Re: [Intel-gfx] [PATCH 08/21] drm/i915/gem: Disallow bonding of virtual engines

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Apr 28, 2021 at 01:17:27PM -0500, Jason Ekstrand wrote:
> On Wed, Apr 28, 2021 at 1:02 PM Matthew Brost <matthew.brost@intel.com> wrote:
> >
> > On Wed, Apr 28, 2021 at 12:46:07PM -0500, Jason Ekstrand wrote:
> > > On Wed, Apr 28, 2021 at 12:26 PM Matthew Brost <matthew.brost@intel.com> wrote:
> > > > Jumping on here mid-thread. For what is is worth to make execlists work
> > > > with the upcoming parallel submission extension I leveraged some of the
> > > > existing bonding code so I wouldn't be too eager to delete this code
> > > > until that lands.
> > >
> > > Mind being a bit more specific about that?  The motivation for this
> > > patch is that the current bonding handling and uAPI is, well, very odd
> > > and confusing IMO.  It doesn't let you create sets of bonded engines.
> > > Instead you create engines and then bond them together after the fact.
> > > I didn't want to blindly duplicate those oddities with the proto-ctx
> > > stuff unless they were useful.  With parallel submit, I would expect
> > > we want a more explicit API where you specify a set of engine
> > > class/instance pairs to bond together into a single engine similar to
> > > how the current balancing API works.
> > >
> > > Of course, that's all focused on the API and not the internals.  But,
> > > again, I'm not sure how we want things to look internally.  What we've
> > > got now doesn't seem great for the GuC submission model but I'm very
> > > much not the expert there.  I don't want to be working at cross
> > > purposes to you and I'm happy to leave bits if you think they're
> > > useful.  But I thought I was clearing things away so that you can put
> > > in what you actually want for GuC/parallel submit.
> > >
> >
> > Removing all the UAPI things are fine but I wouldn't delete some of the
> > internal stuff (e.g. intel_virtual_engine_attach_bond, bond
> > intel_context_ops, the hook for a submit fence, etc...) as that will
> > still likely be used for the new parallel submission interface with
> > execlists. As you say the new UAPI wont allow crazy configurations,
> > only simple ones.
> 
> I'm fine with leaving some of the internal bits for a little while if
> it makes pulling the GuC scheduler in easier.  I'm just a bit
> skeptical of why you'd care about SUBMIT_FENCE. :-)  Daniel, any
> thoughts?

Yeah I'm also wondering why we need this. Essentially your insight (and
Tony Ye from media team confirmed) is that media umd never uses bonded on
virtual engines.

So the only thing we need is the await_fence submit_fence logic to stall
the subsequent patches just long enough. I think that stays.

All the additional logic with the cmpxchg lockless trickery and all that
isn't needed, because we _never_ have to select an engine for bonded
submission: It's always the single one available.

This would mean that for execlist parallel submit we can apply a
limitation (beyond what GuC supports perhaps) and it's all ok. With that
everything except the submit fence await logic itself can go I think.

Also one for Matt: We decided to ZBB implementing parallel submit on
execlist, it's going to be just for GuC. At least until someone starts
screaming really loudly.

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