linux-kernel - Re: Device tree representation of (hotplug) connectors: discussion at ELCE

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Message-ID: <397483db-91de-4ff2-82e3-52ae785bc343@beagleboard.org>
Date: Tue, 16 Sep 2025 17:52:18 +0530
From: Ayush Singh <ayush@...gleboard.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Herve Codina <herve.codina@...tlin.com>
Cc: David Gibson <david@...son.dropbear.id.au>,
 Luca Ceresoli <luca.ceresoli@...tlin.com>,
 Krzysztof Kozlowski <krzk@...nel.org>, devicetree@...r.kernel.org,
 Rob Herring <robh@...nel.org>, Jason Kridner <jkridner@...il.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, devicetree-compiler@...r.kernel.org,
 linux-kernel@...r.kernel.org, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, Andrew Davis <afd@...com>,
 Wolfram Sang <wsa+renesas@...g-engineering.com>
Subject: Re: Device tree representation of (hotplug) connectors: discussion at
 ELCE

On 9/16/25 15:44, Geert Uytterhoeven wrote:

> Hi Hervé,
>
> On Tue, 16 Sept 2025 at 08:46, Herve Codina <herve.codina@...tlin.com> wrote:
>> On Mon, 15 Sep 2025 14:51:41 +1000
>> David Gibson <david@...son.dropbear.id.au> wrote:
>>> On Thu, Sep 11, 2025 at 10:48:28AM +0200, Herve Codina wrote:
>>>>  From the addon board point of view, the only think we can
>>>> say is "me, as an addon board, I need a connector of type 'foo' and a
>>>> connector of type 'bar'".
>>> Agreed.
>>>
>>>> Also, at base board level, statically defined in the DT
>>>> connA is described (type 'foo'), connB and connC are
>>>> described (type 'bar').
>>>>
>>>> The choice to map connA to the type 'foo' connector expected by the addon
>>>> and the choice to map connB or connC to the type 'bar' connector expected by
>>>> the addon can only be done at runtime and probably with the help of a driver
>>>> that have the knowledge of the 3 connectors.
>>> Agreed.
>>>
>>>> I have the feeling that the choice of physical connectors to which the addon
>>>> board is connected to is a human choice when the board is connected.
>>> Yes.  Although if the addons have an EEPROM, or some other sort of ID
>>> register, it may be possible for some connector drivers to probe this.
>> Right, I think we agree that a driver is needed to help in the mapping at
>> least when multiple connectors are involved.
> I agree you need a driver to read an ID EEPROM.
> But why would you need a driver if no ID EEPROM is involved?
> If the connector types on base board and add-on match, it should work.
>
> Gr{oetje,eeting}s,
>
>                          Geert
>

How would a connector be disabled in such a setup? I guess maybe status 
property can be used while applying overlay to check if the connector is 
enabled. But maybe that goes outside the scope of fdtoverlay?

Also, I would assume that most such connectors would want to provide 
some kind of configfs based API to add/remove addon boards.


Best Regards,

Ayush Singh