wl_surface_commit from different thread

Jason Ekstrand jason at jlekstrand.net
Fri Feb 7 06:18:32 PST 2014

Hi Prabhu,
Could you be a little more specific as to what you are doing.  It sounds
like you are either writing a client or trying to write the client-side
wayland bits for a driver stack.  However, it's kind of hard from you
description to tell exactly what you're working on.  Nevertheless, I will
try to answer your question as best I can.

While Weston itself is single-threaded, the libwayland client library can
handle multiple threads rather well.  Look into wl_event_queue which allows
you to manage what events get called on what thread.  Also, any request can
be called from any thread.  The only issue is in the synchronization that
your app may need to do (know when wl_surface.commit occurs relative to
wl_shell_surface.set_maximized for instance).  If you want to get a frame
completion event in another thread, simply call wl_surface.frame,
wl_surface.commit and then add the callback you got from wl_surface.frame
to the wl_event_queue for that thread.

Also, I'm confused by the relationship between your eglSwapBuffers calls
and your wl_surface.commit calls.  eglSwapBuffers should be calling
wl_surface.commit before it returns.  Sometimes this means the underlying
graphics drivers pass sync fences or do other things to keep from doing a
full glFinish and stalling the GPU. Therefore, in most cases, you shouldn't
need to be calling wl_surface.commit manually.  Perhaps I'm just
misunderstanding something?

I hope that helps,
--Jason Ekstrand

On Fri, Feb 7, 2014 at 8:03 AM, Prabhu S <prabhusundar at gmail.com> wrote:

> Hello,
> eglSwapBuffers is a non blocking call. Normally, 3D GPU will be used both
> for content creation and composition, in that case everything will end up
> the GPU unit. So the end frame will be will be synchronized. (assuming 1
> GPU unit).
> I have a scenario, where 3D GPU being used for creating content and blit
> engine used for Weston composition. I'm getting frame completion from 3D
> engine in different thread (not in the same thread eglSwapBuffers calling
> thread). Currently eglSwapBuffers calling thread need to wait for the frame
> completion and then call wl_surface_commit to notify Wayland compositor.
> This takes out most of the performance since GPUs are not active all the
> time.
> wl_surface_commit cannot be called from different thread AFAIK. Some
> advise, suggestions would be appreciated.
> Thanks
> Prabhu
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> wayland-devel at lists.freedesktop.org
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