Re: kern/49964: DRM/KMS panic on Radeon hardware

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

A few iterations of debugging prints later, it looks like

(a) the machine has a good-looking vbios at [0xc0000, 0xdffff], shown
by `dd if=/dev/mem of=/tmp/vbios bs=1 iseek=786432 count=131072', but

(b) for some reason, pci_find_rom reads 0x00 0x00 from 0xc0000 0xc0001
instead of the expected magic numbers 0x55 0xaa, when called via
pci_map_rom/pci_map_rom_md in sys/external/bsd/drm2/include/linux/pci.h.

I'm at a loss about why this might be.

I don't think it should be necessary to enable the ROM address decoder
in the PCI BAR in order to read from 0xc0000 -- that should work even
with pre-PCI code, I think.