There is a semi-related issue on Thinkpad 600Es, where the default 1G
mapping loses and 512 is needed.  Besides using ACPI/etc. to really
find out the situation (can one do that and then go back to APM?), two
thoughts come to mind:

  a quirk table that takes perhaps discovered memory amount and a list
  of pci devices etc. and provides an RBUS_MIN_START value.  I wonder
  if a heuristic from processor type/speed would be good, since it
  would estimate the age of the machine and thus what space is likeley
  to be implemented and not used.

  a way to try various values until one works.

-- 
        Greg Troxel <gdt@ir.bbn.com>