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

[Rob Clark]

On Wed, Feb 22, 2023 at 9:33 AM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
>
>
> On 22/02/2023 17:16, Rob Clark wrote:
> > On Wed, Feb 22, 2023 at 9:05 AM Tvrtko Ursulin
> > <tvrtko.ursulin@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> >>>>>
> >>>>> 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@xxxxxxxxxxxx>
> >>>>> Reviewed-by: Christian König <christian.koenig@xxxxxxx>
> >>>>> ---
> >>>>>    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?

Currently drm/scheduler is the only one using it.  I ended up dropping
use of it in msm since the deadline is stored in the fence-context
instead.  But I think it is better to try to avoid assuming that zero
means not-set.

It could be moved to drm/sched.. I guess there are few enough
implementations at this point to say whether it is something useful to
other drivers or not.

BR,
-R

> Regards,
>
> Tvrtko