PERFORCE change 75921 for review

[M. Warner Losh]

In message: <c228a787d0ab62d16e6c7457b9c5dba5 at xcllnt.net>
            Marcel Moolenaar <marcel at xcllnt.net> writes:
: 
: On Apr 24, 2005, at 10:49 PM, M. Warner Losh wrote:
: 
: > In message: <200504250459.j3P4xWxs057683 at repoman.freebsd.org>
: >             Marcel Moolenaar <marcel at freebsd.org> writes:
: > : http://perforce.freebsd.org/chv.cgi?CH=75921
: > :
: > : Change 75921 by marcel at marcel_nfs on 2005/04/25 04:59:01
: > :
: > : 	Allow VGA resources across PCI-PCI bridges. Required to
: > : 	support a PCI based VGA driver with the VGA hardware
: > : 	behind a PCI-PCI bridge.
: >
: > WE need to make sure that the ISA bit and VGA bits are set properly on
: > those bridges with vga children and peers of bridges with vga
: > children before allowing this...
: 
: Ideally, yes. But since only one PCI device of class display and
: subclass VGA can have the legacy VGA addresses enabled and we're
: not mucking with the system configuration in that respect, we're
: not really doing anything dangerous by allowing the VGA addresses
: without such scrutiny provided we don't try to allocate those
: addresses for any ol' VGA display device that we find on the PCI
: busses in the system.

Actually yes, really, we need to do this.  Most system BIOSes will get
it right, but not all.  We'll likely need to cope with them when we
turn this on.  MS has an excellent paper on this topic.  That paper
seems to imply that one can have multiple VGA devices in a system
behind different bridges.  I guess what I'm saying here is that we'll
need to be careful and chances are good we'll have some people that
have issues which didn't before.

Warner