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

[Jason Ekstrand <jason@jlekstrand.net>]

On Thu, Apr 29, 2021 at 7:16 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Wed, Apr 28, 2021 at 01:58:17PM -0500, Jason Ekstrand wrote:
> > On Wed, Apr 28, 2021 at 12:18 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
> > >
> > > On Wed, Apr 28, 2021 at 5:13 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> > > >
> > > > On Tue, Apr 27, 2021 at 08:51:08AM -0500, Jason Ekstrand wrote:
> > > > > I sent a v2 of this patch because it turns out I deleted a bit too
> > > > > much code.  This function in particular, has to stay, unfortunately.
> > > > > When a batch is submitted with a SUBMIT_FENCE, this is used to push
> > > > > the work onto a different engine than than the one it's supposed to
> > > > > run in parallel with.  This means we can't dead-code this function or
> > > > > the bond_execution function pointer and related stuff.
> > > >
> > > > Uh that's disappointing, since if I understand your point correctly, the
> > > > sibling engines should all be singletons, not load balancing virtual ones.
> > > > So there really should not be any need to pick the right one at execution
> > > > time.
> > >
> > > The media driver itself seems to work fine if I delete all the code.
> > > It's just an IGT testcase that blows up.  I'll do more digging to see
> > > if I can better isolate why.
> >
> > I did more digging and I figured out why this test hangs.  The test
> > looks at an engine class where there's more than one of that class
> > (currently only vcs) and creates a context where engine[0] is all of
> > the engines of that class bonded together and engine[1-N] is each of
> > those engines individually.  It then tests that you can submit a batch
> > to one of the individual engines and then submit with
> > EXEC_FENCE_SUBMIT to the balanced engine and the kernel will sort it
> > out.  This doesn't seem like a use-case we care about.
> >
> > If we cared about anything, I would expect it to be submitting to two
> > balanced contexts and expecting "pick any two" behavior.  But that's
> > not what the test is testing for.
>
> Yeah ditch it.
>
> Instead make sure that the bonded setparam/ctx validation makes sure that
> 1) no virtual engines are used
> 2) no engine used twice
> 3) anything else stupid you can come up with that we should make sure is
> blocked.

I've re-introduced the deletion and I'll add nuking that test to my
IGT series.  I did it as a separate patch as the FENCE_SUBMIT logic
and the bonding are somewhat separate concerns.

As far as validation goes, I don't think we need any more for this
case.  You used FENCE_SUBMIT and didn't properly isolate things such
that the two run on different engines.  Not our problem.

--Jason
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