Special handling of sysfs device resource files?

[Ian Romanick <idr@us.ibm.com>]

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I'm in the process of modifying X to be civilized in it's handling of
PCI devices on Linux.  As part of that, I've modified it to map the
/sys/bus/pci/device/*/resource[0-6] files instead of mucking about with
/dev/mem.

This seems to mostly work, but I am having one problem.  I map the
region by opening the file with O_RDWR, then mmap with
(PROT_READ|PROT_WRITE) and MAP_SHARED.  In all cases, the open and mmap
succeed.  However, for I/O BARs, the resulting pointer from mmap is
invalid.  Any access to it results in a segfault and GDB says it's "out
of range".

The base address of the BAR is page aligned, so its not a problem with
the alignment of mmap vs. the alignment of the BAR.  What else could it
be?  I'm pretty stumped.
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