Wayland generic dmabuf protocol

Pekka Paalanen ppaalanen at gmail.com
Fri Jun 13 07:09:29 PDT 2014

On Fri, 13 Jun 2014 07:47:54 -0400
Rob Clark <robdclark at gmail.com> wrote:

> On Fri, Jun 13, 2014 at 7:04 AM, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > On Mon, 9 Jun 2014 14:06:33 +0300
> > Pekka Paalanen <pekka.paalanen at collabora.co.uk> wrote:
> >
> >> On Mon, 9 Jun 2014 11:00:04 +0200
> >> Benjamin Gaignard <benjamin.gaignard at linaro.org> wrote:
> >>
> >> > On my hardware the patches you have (+ this one on gstwaylandsink
> >> > https://bugzilla.gnome.org/show_bug.cgi?id=711155) allow me to do zero
> >> > copy between the hardware video decoder and the display engine. I
> >> > don't have implemented GPU yet because my hardware is able to do
> >> > compose few video overlays planes and it was enough for my tests.
> >>
> >> Right.
> >>
> >> What I have been thinking is, that the compositor must be able to use
> >> the new wl_buffer and we need to guarantee that before-hand. If the
> >> compositor fails to use a wl_buffer when the client has already
> >> attached it to a wl_surface and it is time to repaint, it is too late
> >> and the user will see a glitch. Recovering from that requires asking
> >> the client to provide a new wl_buffer of a different kind, which might
> >> take time. Or a very rude compositor would just send a protocol error,
> >> and then we'd get bug reports like "the video player just disappears
> >> when I try to play (and ps. I have an old kernel that doesn't support
> >> importing whatever)".
> >>
> >> I believe we must allow the compositor to test the wl_buffer before it
> >> is usable for the client. That is the reason for the roundtrippy design
> >> of the below proposal.
> >>
> >> Because we do not even try to communicate all the possible restrictions
> >> to the client for it to match, we can leave the validation strictly as
> >> a kernel internal issue. Buffer migration inside the kernel might even
> >> magically solve some of the mismatches. It does leave the problem of
> >> what can the client do, if it doesn't fill all the requirements for the
> >> compositor to be able to import the dmabufs. But what restrictions
> >> other than color format we can or should communicate, and where does
> >> user space get them in the first place... *hand-waving*
> >>
> >> But, this also leaves it up to the compositor to choose how/where it
> >> wants to import the dmabufs. If a compositor is usually compositing
> >> with GL, it will try to import with EGL on whatever GPU it is using. If
> >> the compositor uses a software renderer, it can try to mmap the dmabufs
> >> (or try this as a fallback, if the EGL import fails). If the compositor
> >> is absolutely sure it can rely on the hardware display engine to
> >> composite these buffers (note, buffers! You don't know to which
> >> surfaces these buffers will be attached to), it can import directly
> >> with DRM as FB objects, or V4L, or whatever. A compositor with the
> >> fullscreen shell extension but without the sub-surface extension comes
> >> to mind.
> >>
> >> In summary, the compositor must be able to use the wl_buffer in its
> >> default/fallback compositing path. If the wl_buffer is also suitable
> >> for direct scanout, e.g. on an overlay, that is "just" a bonus.
> >>
> >> With the round-trippy design, I am assuming that you can
> >> export-pass-import a set of dmabufs once, and then reuse them as long
> >> as you don't need to e.g. resize them. Is this a reasonable assumption?
> >> Are there any, for instance, hardware video decoders that just insist on
> >> exporting a new buffer for every frame?
> >>
> >> I am tracking the proposal in
> >> http://cgit.collabora.com/git/user/pq/weston.git/log/?h=linux_dmabuf
> >>
> >> So far I added back the event to advertise the supported
> >> drm_fourcc formats, since that is probably quite crucial.
> >
> > Yeah, about that...
> >
> > https://www.khronos.org/registry/egl/extensions/EXT/EGL_EXT_image_dma_buf_import.txt
> > provides no way for the compositor to query, what formats the EGL
> > implementation might support for importing dmabufs. I'm not sure GBM
> > has that yet either.
> >
> > So there is no way a compositor could advertise the set of supported
> > formats, since it has no way of knowing, has it?
> It wouldn't be too hard, I don't think, to add something at gbm level.
> > Any suggested solutions for this? Or would probing (export dmabuf, send
> > to compositor, wait for compositor to ack/reject) for suitable formats
> > be enough?
> Well, compositor could, I suppose, build up a supported formats list
> by getting a dummy buffer somehow (either from display or gpu device,
> it shouldn't really matter), and then iterate though importing that
> one buffer as different formats.  Seems easier than doing it on client
> side with a round trip to compositor each time.  And, well, I'm pretty
> out of date on wl proto stuff, but seems pretty reasonable that the
> compositor could tell the client what formats it supported..

Oh right, the compositor could probe it on its own during start-up.
That sounds good as at least a temporary measure, and if the EGL
extension will never expose them directly. Defining the EGL extension
to have to support the formats that GBM advertises feels a bit awkward.

The basic design principle in Wayland protocol is that you tell the
client all the restrictions before-hand, and if the client still breaks
them, it gets kicked out, no questions asked. The answer to all
failures is to just kill the client, makes failure handling pretty
easy protocol-wise. Both wl_shm and wl_drm pixel format support works
like that AFAIK.

Doing the same for dmabuf is more tricky, since there is a lot more
than just pixel format, which why I thought we might need an explicit
protocol for graceful failure ahead of time.


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