Re: [PATCH 1/2] fdt: Allow stacked overlays phandle references

[Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 07/25/17 21:32, David Gibson wrote:
> On Fri, Jul 14, 2017 at 10:21:01AM +0300, Pantelis Antoniou wrote:
>> Hi Frank,
>>
>> On Thu, 2017-07-13 at 14:40 -0700, Frank Rowand wrote:
>>> On 07/13/17 14:22, Phil Elwell wrote:
>>>> On 13/07/2017 21:07, Frank Rowand wrote:
>>>>> On 07/13/17 12:38, Phil Elwell wrote:
>>>>>
>>
>> [snip]
>>
>>>> hope an inability to solve the problem posed by this advanced usage won't
>>>> prevent a solution to a simpler problem from being accepted.
>>
>> I have waited until people started commenting on this patchset before
>> replying.
>>
>> I think we agree on a few things to keep the discussion moving forward.
>>
>> 1. Stacked overlays are useful and make overlays easier to use.
> 
> Agreed.
> 
>> 2. Changing the overlay symbols format now would be unwise.
> 
> Agreed.  At least, I don't think updating the symbols alone would be
> silly without revisiting everything in the overlay format and making
> something completely new.
> 
>> 3. A number of extensions have been put forward/requested.
>>
>> 3.1. There should be a method to place a symbol on a node that didn't
>> have one originally (due to vendor supplying broken DTB or being
>> generated by firmware at runtime).
> 
> There already is.  An overlay can update *anything* in the base tree,
> including the /__symbols__ node.  Of course you need the exact path of
> the node to tag in the base tree, but you were always going to need
> that.

I haven't tested that, but it should work with the existing dtc and
Linux kernel.

But it will stop working in the future _if_ some changes are made
that I would like:

  - dtc no longer accept node names beginning with underscore as
    valid.
  - dtc supports Pantelis' sugar syntax

My intent behind these changes is to hide the implementation details
of the overlay extensions (__symbols__, __fixups__, __local_fixups__,
fragements nodes, etc) from the normal dts developer.  This should
make it easier to write and understand overlays, and reduce errors
in the dtb.

With those changes, it would not be possible to write an overlay
that applied against node __symbols__ because it would not be
possible to create a label on __symbols__, which would be needed
to reference __symbols__ with the sugar syntax.

-Frank

>> 3.2. Scoping symbol visibility in case of clashes. This can the ability
>> to put multiple path references to a single label/symbol. i.e.
>> foo = "/path/bar", "/path/bar/baz";
>> Resolving the ambiguity would require the caller to provide it's
>> 'location' on the tree. I.e. a device under /path/bar/baz would resolve
>> to the latter. It is a big change semantically.
> 
> I think this would be a nice idea, but trying to do it as a update to
> the existing overlay format will be really difficult verging on
> impossible.
> 
> Better to keep this in mind as a design goal for a new format to
> replace overlays.
> 

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