Subset of power patch: subtractive decoding

[Don Lewis]

On 10 Jan, M. Warner Losh wrote:
> I've teased out the subtractive bridge decoding patches from the
> larger power patches.  Please test them and let me know how things
> go.  This eliminates hw.pci.allow_unsupported_io_range by fixing the
> underlying problem this was kludging around.  Please test it and let
> me know how it is working for you.  It should only make things better,
> and make nothing worse.
> 
> This won't fix the other problems that the more complete power patches
> attempt to fix.
> 
> http://people.freebsd.org/~imp/pci-subtractive.diff

It works on my Thinkpad R40.  I never needed
hw.pci.allow_unsupported_io_range.  It doesn't eliminate my need for
hw.cbb.start_memory=0xC0800000 in /boot/loader.conf.  Without this my
laptop hangs during boot.