[PATCH wayland-protocols v7] Add zwp_linux_explicit_synchronization_v1

Mon Nov 26 15:14:45 UTC 2018

Hi Pekka,

Yes, sorry, I was writing specifically about Weston implementation. In the
merge request from Alexandros the actual compatibility check is in the main
compositor, while compositor doesn't have enough information to decide
whether the selected renderer can handle buffer+fence combination or not.

As for the opaque wl_buffer, that's an internal implementation detail of
Vulkan WSI or EGL so different drivers can choose to do different things
here. It's the 3D driver, and *NOT* the client application, that creates
those buffers and sends attach/damage/commit sequence to the compositor.
The 3D driver makes a decision what type of buffer to use, an EGL or Vulkan
client application doesn't have any means of accessing wl_buffer objects,
it's all hidden away.

On the compositor side buffers are received through one of the protocol
extensions you've mentioned. The compositor has a choice to make. It can
either probe the wl_buffer for known buffer types or leave that task to the
EGL implementation. Let's say a client-side driver uses dma-buf buffers for
its swapchain images. If the compositor knows how to handle dma-buf, it can
either directly access those buffers or it can hand them over to the
compositor-side 3D driver through
eglCreateImage(dpy,  EGL_LINUX_DMA_BUF_EXT, ...). If the compositor doesn't
know the particular type of buffer, it can check if EGL can handle it
either through eglCreateImage(dpy,  EGL_WAYLAND_BUFFER_WL, ...) or
eglQueryWaylandBufferWL(...).
If the client and compositor use the same 3D driver (that's the most likely
scenario) this is bound to work. In multiple-GPU configurations your
mileage may vary though.

If the EGL implementation can use the type of the wl_buffer and can import
the fence fd, you're home. Having said that, I can't think of a way of
figuring out ahead of time what type of wl_buffer the client-side driver is
going to use for its swapchain and whether this particular type will be
accepted by the compositor-side driver.

This is how I use it at the moment: I've written a custom Weston backend
because the code runs on top of an embedded middleware. My backend always
uses GL renderer. The GL renderer has to call eglBindWaylandDisplayWL() at
startup, and the implementation of that API in the 3D driver adds a custom
Wayland protocol extension for sharing buffers. Now the scene is set. When
a Wayland client application starts, the EGL or Vulkan WSI implementation
driver goes for that extension and bails out if unavailable. This way
swapchain buffers from EGL and Vulkan client can be used by Weston's GL
renderer without any knowledge of the embedded platform details.

With regards to using fences directly by the client app, I guess it's the
same principle as drawing into the window. Either client app does
everything "by hand" or lets the Vulkan or EGL/GLES do it. If the app is in
charge, the app manages the window swap chain buffers and synchronization,
otherwise the 3D driver does it. You shouldn't allow more than one thing
managing the Wayland window at the same time. Perhaps you could use wording
similar to Vulkan WSI or EGL window surface when describing what is and
what isn't allowed when it comes to Wayland windows.

Cheers,
Tomek

On Mon, 26 Nov 2018 at 09:22, Pekka Paalanen <ppaalanen at gmail.com> wrote:

> On Fri, 23 Nov 2018 16:26:19 +0000
> Tomek Bury <tomek.bury at gmail.com> wrote:
>
> > Hi Pekka,
> >
> > > I presume you have a driver stack that relies on the opaque EGL
> buffers
> > and not zwp_linux_dmabuf any time soon?
> > Yes, exactly. I've added a protocol extension for sharing those buffers
> and
> > our eglCreateImage() implementation can import such buffers into the
> driver
> > on the compositor end. The buffers are carried by an fd to the compositor
> > that's the only similarity. They're not dma-buf.
> >
> > > Yeah, support for opaque EGL buffers could be added, just need to
> think
> > of a good wording, since acquire fences do not make sense for all buffer
> > types.
>
> > Isn't that renderer's and/or backend's decision? The GL renderer can
> accept
> > fence with any buffer it can send to the 3D driver, so, effectively,
> > anything backed by available EGL image extensions. Someone may add a
> custom
> > backend and/or renderer using whatever hardware or API they have at
> hand. A
> > Vulkan renderer could potentially use fences with anything a Vulkan
> driver
> > is capable of importing. A renderer that does the CPU wait could be
> useful
> > at least for debugging. So I wouln't block the explicit sync at the
> > compositor level based on the white list.
>
> Hi Tomek,
>
> fences do not make sense to all buffer types to begin with, today. My
> objection is to allowing fencing buffer types that cannot be sent to
> the 3D driver, e.g. wl_shm which is usually copied through glTexImage2D
> and friends. We cannot ignore those in the spec language.
>
> A renderer (a compositor really, we're not talking about just Weston)
> decides what buffer types it accepts, yes. This is communicated to
> clients through which buffer factory interface globals are being
> advertised. Each type is a different protocol extension. The fence
> extension OTOH is just a single extension, and currently there is no
> protocol to negotiate which buffer types are usable with acquire
> fences. The first attempt is to define in the spec language what will
> always be supported, provided the buffer factory exists.
>
> The opaque EGL buffer type is really just one type in practise:
> compositors and clients use it through a well-known, single API: EGL.
> It does not matter that there are multiple incompatible EGL
> implementations, it all looks like just one opaque buffer type to
> compositors. I think this makes it easier to extend the fence spec
> wording to require opaque EGL buffers to be supported.
>
> Either the fence protocol spec needs to be clear on what works, or we
> need advertisement events to let clients know in advance what the
> compositor supports. A client sending a fence that the compositor
> cannot use must not be possible; compositor, client, EGL, driver, etc.
> bugs notwithstanding.
>
> Btw. I just realized that if client-side EGL uses the fence extension
> internally, that means the client app code must not attempt to add or
> request fences of its own, because the spec disallows multiple acquire
> fences and multiple release notification requests. I suppose that's
> fine?
>
> Alf, can you come up with changes to the spec wording and Weston to
> require opaque EGL buffers are supported?
>
> On one hand it is actually a little strange to couple opaque EGL
> buffers (a private, EGL implementation specific protocol interface)
> with a generic fencing extension, but maybe that is necessary because
> there is not enough compositor-side GBM and EGL API so that the EGL
> implementation could handle it all in an EGL implementation specific
> interface?
>
>
> Thanks,
> pq
>
> > On Fri, 23 Nov 2018 at 13:47, Pekka Paalanen <ppaalanen at gmail.com>
> wrote:
> >
> > > On Fri, 23 Nov 2018 13:07:37 +0000
> > > Tomek Bury <tomek.bury at gmail.com> wrote:
> > >
> > > > Hi Alexandros,
> > > >
> > > > Sorry for a delay. I've finally got an end-to-end system to test it
> out.
> > > It
> > > > took some time because Weston backend I wrote a while back needed
> serious
> > > > rework to catch up with latest changes.
> > > >
> > > > There's one thing that didn't work for me. In compositor you reject
> > > > anything that isn't a DMA buffer and then in glrenderer you put an
> extra
> > > > assertion. Why? All you do is use an EGL extension in order to import
> > > > external fence_fd. There's no dmabuf dependency there. As long as
> the EGL
> > > > implementation exposes EGL_SYNC_NATIVE_FENCE_ANDROID extension this
> > > should
> > > > "just work" (tm) for the GL renderer. It certainly did for me.
> CPU-based
> > > > renderers can poll() to wait.
> > >
> > > Hi Tomek,
> > >
> > > with Weston it was decided not to implement a poll() based wait at
> > > first as implementing that properly (not blocking the compositor) would
> > > be a big hassle and no-one could see the benefit of it given what
> > > clients could actually produce.
> > >
> > > Therefore the acquire fence can only apply to buffers which can be
> > > pipelined to a GPU. Mesa EGL is using zwp_linux_dmabuf, but the support
> > > could be extended to opaque EGL buffers very well. We just chose to
> > > start small and bring up the infrastructure around fences first.
> > >
> > > Restrictions on buffer types etc. can certainly be lifted in the future
> > > if there are good use cases. I presume you have a driver stack that
> > > relies on the opaque EGL buffers and not zwp_linux_dmabuf any time
> soon?
> > >
> > > Would anyone ever use an acquire fence with wl_shm buffers? That sounds
> > > fundamentally wrong to me as one cannot create fences to be signalled
> > > by userspace AFAIK. Therefore buffers whose wait cannot be pipelined to
> > > the GPU or the display device do not make much sense to me.
> > >
> > > > The type of buffer used is an orthogonal problem. The
> > > > EGL_WL_bind_wayland_display
> > > > extension takes care of GL clients' buffers in GL renderer, for
> anything
> > > > else the renderer needs to know how to get pixels and use whatever
> means
> > > to
> > > > put those pixels on screen.
> > >
> > > Yeah, support for opaque EGL buffers could be added, just need to think
> > > of a good wording, since acquire fences do not make sense for all
> > > buffer types. A compositor must be allowed to raise protocol errors for
> > > fence+buffer combinations it cannot use, which means that clients must
> > > know in advance what they cannot use.
> > >
> > >
> > > Thanks,
> > > pq
> > >
> > > > On Tue, 13 Nov 2018 at 09:33, Tomek Bury <tomek.bury at gmail.com>
> wrote:
> > > >
> > > > > Thanks!
> > > > >
> > > > > On Tue, Nov 13, 2018 at 9:08 AM Alexandros Frantzis <
> > > > > alexandros.frantzis at collabora.com> wrote:
> > > > >
> > > > >> On Mon, Nov 12, 2018 at 12:39:58PM +0000, Tomek Bury wrote:
> > > > >> > On Mon, Nov 12, 2018 at 11:15 AM Daniel Stone <
> daniel at fooishbar.org>
> > >
> > > > >> wrote:
> > > > >> >
> > > > >> > > On Fri, 9 Nov 2018 at 10:48, Pekka Paalanen <
> ppaalanen at gmail.com>
> > >
> > > > >> wrote:
> > > > >> > > > I can add that while pushing upstream, if there are no
> other
> > > changes
> > > > >> > > > coming.
> > > > >> > > >
> > > > >> > > > Reviewed-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk
> >
> > > > >> > > >
> > > > >> > > > You have ensured that the C files generated from this
> revision
> > > build
> > > > >> > > > fine in Weston, right?
> > > > >> > > >
> > > > >> > > > David, Daniel, since your name is in the maintainers, can
> I
> > > have
> > > > >> your
> > > > >> > > > R-b, please?
> > > > >> > >
> > > > >> > > The protocol is:
> > > > >> > > Reviewed-by: Daniel Stone <daniels at collabora.com>
> > > > >> > >
> > > > >> > > The Weston implementation looks pretty good so far, though
> > > there's no
> > > > >> > > full implementation of release yet.
> > > > >> > >
> > > > >> > > Cheers,
> > > > >> > > Daniel
> > > > >> > > _______________________________________________
> > > > >> > > wayland-devel mailing list
> > > > >> > > wayland-devel at lists.freedesktop.org
> > > > >> > > https://lists.freedesktop.org/mailman/listinfo/wayland-devel
> > > > >> > >
> > > > >> >
> > > > >> > HI Daniel,
> > > > >> >
> > > > >> > Where can I find the work-in-progress implementation? I'd like
> to
> > > try it
> > > > >> > out with Broadcom driver which doesn't have implicit
> cross-process
> > > > >> sync. I
> > > > >> > can add the explicit sync protocol implementation on the
> driver
> > > side but
> > > > >> > I'd need a reference to test it against.
> > > > >> >
> > > > >> > Cheers,
> > > > >> > Tomek
> > > > >>
> > > > >> Hi Tomek,
> > > > >>
> > > > >> the WIP implementation can be found here [1]. I hope to push an
> > > update,
> > > > >> including some zwp_buffer_release_v1 correctness fixes, in the
> > > following
> > > > >> days.
> > > > >>
> > > > >> Thanks,
> > > > >> Alexandros
> > > > >>
> > > > >> [1]
> https://gitlab.freedesktop.org/wayland/weston/merge_requests/32
> > > > >>
> > > > >
> > >
> > >
>
>
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