Comment 17 for bug 316514

Binary package hint: xorg

[Problem]
X does not support (or does not support very well) displaying a desktop across more than one video card. When it is possible, a variety of bugs are experienced.

[Discussion]
Prior to the introduction of Xrandr it was (sometimes) possible to configure xorg.conf to display on multiple video cards. One could then use Xinerama, etc. to stitch screens into a contiguous desktop. Of course, any alterations to this setup usually required hand-tuning xorg.conf.

The introduction of Xrandr made a number of things much easier, however it is not good at handling the case of multiple video cards. It can handle dual-head displays, where both monitors are connected to separate outputs on the same video card, but does not work as well with outputs on two different cards.

The fundamental problem (as I understand it) is essentially that X can talk to only one physical "pool" of memory, and each card has its own physical pool. Recent developments including GEM and several kernel changes promise to remedy this by enabling these pools to be aggregated and managed as a single virtual pool.

[Exceptions]
As mentioned above, the problem is most notable with drivers that have switched from Xinerama to Xrandr. If you're using an older driver that still uses the old Xinerama approach, you *might* find it works acceptably.

In my own testing, I've found cases where I could get displays across multiple cards, but I ran into so many different bugs (mouse not working properly, X crashing, display corruption, etc.) that it was essentially unusable. Resolving those issues will likely wait until the aforementioned architecture is fully in place.