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

Terry Barnaby terry1 at beam.ltd.uk
Mon Mar 4 13:24:56 UTC 2024

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 

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. 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 ?

So is there a way to actually display Video on a subsurface many levels 
deep in a surface hierarchy, would setting all of the surfaces up to the 
subsurface just below the desktop top level one work (although not ideal 
as it would mean overriding other software modules surfaces at the 
Wayland level) ?

Or can desynced subsurfaces really only work to one level deep ?

If it is just one subsurface level deep that video can be displayed, I 
guess I will have to get GStreamers waylandsink to create its subsurface 
off the top most surface and add calls to manage its surface from my 
app. Or maybe get waylandsinks subsurface and manipulate it behind 
waylandsinks back. Not sure what this will do to the Qt level though. 
Using the QWidgets subsurface as its base should have allowed isolation 
to a degree between the Qt and wayland sink sub systems, its a much more 
modular approach.

There is no documentation on this Wayland restriction in waylandsink or 
other places, I can try and feed this back.

Oh for X11 !

>> Waylandsink is stuck in a loop awaiting a Wayland
>> callback after committing its subsurface.
>> If the Video Widget is a top level widget (UseWidgetTop=1) this works
>> fine (only one subsurface deep ?).
>> I have tried using both Gstreamer's waylandsink and glimagesink
>> elements, both show the same issue.
>> This seems to suggest that the de-synced subsurface system is not
>> working properly with Weston, I miss-understand how this should work or
>> I have a program error.
>> This has been tested on Fedora37 running Weston 10.0.1 under KDE/Plasma/X11.
>> 1. Should de-synced subsurfaces under other subsurfaces work under
>> Weston 10.0.1 ?
>> 2. Do I miss-understand how this should work ?
>> 3. Do I have some coding issue (sorry the code is a bit complex with
>> Wayland callbacks and C++ timers etc) ?

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