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

[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.

> 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.

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);
> >
>