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

[Christian König]

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.

Regards,
Christian.

> Regards,
>
> Tvrtko
>
> >       DMA_FENCE_FLAG_USER_BITS, /* must always be last member */
> >   };
> >   @@ -257,6 +258,23 @@ 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 is called without &dma_fence.lock held, it can be called
> > +     * multiple times and from any context.  Locking is up to the 
> > callee
> > +     * if it has some state to manage.
> > +     *
> > +     * 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,
> > @@ -583,6 +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);