porting info for FreeBSD's kernel?

[Alfred Perlstein]

* Chuck Robey <chuckr at telenix.org> [090521 14:56] wrote:
> Alfred Perlstein wrote:
> > * Chuck Robey <chuckr at telenix.org> [090518 13:03] wrote:
> >> -----BEGIN PGP SIGNED MESSAGE-----
> >> Hash: SHA1
> >>
> >> I've been googling, trying to see if I can find notes regarding what needs
> >> changing, in what order, to adapt the FreeBSD kernel to a new processor.  Anyone
> >> know where stuff like that can be found?
> > 
> > You need a cross compile toolchain of course, look into how FreeBSD
> > is configured for the various arches.
> > 
> > Then I would suggest looking at the loaders, followed by 
> > kern/init_main.c.  If you trace down init_main.c and some
> > of the early sysinits that should give you an idea.
> > 
> > You might also be able to backtrack using CVS/svn to follow
> > how mips or arm was done.
> > 
> > Note: freebsd has a decent cross-compile setup now, see
> > "make universe" so things should be easier to get started.
> > 
> 
> Thanks.  I will *definitely* read all the parts you hint me at, I won't be
> deleting this mail, and I appreciate it.  I was asking on the llvm maillist
> about Cortex-A8 support.  What I got says that it's not there yet, but it's
> being worked upon, that and the -A9 support (definite differences).  So, any
> crosstools needed today would have to be gcc, from a version at least as new as
> the 4.3 branch (that's where they brought in the -A8 support).
> 
> The tool I got by doing the freeBSD crosstools was 4.2.1, which isn't going to
> do it for the Cortex-A8, and I had someone else (from a FreeBSD list) tell me
> that bringing in a newer version of gcc wasn't extremely likely, that they'd
> want llvm instead.  I see 3 alternatives for a Cortex-A8 port: using a new gcc
> port, waiting on the upgrade of llvm, or maybe deciding that the version the
> llvm that's out now, with the v6 compatibility, would be (for the short term)
> good enough.  Any idea which one to choose?  The only one that interests me is
> for the TI OMAP 3530 (Cortex-A8, among other parts).  Maybe if the currently
> available llvm is good enough, maybe gcc-4.2.1 may creak along well enough for
> the short term?  I need to understand this.
> 
> My own personal Pandora won't probably won't arrive on my doorstep for maybe as
> long as 3 more months, so in the meantime, I think I will be reading all I can
> get my hands on regarding llvm.  Maybe I can really learn enough to make a
> difference.  In school, I concentrated very definitely on OSes (I've written 3
> of them over the years, of quite varying levels of performance), so for
> compilers, I'm relying on my old 1988 version of the Aho/Sethi/Ullman compilers
> book.  If anyone knows a more modern book that will show me enough about
> compilers to be useful, I'd really appreciate the name, maybe Amazon will let me
> get a cheap used version.

I wouldn't sweat the compiler as much as the actual OS code, I think
it should be relatively easy to trick the build to use an external
compiler (ie, don't get caught up in the compiler bootstrap quagmire,
leave that for later...)

Anyhow, you're talking to someone that has studied, but not implemented
a port, so take my advice with a few heaps of salt. :)

Typically what people focus on is:

1) "how am I going to get the first line of dmesg to come up"
2) "how am I going to get to single user mode"
3) "multi user?"
4) cleanup of compiler and bootstrap issues.

If you get sidetracked by #4, you can spend months doing that
instead of just rolling with it when you get there.


-- 
- Alfred Perlstein