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Date: Tue, 21 Sep 2021 08:57:30 -0700
From: Rob Clark <robdclark@...il.com>
To: Rob Clark <robdclark@...il.com>,
dri-devel <dri-devel@...ts.freedesktop.org>,
"moderated list:DMA BUFFER SHARING FRAMEWORK"
<linaro-mm-sig@...ts.linaro.org>,
Christian König <ckoenig.leichtzumerken@...il.com>,
Michel Dänzer <michel@...nzer.net>,
Pekka Paalanen <ppaalanen@...il.com>,
Rob Clark <robdclark@...omium.org>,
David Airlie <airlied@...ux.ie>,
Sumit Semwal <sumit.semwal@...aro.org>,
Christian König <christian.koenig@....com>,
Tian Tao <tiantao6@...ilicon.com>,
Steven Price <steven.price@....com>,
Melissa Wen <mwen@...lia.com>,
Luben Tuikov <luben.tuikov@....com>,
Andrey Grodzovsky <andrey.grodzovsky@....com>,
Boris Brezillon <boris.brezillon@...labora.com>,
Jack Zhang <Jack.Zhang1@....com>,
open list <linux-kernel@...r.kernel.org>,
"open list:DMA BUFFER SHARING FRAMEWORK"
<linux-media@...r.kernel.org>
Cc: Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v3 4/9] drm/scheduler: Add fence deadline support
On Wed, Sep 8, 2021 at 10:45 AM Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Fri, Sep 03, 2021 at 11:47:55AM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > As the finished fence is the one that is exposed to userspace, and
> > therefore the one that other operations, like atomic update, would
> > block on, we need to propagate the deadline from from the finished
> > fence to the actual hw fence.
> >
> > v2: Split into drm_sched_fence_set_parent() (ckoenig)
> >
> > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > ---
> > drivers/gpu/drm/scheduler/sched_fence.c | 34 +++++++++++++++++++++++++
> > drivers/gpu/drm/scheduler/sched_main.c | 2 +-
> > include/drm/gpu_scheduler.h | 8 ++++++
> > 3 files changed, 43 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/scheduler/sched_fence.c b/drivers/gpu/drm/scheduler/sched_fence.c
> > index bcea035cf4c6..4fc41a71d1c7 100644
> > --- a/drivers/gpu/drm/scheduler/sched_fence.c
> > +++ b/drivers/gpu/drm/scheduler/sched_fence.c
> > @@ -128,6 +128,30 @@ static void drm_sched_fence_release_finished(struct dma_fence *f)
> > dma_fence_put(&fence->scheduled);
> > }
> >
> > +static void drm_sched_fence_set_deadline_finished(struct dma_fence *f,
> > + ktime_t deadline)
> > +{
> > + struct drm_sched_fence *fence = to_drm_sched_fence(f);
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&fence->lock, flags);
> > +
> > + /* If we already have an earlier deadline, keep it: */
> > + if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &f->flags) &&
> > + ktime_before(fence->deadline, deadline)) {
> > + spin_unlock_irqrestore(&fence->lock, flags);
> > + return;
> > + }
> > +
> > + fence->deadline = deadline;
> > + set_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &f->flags);
> > +
> > + spin_unlock_irqrestore(&fence->lock, flags);
> > +
> > + if (fence->parent)
> > + dma_fence_set_deadline(fence->parent, deadline);
> > +}
> > +
> > static const struct dma_fence_ops drm_sched_fence_ops_scheduled = {
> > .get_driver_name = drm_sched_fence_get_driver_name,
> > .get_timeline_name = drm_sched_fence_get_timeline_name,
> > @@ -138,6 +162,7 @@ static const struct dma_fence_ops drm_sched_fence_ops_finished = {
> > .get_driver_name = drm_sched_fence_get_driver_name,
> > .get_timeline_name = drm_sched_fence_get_timeline_name,
> > .release = drm_sched_fence_release_finished,
> > + .set_deadline = drm_sched_fence_set_deadline_finished,
> > };
> >
> > struct drm_sched_fence *to_drm_sched_fence(struct dma_fence *f)
> > @@ -152,6 +177,15 @@ struct drm_sched_fence *to_drm_sched_fence(struct dma_fence *f)
> > }
> > EXPORT_SYMBOL(to_drm_sched_fence);
> >
> > +void drm_sched_fence_set_parent(struct drm_sched_fence *s_fence,
> > + struct dma_fence *fence)
> > +{
> > + s_fence->parent = dma_fence_get(fence);
> > + if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT,
> > + &s_fence->finished.flags))
>
> Don't you need the spinlock here too to avoid races? test_bit is very
> unordered, so guarantees nothing. Spinlock would need to be both around
> ->parent = and the test_bit.
>
> Entirely aside, but there's discussions going on to preallocate the hw
> fence somehow. If we do that we could make the deadline forwarding
> lockless here. Having a spinlock just to set the parent is a bit annoying
> ...
>
> Alternative is that you do this locklessly with barriers and a _lot_ of
> comments. Would be good to benchmark whether the overhead matters though
> first.
So, my thinking is that very few (well no) guarantees are made to the
fence implementor that their ->set_deadline() is not called multiple
times, from multiple threads, etc. And no guarantee that a later
deadline is set after an earlier deadline has been set. It is all up
to the set_deadline() implementation to deal with these cases.
So that means we just need the appropriate barrier-fu to ensure
another thread calling drm_sched_fence_set_deadline_finished() sees
fence->parent set before the test_bit. It could mean that the backend
implementation sees the same deadline set twice, but that is fine.
BR,
-R
> -Daniel
>
> > + dma_fence_set_deadline(fence, s_fence->deadline);
> > +}
> > +
> > struct drm_sched_fence *drm_sched_fence_alloc(struct drm_sched_entity *entity,
> > void *owner)
> > {
> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> > index 595e47ff7d06..27bf0ac0625f 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -978,7 +978,7 @@ static int drm_sched_main(void *param)
> > drm_sched_fence_scheduled(s_fence);
> >
> > if (!IS_ERR_OR_NULL(fence)) {
> > - s_fence->parent = dma_fence_get(fence);
> > + drm_sched_fence_set_parent(s_fence, fence);
> > r = dma_fence_add_callback(fence, &sched_job->cb,
> > drm_sched_job_done_cb);
> > if (r == -ENOENT)
> > diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> > index 7f77a455722c..158ddd662469 100644
> > --- a/include/drm/gpu_scheduler.h
> > +++ b/include/drm/gpu_scheduler.h
> > @@ -238,6 +238,12 @@ struct drm_sched_fence {
> > */
> > struct dma_fence finished;
> >
> > + /**
> > + * @deadline: deadline set on &drm_sched_fence.finished which
> > + * potentially needs to be propagated to &drm_sched_fence.parent
> > + */
> > + ktime_t deadline;
> > +
> > /**
> > * @parent: the fence returned by &drm_sched_backend_ops.run_job
> > * when scheduling the job on hardware. We signal the
> > @@ -505,6 +511,8 @@ void drm_sched_entity_set_priority(struct drm_sched_entity *entity,
> > enum drm_sched_priority priority);
> > bool drm_sched_entity_is_ready(struct drm_sched_entity *entity);
> >
> > +void drm_sched_fence_set_parent(struct drm_sched_fence *s_fence,
> > + struct dma_fence *fence);
> > struct drm_sched_fence *drm_sched_fence_alloc(
> > struct drm_sched_entity *s_entity, void *owner);
> > void drm_sched_fence_init(struct drm_sched_fence *fence,
> > --
> > 2.31.1
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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