On 64-bit PowerPC, and possibly other architectures, PCI devices are mapped
above the 32-bit barrier.  When X is built as a 32-bit binary, which is typical
on PowerPC, it cannot access these devices.  X tries to map the BAR as an offset
to /dev/mem.  Since the offset is larger than the offset parameter to mmap, the
wrong address is mapped.  The result is that either random locations in system
memory are used or a bus error is generated (I've seen both).

At least on Linux, the correct solution is to use
/sys/bus/pci/devices/*/resource* instead of /dev/mem.  By doing this, the offset
to mmap would always be zero.