Re: [Mesa-dev] Linux Graphics Next: Userspace submission update

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jun 02, 2021 at 05:38:51AM -0400, Marek Olšák wrote:
> On Wed, Jun 2, 2021 at 5:34 AM Marek Olšák <maraeo@gmail.com> wrote:
> 
> > Yes, we can't break anything because we don't want to complicate things
> > for us. It's pretty much all NAK'd already. We are trying to gather more
> > knowledge and then make better decisions.
> >
> > The idea we are considering is that we'll expose memory-based sync objects
> > to userspace for read only, and the kernel or hw will strictly control the
> > memory writes to those sync objects. The hole in that idea is that
> > userspace can decide not to signal a job, so even if userspace can't
> > overwrite memory-based sync object states arbitrarily, it can still decide
> > not to signal them, and then a future fence is born.
> >
> 
> This would actually be treated as a GPU hang caused by that context, so it
> should be fine.

This is practically what I proposed already, except your not doing it with
dma_fence. And on the memory fence side this also doesn't actually give
what you want for that compute model.

This seems like a bit a worst of both worlds approach to me? Tons of work
in the kernel to hide these not-dma_fence-but-almost, and still pain to
actually drive the hardware like it should be for compute or direct
display.

Also maybe I've missed it, but I didn't see any replies to my suggestion
how to fake the entire dma_fence stuff on top of new hw. Would be
interesting to know what doesn't work there instead of amd folks going of
into internal again and then coming back with another rfc from out of
nowhere :-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch