AFAIK autoconfiguration starts here:
http://bxr.su/FreeBSD/sys/mips/mips/autoconf.c#95 and later it
calls BUS_NEW_PASS(root_bus) which calls bus_set_pass(BUS_PASS_DEFAULT).
Why is it called with maximum possible bus pass level - BUS_PASS_DEFAULT?
Where are bus passes with lower pass value called?
bus_set_pass() walks up from the current bus pass to the requested bus
pass stopping at each level that has a registered driver. The intention
is that eventually we may want to insert SYSINITs in between some of
the passes in which case a SYSINIT would use bus_set_pass with a lower
level and then do some other work, but currently we don't have any of those,
so we are still just passing through all the levels in one SYSINIT.