Introduction and updates from NVIDIA

Thu May 12 09:52:58 UTC 2016

On Thu, 12 May 2016 12:30:50 +0300
Pekka Paalanen <ppaalanen at gmail.com> wrote:

> On Wed, 11 May 2016 16:08:13 -0700
> 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.  
> 
> Hi,
> 
> I've been following this conversation from side with great interest,
> and inspite of sounding stupid, I'd like to note a couple of things.
> 
> I twitch a little every time you mention wl_drm, because it is so easy
> to understand as "the wl_drm protocol specified in Mesa", while for the
> discussion you actually mean "any Wayland-based protocol extension you
> might ever want to write for communicating between the client and the
> server sides of the EGL implementation". Could we use a different word for it, please?

(Argh, sorry, ctrl+enter slip.)

A word like "EGL_WL_bind_wayland_display" that tells what we are talking
about without tying it into a specific implementation.

Weston and apps have zero code for dealing with wl_drm.

> > 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.

Wouldn't that be provided by EGL_WL_bind_wayland_display protocol you
write to suit your needs, very much like you write the streams protocol
but without the burden of forcing everyone to use EGLStreams API?

> > 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.

GBM API is used by compositors to allocate buffers they will composite
into, for putting them via KMS for display.

I get the feeling there is some assumption that Wayland clients might
be using the GBM API. Is there? That is not the case (today). If the EGL
implementation internally uses GBM, that's up to it, but it also means
EGL implementation has all the arbitrary information available from
EGL_WL_bind_wayland_display protocol to do whatever allocation calls it
needs to do.

If you write your EGL_WL_bind_wayland_display protocol to support it,
you can swap the allocation from under an existing wl_buffer without
destroying the wl_buffer. At least I think it's possible, I have never
looked into what cornercases it might raise. But I also do not quite
see why you would need to avoid destroying a wl_buffer and making a new
one based on what your EGL_WL_bind_wayland_display protocol tells you
client-side. wl_buffer is just a handle to arbitrary (meta-)data,
anyway.

Thanks,
pq

> > 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.
> > 
> > Thanks,
> > -James
> >   
> > > Cheers,
> > > Daniel
> > >    
> > _______________________________________________
> > wayland-devel mailing list
> > wayland-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/wayland-devel  
> 

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