Well I believe I have found what the problem is with tri/quad crossfire Eyefinity using 6970's on 3x 30" displays. It's a bandwidth limitation on the crossfire bridges.
If I drop the resolution down from 4800x2560 to 3150x1680, quad-fire Eyefinity magically works again. Using Heaven 2.1 which allows custom resolutions, up till about 4000x2000 quad-Eyefinity works. Above that bandwidth, it crashes.
So AMD decided to make their bridges run serial. So this means cards #4,#3,#2 data all has to pass to card #1 over a single bridge which it obviously can't handle at these high resolutions.
From TechReport: "One of those improvements is the ability to combine CrossFire multi-GPU setups with multi-monitor Eyefinity display surfaces. The appeal here is obvious, since pushing 24 megapixels with a single 5870 GPU is possible and sometimes quite workable, but not for every game. Generating that many pixels at the right quality levels would tax any single graphics chip. Making CrossFire work on this scale presents some challenges, however, as AMD readily admits. The core issue is the fact that the dedicated CrossFire interconnect used for passing completed frames between cards has "only" enough bandwidth to sustain a 2560x1600 display resolution. Even three 1080p displays will exceed its capacity."
Another huge design flaw just like the mixed Displayport/DVI-D Eyefinity screen tear design flaw. AMD has some very poor design decisions which will keep me away from them until they get their act together.
nVidia has the right way to design multi-GPU communications, parallel so that all cards can talk directly with each other:
Good thing I have 3GB GTX 580's on order so I can get rid of these 6970's!Edited by CallsignVega - 1/30/11 at 3:54pm