-fence: Add deadline awareness

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Date:   Wed, 22 Feb 2023 17:04:42 +0000
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Christian König <christian.koenig@....com>,
        Rob Clark <robdclark@...il.com>,
        dri-devel@...ts.freedesktop.org
Cc:     Rob Clark <robdclark@...omium.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Michel Dänzer <michel@...nzer.net>,
        open list <linux-kernel@...r.kernel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        freedreno@...ts.freedesktop.org,
        "open list:SYNC FILE FRAMEWORK" <linux-media@...r.kernel.org>
Subject: Re: [PATCH v4 01/14] dma-buf/dma-fence: Add deadline awareness


On 22/02/2023 15:28, Christian König wrote:
> Am 22.02.23 um 11:23 schrieb Tvrtko Ursulin:
>>
>> On 18/02/2023 21:15, Rob Clark wrote:
>>> From: Rob Clark <robdclark@...omium.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.
>>>
>>> v2: Drop dma_fence::deadline and related logic to filter duplicate
>>>      deadlines, to avoid increasing dma_fence size.  The fence-context
>>>      implementation will need similar logic to track deadlines of all
>>>      the fences on the same timeline.  [ckoenig]
>>> v3: Clarify locking wrt. set_deadline callback
>>>
>>> Signed-off-by: Rob Clark <robdclark@...omium.org>
>>> Reviewed-by: Christian König <christian.koenig@....com>
>>> ---
>>>   drivers/dma-buf/dma-fence.c | 20 ++++++++++++++++++++
>>>   include/linux/dma-fence.h   | 20 ++++++++++++++++++++
>>>   2 files changed, 40 insertions(+)
>>>
>>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
>>> index 0de0482cd36e..763b32627684 100644
>>> --- a/drivers/dma-buf/dma-fence.c
>>> +++ b/drivers/dma-buf/dma-fence.c
>>> @@ -912,6 +912,26 @@ 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)
>>> +{
>>> +    if (fence->ops->set_deadline && !dma_fence_is_signaled(fence))
>>> +        fence->ops->set_deadline(fence, deadline);
>>> +}
>>> +EXPORT_SYMBOL(dma_fence_set_deadline);
>>> +
>>>   /**
>>>    * dma_fence_describe - Dump fence describtion into seq_file
>>>    * @fence: the 6fence to describe
>>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>>> index 775cdc0b4f24..d77f6591c453 100644
>>> --- a/include/linux/dma-fence.h
>>> +++ b/include/linux/dma-fence.h
>>> @@ -99,6 +99,7 @@ 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,
>>
>> Would this bit be better left out from core implementation, given how 
>> the approach is the component which implements dma-fence has to track 
>> the actual deadline and all?
>>
>> Also taking a step back - are we all okay with starting to expand the 
>> relatively simple core synchronisation primitive with side channel 
>> data like this? What would be the criteria for what side channel data 
>> would be acceptable? Taking note the thing lives outside drivers/gpu/.
> 
> I had similar concerns and it took me a moment as well to understand the 
> background why this is necessary. I essentially don't see much other 
> approach we could do.
> 
> Yes, this is GPU/CRTC specific, but we somehow need a common interface 
> for communicating it between drivers and that's the dma_fence object as 
> far as I can see.

Yeah I also don't see any other easy options. Just wanted to raise this 
as something which probably needs some wider acks.

Also what about the "low level" part of my question about the reason, or 
benefits, of defining the deadline bit in the common layer?

Regards,

Tvrtko