PReP and generic PCI resource assignment

[Hollis <hollis@austin.rr.com>]

I'm trying to boot _2_4_devel on a Thinkpad 850. The only problem comes when
the video controller's PCI resource 1 (mapped by the firmware to
0x0..0x00ffffff) is relocated to 0xc1000000..0xc1ffffff. [The symptom is that
all new VGA text is drawn backwards-endian, which I would not have expected
but is very visible.] That's way out of bounds for PReP IO memory, which in
bus addresses can only be 0x0..0x40000000 (in CPU addresses that's
0xc0000000..0xfeffffff or so).

How did it get moved that drastically? The problem comes from generic PCI
code, starting with pci_assign_resource (called from
pcibios_assign_resources). The actual bad line of code is in
kernel/resource.c find_resource():
	new->start = root->start;
where it starts the new resource at the bottom of the parent resource's
range.

The problem is that root in this case is the PCI memory resource of the host
bridge, which correctly starts at 0xc0000000... but NOT in bus terms. In bus
terms it starts at 0x0. (CPU physical 0xc0000000 = PCI bus 0x0 [IO mem].) So
the new resource is assigned 0xc1000000, which is written back to the BAR
with pcibios_update_resource... which is way wrong.

Now if resources could be assigned properly in the first place this code path
wouldn't be taken, but quite a few things fail (request_resource's and
pci_find_parent_resource's) and it's proving difficult to track down why
(maybe I've been staring at this too long).

Anyways, when I change "root->start" above to be 0, the symptom is fixed, but
that's clearly a hack. I don't see how anything PReP (system IO nor IO
memory) could escape generic resource assignment unscathed though.

Thoughts?

-Hollis

** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/