linux-kernel - Re: [PATCH v2 1/7] dt-bindings: Add support for export-symbols node

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Message-ID: <49e1e1fc-412d-4334-8337-16e352a34788@kernel.org>
Date: Wed, 4 Jun 2025 20:35:51 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Herve Codina <herve.codina@...tlin.com>
Cc: David Gibson <david@...son.dropbear.id.au>, Andrew Davis <afd@...com>,
 Ayush Singh <ayush@...gleboard.org>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Arnd Bergmann <arnd@...db.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Saravana Kannan <saravanak@...gle.com>, devicetree@...r.kernel.org,
 devicetree-compiler@...r.kernel.org, linux-kernel@...r.kernel.org,
 Luca Ceresoli <luca.ceresoli@...tlin.com>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v2 1/7] dt-bindings: Add support for export-symbols node

On 28/05/2025 18:57, Herve Codina wrote:
> Hi Krzysztof,
> 
> Thanks a lot for your feedback!
> 
> On Tue, 27 May 2025 20:31:14 +0200
> Krzysztof Kozlowski <krzk@...nel.org> wrote:
> 
>> On 30/04/2025 14:51, Herve Codina wrote:
>>> An export-symbols node allows to export symbols for symbols resolution
>>> performed when applying a device tree overlay.
>>>
>>> When a device tree overlay is applied on a node having an export-symbols
>>> node, symbols listed in the export-symbols node are used to resolve
>>> undefined symbols referenced from the overlay.  
>>
>>
>> I have impression that this is being discussed in three places
>> simultaneously - here, DT spec and DT schema. I don't know how to solve
>> the multiplication, but I will keep answering here, because that's my part.
>>
>>>
>>> This allows:
>>>   - Referencing symbols from an device tree overlay without the need to
>>>     know the full base board. Only the connector definition is needed.
>>>
>>>   - Using the exact same overlay on several connectors available on a given
>>>     board.
>>>
>>> For instance, the following description is supported with the
>>> export-symbols node:
>>>  - Base device tree board A:
>>>     ...
>>>     foo_connector: connector1 {
>>>         export-symbols {
>>>            connector = <&foo_connector>;
>>>         };
>>>     };
>>>
>>>     bar_connector: connector2 {
>>>         export-symbols {
>>>            connector = <&bar_connector>;
>>>         };
>>>     };
>>>     ...  
>>
>> And what would this mean? Which symbol is exported - foo or bar?
> 
> Symbols are exported only when an overlay is applied on the node where the
> export-symbols node is available. Those symbols are visible only from the
> overlay applied. Symbols exported thanks to export-symbols are not global
> to the all device-tree (it is not __symbols__) but local to a node.
> 
> If an overlay is applied at connector1 node, it can use the 'connector'
> symbols and thanks to export-symbols, the 'connector' symbol will be
> resolved to foo_connector.
> 
> If the overlay is applied at connector2 node, the 'connector' symbol is then
> resolved to bar_connector.

OK, this explains a lot. Unless I missed it, would be nice to include it
in binding description.


...



...

>>> +patternProperties:
>>> +  "^[a-zA-Z_]?[a-zA-Z0-9_]*$":  
>>
>> This messes up with coding style which I would prefer keep intact.
>> Basically these properties will be using label style.
> 
> Yes, those properties remap phandles.
> 
> Their names are the name of the label used from the overlay and their
> values are the phandle mapped.
> 
> You already have this kind properties using label style in __symbols__,
> __fixups__, __local_fixups__ nodes.

I have them in DTB, but I don't have these in DTS. The exported-symbols
would be in the DTS and that is what coding style is about.



Best regards,
Krzysztof