I have a Radeon HD4850 card, which works great with the free radeon (aka xf86-video-ati) driver - which is to say DRI and XVideo work great, and that's all I need. (I do my gaming in Windows.)
Recently I bought a second 4850 card and set them up in dual Crossfire mode. DRI and XV stopped working, saying in the xorg log that DRI was not supported with "vga arb" (i.e. vga arbitrartion between several video cards). I only wanted to use one card - no crossfire - but the mere presence of another card prevented DRI from working.
Some Googling revealed that this is a known condition, which holds for DRI with any two cards with any drivers (even when only one card is active). It's not going to fixed soon, apparently, and presumably when it *does* get fixed it'll be for DRI2 only.
The solution? Disable the second card completely, on the PCI bus level. First I find out the PCI bus ID of the cards:
Note that the main bus IDs are 1 and 2, because these are PCIe cards, and each PCIe slot has its own PCI bridge.
[danarmak@planet data]$ lspci|grep VGA
01:00.0 VGA compatible controller: ATI Technologies Inc RV770 [Radeon HD 4850]
02:00.0 VGA compatible controller: ATI Technologies Inc RV770 [Radeon HD 4850]
And here's how to remove a card from the system:
echo 1 > /sys/bus/pci/devices/0000:02:00.0/removeAfter executing that, the card is gone from lspci's output and the directory /sys/bus/pci/devices/0000:02:00.0 is gone as well. If you want it to reappear (without rebooting), do:
Googling will give you refs to other names in /sys, which I don't have. I assume the names changed in a recent kernel version. As of 2.6.32, the above 'remove' file is the one to use.
echo 1 > /sys/bus/pci/rescan