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

[Rob Clark]

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:

BR,
-R

> Regards,
>
> Tvrtko