[Intel-gfx] [PATCH 3/7] drm/etnaviv: Don't break exclusive fence ordering

[Daniel Vetter]

On Wed, Jul 7, 2021 at 10:54 AM Lucas Stach <l.stach at pengutronix.de> wrote:
>
> Hi Daniel,
>
> I'm feeling like I miss a ton of context here, so some maybe dumb
> questions/remarks below.
>
> Am Dienstag, dem 06.07.2021 um 12:12 +0200 schrieb Daniel Vetter:
> > There's only one exclusive slot, and we must not break the ordering.
> >
> > A better fix would be to us a dma_fence_chain or _array like e.g.
> > amdgpu now uses, but it probably makes sense to lift this into
> > dma-resv.c code as a proper concept, so that drivers don't have to
> > hack up their own solution each on their own. Hence go with the simple
> > fix for now.
> >
> > Another option is the fence import ioctl from Jason:
> >
> > https://lore.kernel.org/dri-devel/20210610210925.642582-7-jason@jlekstrand.net/
>
> Sorry, but why is the fence import ioctl a alternative to the fix
> proposed in this patch?

It's not an alternative to fixing the issue here, it's an alternative
to trying to fix this here without adding more dependencies. Depends a
bit what exactly you want to do, I just linked this for the bigger
picture.


>
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: Lucas Stach <l.stach at pengutronix.de>
> > Cc: Russell King <linux+etnaviv at armlinux.org.uk>
> > Cc: Christian Gmeiner <christian.gmeiner at gmail.com>
> > Cc: etnaviv at lists.freedesktop.org
> > ---
> >  drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c b/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
> > index 92478a50a580..5c4fed2b7c6a 100644
> > --- a/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
> > +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
> > @@ -178,18 +178,20 @@ static int submit_fence_sync(struct etnaviv_gem_submit *submit)
> >       for (i = 0; i < submit->nr_bos; i++) {
> >               struct etnaviv_gem_submit_bo *bo = &submit->bos[i];
> >               struct dma_resv *robj = bo->obj->base.resv;
> > +             bool write = bo->flags & ETNA_SUBMIT_BO_WRITE;
> >
> > -             if (!(bo->flags & ETNA_SUBMIT_BO_WRITE)) {
> > +             if (!(write)) {
>
> No parenthesis around the write needed.
>
> >                       ret = dma_resv_reserve_shared(robj, 1);
> >                       if (ret)
> >                               return ret;
> >               }
> >
> > -             if (submit->flags & ETNA_SUBMIT_NO_IMPLICIT)
> > +             /* exclusive fences must be ordered */
>
> I feel like this comment isn't really helpful. It might tell you all
> you need to know if you just paged in all the context around dma_resv
> and the dependency graph, but it's not more than noise to me right now.
>
> I guess the comment should answer the question against what the
> exclusive fence we are going to add needs to be ordered and why it
> isn't safe to skip implicit sync in that case.

The full-length comment is the doc patch, which is last in the series.
Essentially the rule is that your not allowed to drop fences on the
floor (which you do if you just set a new write fence and not take any
of the existing fences as dependencies), at least for shared buffers.
But since it's easier to be consistent I think it's best if we do this
just across the board.

Like the commit message explains, there's a few ways to fix this:
- just treat it as implicit synced
- chain the fences together - that way you don't sync, but also
there's no fence that's being lost. This is what amdgpu does, and also
what the new import ioctl does.

2nd option is a lot more involved, and since all the dma-resv stuff is
a bit under discussion, I went with the most minimal thing possible.
-Daniel

>
> Regards,
> Lucas
> > +             if (submit->flags & ETNA_SUBMIT_NO_IMPLICIT && !write)
> >                       continue;
> >
> >               ret = drm_sched_job_await_implicit(&submit->sched_job, &bo->obj->base,
> > -                                                bo->flags & ETNA_SUBMIT_BO_WRITE);
> > +                                                write);
> >               if (ret)
> >                       return ret;
> >       }
>
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch