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

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

On 07/13/17 12:38, Phil Elwell wrote:

(I moved Phil's reply to after the email he replied to.)

> On 13 Jul 2017 8:32 pm, "Frank Rowand" <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> 
>> On 07/03/17 02:06, David Gibson wrote:
>>> On Wed, Jun 14, 2017 at 05:52:25PM +0300, Pantelis Antoniou wrote:
>>>> This patch enables an overlay to refer to a previous overlay's
>>>> labels by performing a merge of symbol information at application
>>>> time.
>>>
>>> This seems to be doing things the hard way.
>>>
>>> You're essentially extending the semantics of overlay application to
>>> add the symbol merging.  You've implemented these extended semantics
>>> in libfdt, which is all very well, but that's not the only overlay
>>> application implementation.
>>>
>>>
>>> It seems to me a better approach would be to change dtc's -@
>>> implementation, so that in /plugin/ mode instead of making a global
>>> __symbols__ node, it puts it into the individual fragments.  That way
>>> the existing overlay application semantics will update the __symbols__
>>> node.
>>
>> If the __symbols__ node was inside a fragment, then the existing
>> code would add (or update) a __symbols__ node located at the location
>> pointed to by the fragment's target path, instead of updating the
>> node /__symbols__.
>>
>> It makes sense to me to have only one global __symbols__ node instead
>> of several.
>>
>> If there is a global __symbols__ node then we have a single name
>> space for symbols.
>>
>> If there are multiple __symbols__ nodes spread throughout the tree,
>> then to me that would imply different name spaces spread throughout
>> the tree, where namespaces are determined by fragments.  This sounds
>> confusing to me.  Or if the intent is to have a single name space
>> then the __symbols__ information would be scattered throughout the
>> tree instead of located in a single node.
>>
>> My current patch (under review), targeted for Linux 4.13-rc1, puts
>> an overlay's __symbols__ node properties into the overlay's
>> changeset, so they get added when the overlay is loaded and
>> removed when the overlay is unloaded.

> Can we also consider a mechanism for overlay-local symbols, i.e. symbols
> that are used purely to create links within an overlay - perhaps using a
> particular naming convention? This would make it easier to instantiate an
> overlay multiple times without having to uniquify all symbols, and it would
> avoid polluting the global namespace without reason.
> 
> Phil

That is essentially the result you get if you compile the overlay dts
without '-@'.  There will be no __sympls__ node created even if there
are symbols within the overlay.

This is important if the overlay is for an add-on board which might
have several instances plugged into different sockets on the base
system.

But Phil does bring up an interesting use case.  If the add-on board
("level one add-on") in turn has a socket that an additional board
("level two add-on") can be plugged into, then the level two add-on
overlay might have a need to reference a symbol from the overlay
for the level one add-on.  And since there may be multiple level one
add-on cards in the system, the overlay for each of the level one
add-ons would need to export its symbols in a name space only
visible to the level two add-on plugged into that specific level
on add-on.

-Frank
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