FDTbus and nexus

[Nathan Whitehorn]

As many of you may have noticed, I've been working on trying to bring 
the code in /sys/dev/fdt a little bit closer to the code in /sys/dev/ofw 
as well as improving standards-compliance. One motivation for this is to 
be able to build kernels that support both real Open Firmware and FDT at 
the same time, with the same drivers, which is not currently possible, 
largely due to differences in bus enumeration.

The remaining primary difference between FDT and OF systems is that the 
root of the OF hierarchy is attached through fdtbus on one and 
enumerated through nexus on the other. Of these two, I think nexus is 
the cleaner approach here since it means we can get rid of things like 
fdt_bs_tag and isolate the MD resource allocation code to the MD nexus 
driver, as well as increasing flexibility with things like interrupt 
mapping (nexus and individual PIC drivers can then handle the mappings 
in /sys/dev/fdt/fdt_ARCH.c).

The patch at http://people.freebsd.org/~nwhitehorn/nexus.diff implements 
this change by subclassing the ARM and MIPS nexus drivers from ofw_nexus 
(the same thing fdtbus subclasses, and what is already done on PowerPC). 
fdtbus is made optional for now as a consideration for out-of-tree code 
that I don't know about, and the set of things attaching to fdtbus is 
made to attach to nexus instead. The patch is quite minimal, aside from 
rearranging the order of some ARM and MIPS nexus methods for easier 
#ifdef, and has been tested with no issues on both PowerPC (on both FDT 
and non-FDT systems) and ARM. The MIPS support is only compile-tested, 
however, so I would really appreciate some testing there.
-Nathan