Plumbing explicit synchronization through the Linux ecosystem

Adam Jackson ajax at
Wed Mar 11 23:02:37 UTC 2020

On Wed, 2020-03-11 at 12:31 -0500, Jason Ekstrand wrote:

>  - X11: With present, it has these "explicit" fence objects but
> they're always a shmfence which lets the X server and client do a
> userspace CPU-side hand-off without going over the socket (and
> round-tripping through the kernel).  However, the only thing that
> fence does is order the OpenGL API calls in the client and server and
> the real synchronization is still implicit.

I'm pretty sure "the only thing that fence does" is an implementation
detail. PresentPixmap blocks until the wait-fence signals, but when and
how it signals are properties of the fence itself. You could have drm
give the client back a fence fd, pass that to xserver to create a fence
object, and name that in the PresentPixmap request, and then drm can do
whatever it wants to signal the fence.

> From my perspective, as a Vulkan driver developer, I have to deal with
> the fact that Vulkan is an explicit sync API but Wayland and X11
> aren't.

I'm quite sure we can give you an explicit-sync X11 API. I think you
may already have one.

- ajax

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