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

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Message-ID: <ed6beb97-12f1-4d71-b4dc-b34d4d611b81@beagleboard.org>
Date: Wed, 18 Jun 2025 15:24:07 +0530
From: Ayush Singh <ayush@...gleboard.org>
To: Herve Codina <herve.codina@...tlin.com>,
 Krzysztof Kozlowski <krzk@...nel.org>,
 David Gibson <david@...son.dropbear.id.au>, Rob Herring <robh@...nel.org>
Cc: Andrew Davis <afd@...com>, Geert Uytterhoeven <geert@...ux-m68k.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 6/18/25 15:02, Herve Codina wrote:
> Hi Krzysztof,
>
> On Wed, 4 Jun 2025 20:35:51 +0200
> Krzysztof Kozlowski <krzk@...nel.org> wrote:
>
> ...
>
>>> 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.
> Sure, I will add something in the next iteration.
>
> ...
>
>>>>> +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.
>>
> I think export-symbols has to be in DTS.
> Maybe it could be described in an other way in order to avoid the coding style
> issue you reported.
>
> Hardware:
>    i2c0 from SoC --------- connector 1, I2C A signals
>    i2c1 from SoC --------- connector 1, I2C B signals
>
>    connector1 {
>        export-symbols {
> 	  i2c_a = <&i2c0>;
> 	  i2c_b = <&i2c1>;
>        };
>    };
>
> In order to avoid the coding style issue, this could be replace
> with:
>   connector1 {
>        export-symbols {
> 	  symbol-names = "i2c_a", "i2c_b";
> 	  symbols = <&i2c0>, <&i2c1>;
>        };
>    };
>
> Krzysztof, Rob, do you think this could be accepted ?
>
> Ayush, David, do you thing this could be easily implemented in fdtoverlay ?
>
> Best regards,
> Hervé
>

Well, it is possible.

However, on connectors like pb2 header, there will be 50-100 export 
symbols. So it will start becoming difficult to maintain.

Additionally, the further away we move from __symbols__ style, the more 
difficult the implementation will become since we can currently very 
easily piggy-back on __symbols__ resolution implementation.


Best Regards,

Ayush Singh