[LinuxBIOS] Fast path resume, high-level design

[Jim Gettys]

> On 5/3/07, Peter Stuge <stuge-linuxbios at cdy.org> wrote:
> > On Thu, May 03, 2007 at 05:55:39PM -0400, Jim Gettys wrote:
> > > Mitch wrote a fast path resume from RAM for the OLPC Geode
> > > hardware.
> >
> > The fastest path is obviously attractive also in LinuxBIOS.
> >
> >
> > > Once you've done that, you find that you've done just about
> > > everything to initialize the hardware in the first place; so that
> > > code is now used both at boot time and at suspend from RAM time.
> >
> > This is an interesting observation.
> >
> >
> > Perhaps all chip init would best be written in some macro language?
> > (one specific for the task?)
> 
> We've talked about this. It only really works on OLPC because OLPC is
> so incredibly simple. Add an opteron of the type that stefan used to
> port to -- variable number of CPUs, 31 PCI busses, etc. etc. -- and
> it's a bit harder.
> 
> Also, let's pretend that someday there is an OLPC with SPD. The
> problem starts to get ugly again as you add back in SMBUS support,
> etc. etc. -- all that stuff that linuxbios can do pretty well.
> 
> I also still believe, having watched how things played out, that I
> want linux as my bios. Nothing in what I have seen on OLPC changes my
> belief, and some things that have happened on OLPC strongly confirm
> it.
> 
> Again, the OLPC code can be found at openbios.org, it's pretty
> trivial, and works mainly because the OLPC hardware has hardcoded dram
> timing, etc. etc. And, as Jordan pointed out, things can fall apart
> when some of these constants change.
> 

Heh.  Accusing a Geode as being "simple" is a false accusation....  Rather...

As Ron points out, with the exception of SD and USB (at fixed locations), the XO-1
is a *fixed configuration* system.

There are additional complications.  Since fast resume is *so* important to us,
we are sometimes going to initialize hardware in OFW early in resume, so that Linux
PM will not have to start the process and then wait (or wait as long) to be able to
complete hardware re-initialization of that particular device.

Mark Foster described (at the Power management summit a year ago) a more general scheme
for paralleling initialization using tables and data structures.  Something
like that might be adaptable enough for more complex systems: but that isn't what's
been implemented here; we haven't needed to do so.
                                         Regards,
                                             - Jim

-- 
Jim Gettys
One Laptop Per Child