Re: [Intel-gfx] [PATCH 16/21] drm/i915/gem: Delay context creation

[Jason Ekstrand <jason@jlekstrand.net>]

On Fri, Apr 30, 2021 at 1:53 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Thu, Apr 29, 2021 at 11:35 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
> >
> > On Thu, Apr 29, 2021 at 2:07 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> > >
> > > On Thu, Apr 29, 2021 at 02:01:16PM -0500, Jason Ekstrand wrote:
> > > > On Thu, Apr 29, 2021 at 1:56 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > > On Thu, Apr 29, 2021 at 01:16:04PM -0500, Jason Ekstrand wrote:
> > > > > > On Thu, Apr 29, 2021 at 10:51 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > > > > > +     ret = set_proto_ctx_param(file_priv, pc, args);
> > > > > > >
> > > > > > > I think we should have a FIXME here of not allowing this on some future
> > > > > > > platforms because just use CTX_CREATE_EXT.
> > > > > >
> > > > > > Done.
> > > > > >
> > > > > > > > +     if (ret == -ENOTSUPP) {
> > > > > > > > +             /* Some params, specifically SSEU, can only be set on fully
> > > > > > >
> > > > > > > I think this needs a FIXME: that this only holds during the conversion?
> > > > > > > Otherwise we kinda have a bit a problem me thinks ...
> > > > > >
> > > > > > I'm not sure what you mean by that.
> > > > >
> > > > > Well I'm at least assuming that we wont have this case anymore, i.e.
> > > > > there's only two kinds of parameters:
> > > > > - those which are valid only on proto context
> > > > > - those which are valid on both (like priority)
> > > > >
> > > > > This SSEU thing looks like a 3rd parameter, which is only valid on
> > > > > finalized context. That feels all kinds of wrong. Will it stay? If yes
> > > > > *ugh* and why?
> > > >
> > > > Because I was being lazy.  The SSEU stuff is a fairly complex param to
> > > > parse and it's always set live.  I can factor out the SSEU parsing
> > > > code if you want and it shouldn't be too bad in the end.
> > >
> > > Yeah I think the special case here is a bit too jarring.
> >
> > I rolled a v5 that allows you to set SSEU as a create param.  I'm not
> > a huge fan of that much code duplication for the SSEU set but I guess
> > that's what we get for deciding to "unify" our context creation
> > parameter path with our on-the-fly parameter path....
> >
> > You can look at it here:
> >
> > https://gitlab.freedesktop.org/jekstrand/linux/-/commit/c805f424a3374b2de405b7fc651eab551df2cdaf#474deb1194892a272db022ff175872d42004dfda_283_588
>
> Hm yeah the duplication of the render engine check is a bit annoying.
> What's worse, if you tthrow another set_engines on top it's probably
> all wrong then. The old thing solved that by just throwing that
> intel_context away.

I think that's already mostly taken care of.  When set_engines
happens, we throw away the old array of engines and start with a new
one where everything has been memset to 0.  The one remaining problem
is that, if userspace resets the engine set, we need to memset
legacy_rcs_sseu to 0.  I've added that.

> You're also not keeping the engine id in the proto ctx for this, so
> there's probably some gaps there. We'd need to clear the SSEU if
> userspace puts another context there. But also no userspace does that.

Again, I think that's handled.  See above.

> Plus cursory review of userspace show
> - mesa doesn't set this
> - compute sets its right before running the batch
> - media sets it as the last thing of context creation
>
> So it's kinda not needed. But also we're asking umd to switch over to
> CTX_CREATE_EXT, and if sseu doesn't work for that media team will be
> puzzled. And we've confused them enough already with our uapis.
>
> Another idea: proto_set_sseu just stores the uapi struct and a note
> that it's set, and checks nothing. To validate sseu on proto context
> we do (but only when an sseu parameter is set):
> 1. finalize the context
> 2. call the real set_sseu for validation
> 3. throw the finalized context away again, it was just for validating
> the overall thing
>
> That way we don't have to consider all the interactions of setting
> sseu and engines in any order on proto context, validation code is
> guaranteed shared. Only downside is that there's a slight chance in
> behaviour: SSEU, then setting another engine in that slot will fail
> instead of throwing the sseu parameters away. That's the right thing
> for CTX_CREATE_EXT anyway, and current userspace doesn't care.
>
> Thoughts?

I thought about that.  The problem is that they can set_sseu multiple
times on different engines.  This means we'd have to effectively build
up an arbitrary list of SSEU set operations and replay it.  I'm not
sure how I feel about building up a big data structure.

> > I'm also going to send it to trybot.
>
> If you resend pls include all my r-b, I think some got lost in v4.

I'll try and dig those up.

> Also, in the kernel at least we expect minimal commit message with a
> bit of context, there's no Part-of: link pointing at the entire MR
> with overview and discussion, the patchwork Link: we add is a pretty
> bad substitute. Some of the new patches in v4 are a bit too terse on
> that.

Yup.  I can try to expand things a bit more.

> And finally I'm still not a big fan of the add/remove split over
> patches, but oh well.

I'm not either but working through all this reminded me of why I
didn't do it more gradual.  The problem is ordering.  If add and
remove at the same time and do it one param at a time, we'll end up
with a situation in the middle where some params will only be allowed
to be set on the proto-ctx and others will force a proto-ctx ->
context conversion.  If, for instance, one UMD sets engines first and
then VMs and another sets VMs first and then engines, there's no way
to do a gradual transition without breaking one of them.  Also, we
need to handle basically all the setparam complexity in order to
handle creation structs and, again, those can come in any order.

I hate it, I just don't see another way. :-(

--Jason
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