Re: Virtualization difficulty -- phandles

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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On Mon, Jul 24, 2017 at 10:09:48AM -0700, Frank Rowand wrote:
> Hi David,
> 
> (Adding Pantelis and Tom, since I'm going somewhat off-topic from
> the original thread, and they are impacted by what I am asking.)
> 
> On 07/15/17 22:35, David Gibson wrote:
> > On Thu, Jul 13, 2017 at 09:47:01AM -0700, Florian Fainelli wrote:
> >> On 07/12/2017 09:23 PM, Cyril Novikov wrote:
> >>> On 7/12/2017 10:10 AM, Florian Fainelli wrote:
> >>>> On 07/11/2017 11:15 PM, Cyril Novikov wrote:
> >>>>> Hi, all!
> >>>>>
> 
> < snip >
> 
> >   The
> > phandle fixup information goes into the special __local_fixups__ and
> > __fixups__ nodes (which have gratuitiously different format, but
> > that's a rant for elsewhere).
> 
> < snip >
> 
> And in another email, David describes the __local_fixups__ format
> nicely, so I'll just copy that here instead of re-inventing it:
> 
> 
> > Well, I don't want to invent a new encoding if we can possibly avoid
> > it.  The current encoding used for overlay generation looks like this
> > 
> > / {
> > 	target: node@0 {
> > 	};
> > 	node@1 {
> > 		ref = <&target>;
> > 	};
> > 	__local_fixups__ = {
> > 		node@1 {
> > 			ref = <0>;
> > 		};
> > 	};
> > };
> > 
> > Basically, __local_fixups__  has a subtree which paralells the main
> > tree.  Each property found under __local_fixups__ is a list of offsets
> > at which phandle references appear in the corresponding property in
> > the main tree.
> 
> I share your desire to rant about the different formats between
> __local_fixups__ and __fixups__.  But I have not come up with an
> alternate format for __local_fixups__ that makes me happy.  The
> best format that I have come up with so far would be:

Well to fix it minimally, I'd go the other way - make __fixups__ look
like __local_fixups__ but augmented with labels.  Strings that need
parsing aren't a normal thing in the DT.

> / {
> 	target: node@0 {
> 	};
> 	node@1 {
> 		ref = <&target>;
>                 ref2 = <&target 42 &target_2>;
> 	};
>         target_2: node@2 {
>         };
> 	__local_fixups__ = {
> 		x1 = <"node@1/ref" 0>;
>                 x2 = <"node@1/ref2" 0 8>;
> 		};
> 	};
> };
> 
> x1 and x2 are abitrary property names.
> The format of each __local_fixups__ property is
>    - path of property referencing a phandle
>    - list of offsets of the phandle in the property
> 
> As another alternative, Grant was thinking about adding
> a new block to the FDT format to contain the phandle
> information.  That would remove the need to come up
> with a convoluted dts syntax, but adds in the problem
> of bootloaders corrupting the new block if they were
> not aware of it.  He had thoughts about versioning
> and checksums to detect the corruption it if did
> occur.
> 
> If we were starting from scratch, do you have any other
> approach that might be fruitful?  It seems like maybe
> I am missing something that requires thinking outside
> the box.

I thought about this the other day a bit.  If going from scratch, I
think the way to do it would be to add a new FDT_REF tag to the
structure block stream.  After the FDT_PROP tag and its contents,
you'd have an arbitrary number of FDT_REF tags, each giving an offset
in the preceding property  and a label to fix it up to match.  Not
sure if you'd want separate FDT_REF and FDT_LOCAL_REF or just use an
empty label to describe a local ref.

This would also allow for extension to say FDT_PATH_REF to insert
paths rather than phandles (i.e. a runtime equivalent of prop = &foo;
rather than prop = < &foo >;).

For encoding the fragments of an overlay, I'd suggest giving them
simply as separate subtrees in the structure block, all before the
FDT_END tag. At the moment there has to be only a single subtree
before the FDT_END, and the top-level FDT_BEGIN is expected to have an
empty name.  We can extend that to overlays by allowing multiple
subtrees, and making the top-level "name" the target label instead.

Incidentally, I'd take "label" in all the above to be represented as
an old-style OF path.  That is, either an absolute path /foo/bar/baz,
or a path relative to an alias, alias/foo/bar/baz.  That means we can
just use the existing defined /aliases, rather than re-inventing it as
__symbols__.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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