[RFC 1/4] dma-fence: Add deadline awareness

[Rob Clark]

On Tue, Jul 27, 2021 at 12:11 AM Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
>
> Am 27.07.21 um 01:38 schrieb Rob Clark:
> > From: Rob Clark <robdclark at 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.
> >
> > Signed-off-by: Rob Clark <robdclark at chromium.org>
> > ---
> >   drivers/dma-buf/dma-fence.c | 39 +++++++++++++++++++++++++++++++++++++
> >   include/linux/dma-fence.h   | 17 ++++++++++++++++
> >   2 files changed, 56 insertions(+)
> >
> > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> > index ce0f5eff575d..2e0d25ab457e 100644
> > --- a/drivers/dma-buf/dma-fence.c
> > +++ b/drivers/dma-buf/dma-fence.c
> > @@ -910,6 +910,45 @@ 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)
> > +{
> > +     unsigned long flags;
> > +
> > +     if (dma_fence_is_signaled(fence))
> > +             return;
> > +
> > +     spin_lock_irqsave(fence->lock, flags);
> > +
> > +     /* If we already have an earlier deadline, keep it: */
> > +     if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags) &&
> > +         ktime_before(fence->deadline, deadline)) {
> > +             spin_unlock_irqrestore(fence->lock, flags);
> > +             return;
> > +     }
> > +
> > +     fence->deadline = deadline;
> > +     set_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags);
> > +
> > +     spin_unlock_irqrestore(fence->lock, flags);
> > +
> > +     if (fence->ops->set_deadline)
> > +             fence->ops->set_deadline(fence, deadline);
> > +}
> > +EXPORT_SYMBOL(dma_fence_set_deadline);
> > +
> >   /**
> >    * dma_fence_init - Initialize a custom fence.
> >    * @fence: the fence to initialize
> > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> > index 6ffb4b2c6371..4e6cfe4e6fbc 100644
> > --- a/include/linux/dma-fence.h
> > +++ b/include/linux/dma-fence.h
> > @@ -88,6 +88,7 @@ struct dma_fence {
> >               /* @timestamp replaced by @rcu on dma_fence_release() */
> >               struct rcu_head rcu;
> >       };
> > +     ktime_t deadline;
>
> Mhm, adding the flag sounds ok to me but I'm a bit hesitating adding the
> deadline as extra field here.
>
> We tuned the dma_fence structure intentionally so that it is only 64 bytes.

Hmm, then I guess you wouldn't be a fan of also adding an hrtimer?

We could push the ktime_t (and timer) down into the derived fence
class, but I think there is going to need to be some extra storage
*somewhere*.. maybe the fence signaler could get away with just
storing the nearest upcoming deadline per fence-context instead?

BR,
-R

> Regards,
> Christian.
>
> >       u64 context;
> >       u64 seqno;
> >       unsigned long flags;
> > @@ -99,6 +100,7 @@ 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,
> >       DMA_FENCE_FLAG_USER_BITS, /* must always be last member */
> >   };
> >
> > @@ -261,6 +263,19 @@ struct dma_fence_ops {
> >        */
> >       void (*timeline_value_str)(struct dma_fence *fence,
> >                                  char *str, int size);
> > +
> > +     /**
> > +      * @set_deadline:
> > +      *
> > +      * Callback to allow a fence waiter to 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.
> > +      *
> > +      * This callback is optional.
> > +      */
> > +     void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
> >   };
> >
> >   void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
> > @@ -586,6 +601,8 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
> >       return ret < 0 ? ret : 0;
> >   }
> >
> > +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline);
> > +
> >   struct dma_fence *dma_fence_get_stub(void);
> >   struct dma_fence *dma_fence_allocate_private_stub(void);
> >   u64 dma_fence_context_alloc(unsigned num);
>