Re: OpenFirmware and Mac-on-Linux

[Samuel Rydh]

      On Sat, Oct 25, 2003 at 02:45:19PM +0200, Stefan Reinauer wrote:
 Yes, I think
so. I'm not sure exactly how much work is involved but
I will give it a go (it is not necessary to do a full OF implementation
right away...).
 I can have a look on integrating the MOL OF functionality into 
 BeginAgain, but for putting BeginAgain into MOL I'd prefer if you could 
 try this first.
Certainly. I don't think putting the MOL OF code directly into
BeginAgain is such a good idea, at least not the current incarnation
of the of-mol code. My thought was to add the BeginAgain kernel
to of-mol and then gradually start to use it for OF client services
and for user interaction.

 For MOL, the
device tree is loaded from a config file (for MacOS, it is
essentially a copy of a "real" device tree). Bus probing and
similar things are not needed (although one could implement this
if one really wants to). Cloning the device tree at OF startup is 
sufficient
(just like most parts of the device tree is hardcoded in ROM in most 
machines).
 Ah.. these device tree files I have seen. It's a really quick way of 
 setting resources up for different kinds of systems. Are the different
 device trees needed for different purposes/machines?
Yes, different device trees are used for different purposes.

The Mac OS 9 device-tree originates from a B&W G3. Since only
certain PCI-bridges and interrupt controllers are suppoerted by
the OS, I have to use those in the device-tree. MacOS 9 also
expects certain legacy hardware to be present (the via-cuda for
instance).

In general, I prefer to use virtual MOL hardware rather then emulated
real-world hardware since this is much more efficient (the MOL OS side
driver just makes "system calls" to MOL rather than accessing hardware
through PCI register).

The Mac OS X device tree has essentially been built from scratch. It
contains more virtual MOL hardware than the Mac OS 9 tree. Since OSX
is a much more open architecture, it is possible to write custom MOL
drivers for things like interrupt controllers so there is no need
to emulate any hardware at all.

  Or is it just a matter of emulating the 
 kind of machine that you have Mol running on?
No, MOL is completely hardware independent (except that 601 based
CPUs are unsupported).

  Do you have scripts for cloning the device tree in
OpenFirmware?
No, not in OpenFirmware. Under linux, a copy of the
device tree is available under /proc/device-tree so it is just a matter
of reformatting things into a format more suitable for MOL.

  The device tree should actually be hardcoded for all
onboard devices 
 and only expansion cards like PCI should be dynamically added.
Yes, this is essentially what MOL does. Only a few minor tweaks are
done to the device tree at startup (MOL supports three parallell
virtual network cards, but typically only a single one is used).

  In BeginAgain I've implemented a plugin 
 system that allows to emulate PCI cards in a simple manner. I planned to 
 do the Framebuffer output this way, but since OpenBIOS lacks a PCI 
 driver and a framebuffer driver you get nothing but a black window that 
 you can write to with lowlevel functions..
 
 If you don't disagree I'd like to move this discussion to 
 openbios(a)lists.openbios.org in future.
Sure.

/Samuel