Introduction and updates from NVIDIA

[Jonas Ådahl]

On Mon, Apr 04, 2016 at 04:27:56PM +0100, Daniel Stone wrote:
> Hi,
> 
> On 2 April 2016 at 01:28, Andy Ritger <aritger at nvidia.com> wrote:
> > On Tue, Mar 29, 2016 at 05:44:41PM +0100, Daniel Stone wrote:
> >> On 23 March 2016 at 00:12, Andy Ritger <aritger at nvidia.com> wrote:
> >> > Also, mailbox mode versus FIFO mode should essentially equate to Vsync
> >> > off versus Vsync on, respectively.  It shouldn't have anything to do
> >> > with the benefits of streams, but mailbox mode is a nice feature for
> >> > benchmarking games/simulations or naively displaying your latest &
> >> > greatest content without tearing.
> >>
> >> I agree it's definitely a nice thing to have, but it does bring up the
> >> serialisation issue: we expect any configuration performed by the
> >> client (say, wl_surface::set_opaque_area to let the compositor know
> >> where it can disable blending) to be fully in-line with buffer
> >> attachment. The extreme case of this is resize, but there are quite a
> >> few valid cases where you need serialisation.
> >>
> >> I don't know quite off the top of my head how you'd support mailbox
> >> mode with Streams, given this constraint - you need three-way feedback
> >> between the compositor (recording all associated surface state,
> >> including subsurfaces), clients (recording the surface state valid
> >> when that buffer was posted), and the Streams implementation
> >> (determining which frames to dequeue, which to discard and return to
> >> the client, etc).
> >
> > It is possible we don't get that all completely right in our implementation, yet.
> 
> Again this comes down to the synchronisation. In this case, assuming a
> mailbox stream:
>   - wl_egl_surface_resize(w1, h1)
>   - gl*()
>   - eglSwapBuffers() <- commit 1
>   - wl_egl_surface_resize(w2, h2)
>   - gl*()
>   - eglSwapBuffers() <- commit 2
> 
> For this, you would need some kind of synchronisation, to ensure that
> processing commit 1 didn't pick up on the differently-sized frames for
> commit 2.

Just to point out the obvious, using wl_egl_surface_resize as a
barrier/separator/synchronization-triggerer is not enough for this. The
mailbox vs FIFO mode needs to be tightly coupled with subsurface
asynchronous/synchronous mode. For example, ignoring the resize calls
in the above code, commit 1 and commit 2 might have drawn UI elements
that are expected to be aligned with subsurface that were moved.


Jonas