Wayland debugging with Qtwayland, gstreamer waylandsink, wayland-lib and Weston

Pekka Paalanen pekka.paalanen at haloniitty.fi
Mon Mar 4 15:50:07 UTC 2024

On Mon, 4 Mar 2024 14:51:52 +0000
Terry Barnaby <terry1 at beam.ltd.uk> wrote:

> On 04/03/2024 14:14, Pekka Paalanen wrote:
> > On Mon, 4 Mar 2024 13:24:56 +0000
> > Terry Barnaby <terry1 at beam.ltd.uk> wrote:
> >  
> >> On 04/03/2024 09:41, Pekka Paalanen wrote:  
> >>> On Mon, 4 Mar 2024 08:12:10 +0000
> >>> Terry Barnaby <terry1 at beam.ltd.uk> wrote:
> >>>     
> >>>> While I am trying to investigate my issue in the QtWayland arena via the
> >>>> Qt Jira Bug system, I thought I would try taking Qt out of the equation
> >>>> to simplify the application a bit more to try and gain some
> >>>> understanding of what is going on and how this should all work.
> >>>>
> >>>> So I have created a pure GStreamer/Wayland/Weston application to test
> >>>> out how this should work. This is at:
> >>>> https://portal.beam.ltd.uk/public//test022-wayland-video-example.tar.gz
> >>>>
> >>>> This tries to implement a C++ Widget style application using native
> >>>> Wayland. It is rough and could easily be doing things wrong wrt Wayland.
> >>>> However it does work to a reasonable degree.
> >>>>
> >>>> However, I appear to see the same sort of issue I see with my Qt based
> >>>> system in that when a subsurface of a subsurface is used, the Gstreamer
> >>>> video is not seen.
> >>>>
> >>>> This example normally (UseWidgetTop=0) has a top level xdg_toplevel
> >>>> desktop surface (Gui), a subsurface to that (Video) and then waylandsink
> >>>> creates a subsurface to that which it sets to de-sync mode.
> >>>>
> >>>> When this example is run with UseWidgetTop=0 the video frames from
> >>>> gstreamer are only shown shown when the top subsurface is manually
> >>>> committed with gvideo->update() every second, otherwise the video
> >>>> pipeline is stalled.  
> >>> This is intentional. From wl_subsurface specification:
> >>>
> >>>         Even if a sub-surface is in desynchronized mode, it will behave as
> >>>         in synchronized mode, if its parent surface behaves as in
> >>>         synchronized mode. This rule is applied recursively throughout the
> >>>         tree of surfaces. This means, that one can set a sub-surface into
> >>>         synchronized mode, and then assume that all its child and grand-child
> >>>         sub-surfaces are synchronized, too, without explicitly setting them.
> >>>
> >>> This is derived from the design decision that a wl_surface and its
> >>> immediate sub-surfaces form a seamlessly integrated unit that works
> >>> like a single wl_surface without sub-surfaces would. wl_subsurface
> >>> state is state in the sub-surface's parent, so that the parent controls
> >>> everything as if there was just a single wl_surface. If the parent sets
> >>> its sub-surface as desynchronized, it explicitly gives the sub-surface
> >>> the permission to update on screen regardless of the parent's updates.
> >>> When the sub-surface is in synchronized mode, the parent surface wants
> >>> to be updated in sync with the sub-surface in an atomic fashion.
> >>>
> >>> When your surface stack looks like:
> >>>
> >>> - main surface A, top-level, root surface (implicitly desynchronized)
> >>>     - sub-surface B, synchronized
> >>>       - sub-surface C, desynchronized
> >>>
> >>> Updates to surface C are immediately made part of surface B, because
> >>> surface C is in desynchronized mode. If B was the root surface, all C
> >>> updates would simply go through.
> >>>
> >>> However, surface B is a part of surface A, and surface B is in
> >>> synchronized mode. This means that the client wants surface A updates to
> >>> be explicit and atomic. Nothing must change on screen until A is
> >>> explicitly committed itself. So any update to surface B requires a
> >>> commit on surface A to become visible. Surface C does not get to
> >>> override the atomicity requirement of surface A updates.
> >>>
> >>> This has been designed so that software component A can control surface
> >>> A, and delegate a part of surface A to component B which happens to the
> >>> using a sub-surface: surface B. If surface B parts are further
> >>> delegated to another component C, then component A can still be sure
> >>> that nothing updates on surface A until it says so. Component A sets
> >>> surface B to synchronized to ensure that.
> >>>
> >>> That's the rationale behind the Wayland design.
> >>>
> >>>
> >>> Thanks,
> >>> pq  
> >> Ah, thanks for the info, that may be why this is not working even in Qt
> >> then.
> >>
> >> This seems a dropoff in Wayland to me. If a software module wants to
> >> display Video into an area on the screen at its own rate, setting that
> >> surface to de-synced mode is no use in the general case with this
> >> policy.  
> > It is of use, if you don't have unnecessary sub-surfaces in synchronized
> > mode in between, or you set all those extra sub-surfaces to
> > desynchronized as well.  
> Well they may not be necessary from the Wayland perspective, but from 
> the higher level software they are useful to modularise/separate/provide 
> a join for the software modules especially when software modules are 
> separate like Qt and GStreamer.

Sorry to hear that.

> >> I would have thought that if a subsurface was explicitly set to
> >> de-synced mode then that would be honoured. I can't see a usage case for
> >> it to be ignored and its commits synchronised up the tree ?  
> > Resizing the window is the main use case.
> >
> > In order to resize surface A, you also need to resize and paint surface
> > B, and for surface B you also need to resize and paint surface C. Then
> > you need to guarantee that all the updates from surface C, B and A are
> > applied atomically on screen.
> >
> > Either you have component APIs good enough to negotiate the
> > stop-resize-paint-resume on your own, or if the sub-components are
> > free-running regardless of frame callbacks, component A can just
> > temporarily set surface B to synchronized, resize and reposition it,
> > and resume.  
> I would have thought that the Wayland server could/would synchronise 
> screen updates when a higher level surface is resized/moved by itself.

If the whole window is moved, yes. Clients won't observe the
window moving even if they wanted to.

But a compositor cannot resize anything. Resizing always requires the
client to respond with the surface drawn in the new size before it can
actually happen. Or a whole bunch of surfaces atomically, if you use

> As the software components are separately developed systems it is 
> difficult to sync between them without changing them, but may be possible.

Yes, Wayland does many things differently than older toolkits


> > Is Gst waylandsink API the kind that it internally creates a new
> > wl_surface for itself and makes it a sub-surface of the given surface,
> > or is there an option to tell Gst to just push frames into a given
> > wl_surface?
> >
> > If the former, then waylandsink is supposed to somehow give you an API
> > to set the sub-surface position and z-order wrt. its parent and
> > siblings. If the latter, you would create wl_subsurface yourself and
> > keep control of it to set the sub-surface position and z-order.
> >
> > Either way, the optimal result is one top-level wl_surface, with one
> > sub wl_surface drawn by Gst, and no surfaces in between in the
> > hierarchy.  
> Yes, the Gst waylandsink API creates a new subsurface for itself from 
> the GUI's managed surface to separate itself from the GUI (Qt/Gnomes) 
> surfaces. It doesn't allow you to provide a surface to directly use. I 
> don't think it allows the surface to be moved/resized although it can 
> display video at an offset and size as far as I know (although it may 
> actually change the surface to do this I will have a look). It doesn't 
> allow the z-order to be changed I think. It expects the GUI to change 
> its surface and I guess assumes its subsurface would effectively move in 
> z and xy position due to the GUI moving/raising/lowering its surface 
> (the parent) in a similar manner to how X11 would have done this.

Sounds like gst waylandsink is lacking z-ordering API.

Wayland sub-surfaces are very different from X11 windows. One
fundamental difference is that sub-surfaces can extend beyond their
parent's area. Another is that sub-surfaces always have their own
storage (because you have to explicitly attach wl_buffers to them),
they cannot address the parent's storage like in X11. And more.

X11 windows were perhaps meant for individual widgets like buttons to
optimise drawing and input handling. Wayland sub-surfaces are meant for
things that need a separate wl_buffer in order to be off-loaded to DRM
KMS hardware for direct scanout. It's like the opposite ends of the
granularity spectrum of off-loading things to the display server.

> I will try the middle desync and/or this method by managing the 
> waylandsink surface outside of waylandsink if I can and if it doesn't 
> mess up either Qt's or waylandsink's operations.
> Thanks for the input.

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