PCI rework

[Benjamin Herrenschmidt]

> Not sure if it matters for this discussion, but the radeon driver can't
> always map the full size of a BAR, and sometimes it wants to have a
> single mapping across two BARs to cover all of video RAM.

The single mapping accross 2 BARs is too ugly for words :) Could be done
using mmap tricks with MAP_FIXED but I wouldn't bother going there...
We don't handle that currently anyway.

Also, mapping part of a BAR is another issue. We can always _map_ it all
in the sense creating a page table mapping from a given range of user
virtual addresse to a given range of bus addresses. Wether the radeon
chip will actually decode accesses to the entire range or not is a
different matter :)

Ben.