They don't work, as per Ben Herrenschmidt:

 "device->bus may be a host bridge, which has no bus->self -> Ooops.

  Unfortunately, there is no sane way to match a host bridge with it's
  eventual "self" device if it has any. The only way would be to scan for
  devices of class host bridge, but that isn't even 100% reliable.

  The result is that the self device (AGP bridge device) is generally a
  sibling of the actual AGP card, which is source of interesting problems,
  especially with power management."

Cset exclude: davej@delerium.kernelslacker.org|ChangeSet|20050223022533|52155