Multiple output devices in weston

[Pekka Paalanen]

On Mon, 07 Dec 2015 13:29:02 -0500
anders at acorn.pw wrote:

> From the man page for weston-drm:
> 
> >Combining multiple graphics devices are not supported yet.  
> 
> Is there any plan for implementing this or no interest? What would be
> involved in adding support for multiple output devices?

Hi,

I'm not aware of a solid plan written anywhere. Surely there is
interest, but not high enough on anyone's TODO so far to my knowlege.

This question is two-fold:
- compositor support
- full client support

Compositor support would involve being able to open multiple DRM KMS
device nodes and lighting up all outputs across several cards. This is
the smaller part of the two. With DRM-backend and the Pixman-renderer,
it should be mostly just typing. With GL-renderer you will need to start
considering how to get one GPU use the textures from another, or how to
get the other GPU scan out the image rendered with this GPU. All the
infrastructure for that should be in place though, as PRIME and
reverse-PRIME are already working things on Xorg AFAIK. The question
left open is which GPU does a client use.

Full client support is more complicated. The compositor opens one or
more GPU devices and supports EGL-clients on any set of them. Clients
would need to be informed somehow which devices can be supported.
Clients also need API for choosing the GPU to render on (there are
several EGL extensions brewing). A client might even choose a GPU the
compositor is not using at all, in which case you need an alternative
buffer sharing path, e.g. dmabuf. To use dmabuf, you then need a way to
export rendered dmabufs from EGL, or rather create compositor-supported
dmabufs somehow and import them to EGL for rendering in the client. How
to hide all that behind a generic EGL API is certainly unclear to me.

Full client support touches a lot of things, and on some areas there is
work going on already, if slow on interest, for example:
http://patchwork.freedesktop.org/patch/55444/
Support code in libdrm seems to have already landed.


Thanks,
pq
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