[Mesa-dev] [PATCH 2/3] iris: Create a composite context for both compute and render pipelines

[Chris Wilson]

Quoting Kenneth Graunke (2019-03-26 05:52:10)
> On Monday, March 25, 2019 3:58:59 AM PDT Chris Wilson wrote:
> > iris currently uses two distinct GEM contexts to have distinct logical
> > HW contexts for the compute and render pipelines. However, using two
> > distinct GEM contexts implies that they are distinct timelines, yet as
> > they are a single GL context that implies they belong to a single
> > timeline from the client perspective. Currently, fences are occasionally
> > inserted to order the two timelines. Using 2 GEM contexts, also implies
> > that we keep 2 ppGTT for identical buffer state. If we can create a
> > single GEM context, with the right capabilities, we can have a single
> > VM, a single timeline, but 2 logical HW contexts for the 2 pipelines.
> > 
> > This is allowed through the new context interface that allows VM to be
> > shared, timelines to be specified, and for the logical contexts to be
> > constructed as the user desires.
> > 
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Cc: Kenneth Graunke <kenneth at whitecape.org>
> > ---
> >  src/gallium/drivers/iris/iris_batch.c   | 16 ++-----
> >  src/gallium/drivers/iris/iris_batch.h   |  5 +--
> >  src/gallium/drivers/iris/iris_context.c | 56 ++++++++++++++++++++++++-
> >  3 files changed, 60 insertions(+), 17 deletions(-)
> 
> Hi Chris,
> 
> I don't think that I want the single timeline option.  It seems like
> we've been moving away from implicit sync for a long time, and the
> explicit sync code we have is pretty straightforward and seems to do
> the trick.  Jason and I also chatted briefly, and we don't necessarily
> want to a strict submission-order between render/compute.

I disagree if you think this means more implicit sync. It is setting up
the GEM context to an exact match of the GL context, by _explicit_
control of the timeline. Then the fences you do export from inside the
GL context do not need to be faked to be a composite of the pair of
contexts. You still have explicit fences, and you have explicit control
over the definition of their timeline.

> Separating the VMA from the context state image seems like absolutely
> the right thing to do - as you said, they're separate in hardware,
> and no real reason to tie it together.  I would be in favor of new
> uABI for that.
> 
> I don't think there will be much overhead reduction from sharing the
> VMA here though.  It's very plausible that the compositor might want
> to run between render and compute batches, at which point we end up
> doing page directory loads anyway.  I have also heard rumors about bit
> 47 becoming magical at some point which may prohibit us from sharing...

Yeah, but that doesn't actually affect the context setup, just how you
decide to use it in end. And by that point, you'll be forced into using
this new uABI anyway or something entirely different :-p
 
> Context cloning seems OK, but I'm always pretty hesitant to add new
> uABI unless it's strictly necessary.  In this case, we can do the same
> thing with a little bit of userspace code, so I'm not sure it's worth
> adding that...

Actually you cannot do the same without some of the new uABI either,
since previously you did not have all the parameters exposed.
 
> I would love to see an iris patch to use the new
> I915_CONTEXT_PARAM_RECOVERABLE option without the other dependencies.

https://gitlab.freedesktop.org/ickle/mesa/commit/84d9cb1d8d98a50dcceea19ccbc3836b15cf73ae
-Chris