Wayland For Very Large Wall of Screens

Jasper St. Pierre jstpierre at mecheye.net
Tue Sep 30 08:44:04 PDT 2014

It's a great question, with a complicated answer. Part of this is the fault
of the DRM kernel interface, which is being improved. Part of it is the
fault of GL/EGL, which really doesn't have proper multi-GPU support.

GPUs are exposed through the kernel as /dev/dri/card* devices, with the
first GPU being "card0", the second being "card1", and so on. All the
compositors I know of either hardcode "/dev/dri/card0" or look for a
"primary GPU".

GL/EGL on DRM devices can only be booted up on one card at a time due to
the way the interface works. I'm not sure if there's any plans to change
this. (It's possible to create multiple EGLDisplays and switch between them
to render, but this is really ridiculous, and EGL_WL_bind_display won't
work with that). That means that you're left with software rendering. Which
is totally possible to do, but Weston still doesn't support that.

Multi-GPU has been on the radar for a long time. Perhaps the progress of
Mantle/GL5 will help solve this on the hardware rendering side. And then we
can start implementing it in compositors.

There's also, then, lots of technicalities: What happens if I have a
surface that's straddled between two outputs -- who renders it? There is a
future kernel interface, called DMA-BUF, to allow passing buffers between
GPUs like this, which should help solve it.

On Wed, Aug 20, 2014 at 5:24 PM, Linux Luser <linuxluser at gmail.com> wrote:

> Xorg has been notorious for it's lack of multi-monitor support. Xinerama
> was of course shimmed into place to help make one xscreen out of many.
> Xinerama has had several drawbacks, however, one being performance, which
> is usually compensated for in the industry by purchasing beefier graphics
> cards.
> Interestingly, XRandR claims to be the Xinerama replacement but doesn't
> actually do what Xinerama does when multiple GPUs are involved. So right
> now, it's a bit of mystery as to how to get a single, large desktop spanned
> across many screens across many GPUs.
> My question is, how does Wayland address multiple screens connected from
> multiple graphics cards? Is this something that is completely and totally
> handled by the compositor? If so, what compositors support a single dekstop
> across multiple GPUs using Wayland?
> --
> dave
> _______________________________________________
> wayland-devel mailing list
> wayland-devel at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/wayland-devel

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