Sharing surfaces and multiple drivers

[Jon Smirl]

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. 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?

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

-- 
Jon Smirl
jonsmirl at gmail.com