ppc64 has a global called "pci_probe_only" which, when set, prevents the
arch PCI code from calling pci_assign_unassigned_resources().  This was
cleared by pmac so far, but a bug in the definition of the variable make
that ineffective until 2.6.10, and so we never called
pci_assign_unassigned_resources().

With 2.6.10, that bug was fixed and so we now call it, which results in
some problems.  Some devices who have perfectly valid assigned addresses by
firmware end up beeing moved around anyway, which is a BAD thing can can
break boot on some machines since it breaks the relationship between
addresses in Open Firmware device-tree and actual location of PCI devices. 
(Some low level things like the PIC are ioremap'ed based on their OF
address, way before the PCI based ASIC hosting them has been found).  This
also break the "offb" default framebuffer driver since the video card ends
up beeing moved around as well.

For now, the fix is to set pci_probe_only on pmac, thus reverting to the
old behaviour.  In the long run, it would be interesting to "fix"
pci_assign_unassigned_resources() so that it does what it's name claims,
and only assigns things that have been left unassigned instead of moving
things around gratuituously...
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>