Re: RFC: Encoding property deletion in FDT

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On 05/02/18 19:20, David Gibson wrote:
> On Wed, May 02, 2018 at 10:19:58AM +0100, Phil Elwell wrote:
>> David et al.,
>>
>> I've mentioned before the problem posed for overlays by boolean properties, i.e.
>> that a boolean property that is "true" in a base DTB cannot be made "false" by an
>> overlay because doing so requires that the property be deleted. A solution for this
>> problem would be to define a new FDT tag - FDT_DEL_PROP, say - that is used to encode
>> any /delete-property/ found in a node during overlay compilation. When the overlay is
>> applied, the named property would be deleted if present.
>>
>> A heuristic would be needed to decide whether this property should be encoded or just
>> acted on immediately - the use of the '-@' command line parameter would seem to fit the
>> bill.
>>
>> Although one might consider extending this mechanism to cover node deletion, in practice
>> I think this would be too problematic in terms of broken phandle references etc., and in
>> most cases 'status = "disabled"' achieves the same objective, so I'm not proposing this
>> be added.
>>
>> Is such a change something you would consider supporting, or do you have an alternative
>> preferred solution?
> 
> I'd certainly consider it within the right context.
> 
> The difficulty is that this obviously requires changing the things
> that accept the fdt to understand the new format.  And if we're going
> to do that, there's a bunch of other things we should change as well.
> I've previously had a discussion with Frank Rowand about how we could
> do a much saner encoding of the fixups using new tags, rather than the
> currently rather ghastly encoding in special properties and nodes.
> 
> So maybe add this as one feature to put in a hypothetical v32 (or
> whatever) fdt format.

I like this idea.

On the question of current users of FDT objects, I think the problem is limited to
only overlays, so users of base FDTs should not be impacted.  That at least reduces
the amount of code that needs to be updated.

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