Sharing surfaces and multiple drivers

[Brian Paul]

Jon Smirl wrote:
> I haven't worked with PCI Express yet so I haven't figured out how
> video works on it. How do surfaces work on PCI Express? There are
> motherboards now (big $$, quad Opteron, Nforce4) that can support 16
> full 16 lane slots. That lets you plug 16 top of the line cards into
> the system.

I don't think there's any motherboards quite like that yet.  The max 
I've seen is something like support for two x16 PCI Express graphics 
cards, plus a few more lanes for a few other slots.


> Since AGP is gone on PCI Express I believe that PCI
> Express video cards can treat all of system memory as if it were AGP
> space. This creates yet another environment for sharing surfaces. In
> this environment surfaces that aren't in VRAM are sharable by all
> screens.
> 
> Sharable surfaces has lots of impact on the compositing manager. For
> example where should the surface reside for a window that is going to
> be composed onto two screens on two different cards.
> 
> Should you use hardware to draw to VRAM and copy it out for the other card?
> Should you copy it to system memory or VRAM on the other card?
> Or draw it in system memory using hardware?
> Or draw it in system memory using software?

Good questions.

Perhaps a layered EGL extension could be developed to support sharing 
of surfaces between devices.


> The compositing manager will probably need to measure these on startup
> and then adjust its algorithms.
> 

-Brian