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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 22/02/2023 17:16, Rob Clark wrote:
> On Wed, Feb 22, 2023 at 9:05 AM Tvrtko Ursulin
> <tvrtko.ursulin@linux.intel.com> wrote:
>>
>>
>> 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@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.
>>>>>
>>>>> 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@chromium.org>
>>>>> Reviewed-by: Christian König <christian.koenig@amd.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?
> 
> We could leave DMA_FENCE_FLAG_HAS_DEADLINE_BIT out, but OTOH managing
> a bitmask that is partially defined in core enum and partially in
> backend-driver has it's own drawbacks, and it isn't like we are
> running out of bits.. :shrug:

There is DMA_FENCE_FLAG_USER_BITS onwards which implementations could 
use to store their stuff?

And if we skip forward to "drm/scheduler: Add fence deadline support" 
that's the only place bit is used, right? Would it simply work to look 
at drm_sched_fence->deadline == 0 as bit not set? Or you see a need to 
interoperate with other fence implementations via that bit somehow?

Regards,

Tvrtko