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Subject: Re: common flatdevtree code
From: Hollis Blanchard <hollisb@us.ibm.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
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Date: Wed, 27 Sep 2006 18:52:19 -0500
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Cc: linuxppc-dev <Linuxppc-dev@ozlabs.org>, Paul Mackerras <paulus@samba.org>
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On Thu, 2006-09-28 at 01:41 +0200, Segher Boessenkool wrote:
> > I talked to David Gibson about this and he pointed out that there is a
> > problem with using a void * to identify nodes (i.e. returned from
> > ft_find_device and passed to ft_set_prop etc.).  The problem is that
> > if we reallocate the space for the tree, then any void * handles that
> > the user of the library has are then invalid.  Perhaps we have to use
> > offsets from the beginning of the struct region instead - the offset
> > for a node will be stable across changes of the node or any of its
> > descendents, at least.
> 
> Or unpack the tree before you operate on it.  You can pack it
> again later if you need to pass it as a flat tree again (or the
> bootwrapper could implement a real OF client interface, which
> is useful for many more things!)
> 
> A flat contiguous blob of bytes is convenient to pass around,
> but not such a great data structure for basically anything else ;-)

I agree; I think this data structure's design is explicitly static, and
now we're trying to dynamically edit it.

We essentially need to do unpack/pack now whenever the size of the
structure changes (e.g. changing a property size, adding or removing
anything). Making that explicit (and one-time) might be a good idea:
        tree = ft_unpack(blob);
        n = ft_find_node(tree, "/node");
        ft_set_prop(n, "property", value, len);
        ...
        blob = ft_pack(tree);

-- 
Hollis Blanchard
IBM Linux Technology Center