Introduction and updates from NVIDIA

Wed May 11 23:55:35 UTC 2016

On Wed, May 11, 2016 at 7:08 PM James Jones <jajones at nvidia.com> wrote:

> On 05/11/2016 02:31 PM, Daniel Stone wrote:
> > Hi James,
> >
> > On 11 May 2016 at 21:43, James Jones <jajones at nvidia.com> wrote:
> >> On 05/04/2016 08:56 AM, Daniel Stone wrote:
> >>> Right - but as with the point I was making below, GBM _right now_ is
> >>> more capable than Streams _right now_. GBM right now would require API
> >>> additions to match EGLStreams + EGLSwitch + Streams/KMS-interop, but
> >>> the last two aren't written either, so. (More below.)
> >>
> >> The current behavior that enables this, where basically all Wayland
> buffers
> >> must be allocated as scanout-capable, isn't reasonable on NVIDIA
> hardware.
> >> The requirements for scanout are too onerous.
> >
> > I think we're talking past each other, so I'd like to pare the
> > discussion down to these two sentences, and my two resultant points,
> > for now:
> >
> > I posit that the Streams proposal you (plural) have put forward is, at
> > best, no better at meeting these criteria:
> >    - there is currently no support for direct scanout from client
> > buffers in Streams, so it must always pessimise towards GPU
> > composition
> >    - GBM stacks can obviously do the same: implement a no-op
> > gbm_bo_import, and have your client always allocate non-scanout
> > buffers - presto, you've matched Streams
> >
> > I posit that GBM _can_ match the capability of a hypothetical
> > EGLStreams/EGLSwitch implementation. Current _implementations_ of GBM
> > cannot, but I posit that it is not a limitation of the API it exposes,
> > and unlike Streams, the capability can be plumbed in with no new
> > external API required.
> >
> > These seem pretty fundamental, so ... am I missing something? :\ If
> > so, can you please outline fairly specifically how you think
> > non-Streams implementations are not capable of meeting the criteria in
> > your two sentences?
>
> I respect the need to rein in the discussion, but I think several
> substantive aspects have been lost here.  I typed up a much longer
> response below, but I'll try to summarize in 4 sentences:
>
> GBM could match the allocation aspects of streams used in Miguel's first
> round of patches.  However, I disagree that its core API is sufficient
> to match the allocation capabilities of EGLStream+EGLSwitch where all
> producing and consuming devices+engines are known at allocation time.
> Further, streams have additional equally valuable functionality beyond
> allocation that GBM does not seem intended to address.  Absent
> agreement, I believe co-existence of EGLStreams and GBM+wl_drm in
> Wayland/Weston is a reasonable path forward in the short term.
>
> The longer version:
>
> GBM alone can not perform as well as EGLStreams unless it is extended
> into something more or less the same as EGLStreams, where it knows
> exactly what engines are being used to produce the buffer content (along
> with their current configuration), and exactly what
> engines/configuration are being used to consume it.  This implies
> allocating against multiple specific objects, rather than a device and a
> set of allocation modifier flags, and/or importing an external
> allocation and hoping it meets the current requirements.  From what I
> can see, GBM fundamentally understands at most the consumer side of the
> equation.
>
> Suppose however, GBM was taught everything streams know implicitly about
> all users of the buffers at allocation time.  After allocation, GBM is
> done with its job, but streams & drivers aren't.
>
> The act of transitioning a buffer from optimal "producer mode" to
> optimal "consumer mode" relies on all the device & config information as
> well, meaning it would need to be fed into the graphics driver (EGL or
> whatever window system binding is used) by each window system the
> graphics driver was running on to achieve equivalent capabilities to
> EGLStream.
>
> Fundamentally, the API-level view of individual graphics buffers as raw
> globally coherent & accessible stores of pixels with static layout is
> flawed.  Images on a GPU are more of a mutating spill space for a
> collection of state describing the side effects of various commands than
> a 2D array of pixels.  Forcing GPUs to resolve an image to a 2D array of
> pixels in any particular layout can be very inefficient.  The
> GL+GLX/EGL/etc. driver model hides this well, but it breaks down in a
> few cases like EGLImage and GLX_EXT_texture_from_pixmap, the former not
> really living up to its implied potential because of this, and the
> latter mostly working only because it has a very limited domain where
> things can be shared, but still requires a lot of platform-specific code
> to support properly.  Vulkan brings a lot more of this out into the open
> with its very explicit image state transitions and limitations on which
> engines can access an image in any given state, but that's just within
> the Vulkan API itself (I.e., strictly on a single GPU and optionally an
> associated display engine within the same driver & process) so far.
>
> The EGLStream encapsulation takes into consideration the new use cases
> EGLImage, GBM, etc. were intended to address, and restores what I
> believe to be the minimal amount of the traditional GL+GLX/EGL/etc.
> model, while still allowing as much of the flexibility of the "a bunch
> of buffers" mental model as possible.  We can re-invent that with GBM
> API adjustments, a set of restrictions on how the buffers it allocates
> can be used, and another layer of metadata being pumped into drivers on
> top of that, but I suspect we'd wind up with something that looks very
> similar to streams.
>
> We're both delving into future developments and hypotheticals to some
> degree here.  If we can't agree now on which direction is best, I
> believe the right solution is to allow the two to co-exist and compete
> collegially until the benefits of one or the other become more apparent.
>
  The Wayland protocol and Weston compositor were designed in a manner
> that makes this as painless as possible.  It's not like we're going to
> get a ton of Wayland clients that suddenly rely on EGLStream.  At worst,
> streams lose out and some dead code needs to be deleted from any
> compositors that adopted them.  As we discussed, there is some
> maintenance cost to having two paths, but I believe it is reasonably
> contained.
>

Hi,

I've been following this thread for some time, and you've raised some
interesting points. This one in particular concerns me, however. As I
understand it, you're proposing your stream-based approach which would
exist alongside the current standard (and universally-used) GBM.
Additionally, in order to run on your specific brand of hardware, all
toolkit and compositor authors would need to implement your proposed
streams functionality otherwise only software rendering would be available?

If this is true then it seems a bit strange to me that, despite still
speaking in hypothetical terms about future developments in both GBM and
streams, you're stating that GBM cannot be improved to match the
functionality of your proposed approach and are instead advocating that
everyone who has already written support for GBM now also support streams.

As someone with more than a casual interest in both toolkit and compositor
development, I'd like to see the best approach succeed, but I don't see any
fundamental blocker to providing the functionality you've described in GBM,
and I'm not overly enthusiastic about someone requiring even more work from
those who write toolkits and compositors, especially when having "full"
Wayland support is already such an enormous undertaking.

If I'm misunderstanding things, I'd appreciate some clarifications.

Thanks,
Mike

> Thanks,
> -James
>
> > Cheers,
> > Daniel
> >
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