On FreeBSD/i386, when you use the SYS_RES_MEMORY resource to allocate

a PCI memory mapped region, rman_get_bushandle() returns what happens
to be a kernel virtual address pointing to the base of the PCI shared
memory window. However this is not the behavior on all platforms:
the only thing you should do with the bushandle is pass it to the
bus_spare_read()/bus_space_write() routines. If you actually do want
the kernel virtual address of the base of the PCI memory window, you
need to use rman_get_virtual().

The problem is that at the moment, rman_get_virtual() returns a physical
address, which is bad. In order to get the kernel virtual address we
need, we have to play with it a little.

Presumeably this behavior will be changed, but in the meantime the
Tigon driver won't work. So for the moment, I'm adding a kludge to
make things happy on the alpha: the correct kernel virtual address
is calculated from the value returned by rman_get_virtual(). This
should be removed once rman_get_virtual() starts doing the right
thing.

This should make the Tigon actuall work on the alpha now.

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