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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [RFC] Machine description as data
Date: Tue, 17 Feb 2009 17:44:34 +0000
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To: qemu-devel@nongnu.org
Cc: devicetree-discuss@ozlabs.org, Markus Armbruster <armbru@redhat.com>, David Gibson <david@gibson.dropbear.id.au>

> > The machines I care for come with many optional and configurable parts.
> > We select the basic machine type with command line option -M, and
> > configure the rest with more command line options.  I figure we want to
> > keep supporting these options, at least for a while.
> >
> > I believe the best way to deal with that is start with a basic tree
> > selected by -M, then modify it according to the other options.  So,
> > there's a fair amount of configuration tree mutation.
>
> Yeah, you're probably right.  Although, in some cases the amount of
> complex tree mutation can be cut down by thinking about things in the
> right order.  For example if you have a bunch of optional devices,
> rather than adding them one by one (with all the required properties)
> to the skeleton tree, you can instead have the skeleton tree be the
> all-bells-and-whistles variant then delete the subtrees that aren't
> present.  libfdt even has a function to replace subtrees with nops
> instead of eliding them, which means the offsets of other nodes won't
> change.

I'm not so sure this is a vital feature. The current commandline options only 
provide the absolute bare minimum mutation for a basic PC machine, and don't 
even do that particularly well. I'm inclined to say we should punt 
significant machine config modification/generation to an external tool.

Paul