[Intel-gfx] sw_sync deadlock avoidance, take 3

[Daniel Stone]

Hi all,

On Wed, 15 Jul 2020 at 12:57, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Wed, Jul 15, 2020 at 1:47 PM Daniel Stone <daniel at fooishbar.org> wrote:
> > On Wed, 15 Jul 2020 at 12:05, Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl> wrote:
> > > Yes, this is used as part of the Android stack on Chrome OS (need to
> > > see if ChromeOS specific, but
> > > https://source.android.com/devices/graphics/sync#sync_timeline
> > > suggests not)
> >
> > Android used to mandate it for their earlier iteration of release
> > fences, which was an empty/future fence having no guarantee of
> > eventual forward progress until someone committed work later on. For
> > example, when you committed a buffer to SF, it would give you an empty
> > 'release fence' for that buffer which would only be tied to work to
> > signal it when you committed your _next_ buffer, which might never
> > happen. They removed that because a) future fences were a bad idea,
> > and b) it was only ever useful if you assumed strictly
> > FIFO/round-robin return order which wasn't always true.
> >
> > So now it's been watered down to 'use this if you don't have a
> > hardware timeline', but why don't we work with Android people to get
> > that removed entirely?
>
> I think there's some testcases still using these, but most real fence
> testcases use vgem nowadays. So from an upstream pov there's indeed
> not much if anything holding us back from just deleting this all. And
> would probably be a good idea.

It looks like this is just a docs hangover which can be fixed; sw_sync
is no longer part of the unified Android kernel image, so it can no
longer be relied on post-Treble. So let's just continue on the
assumption that sw_sync is not anything anyone can rely on.

Cheers,
Daniel