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

[Ayush Singh <ayush@beagleboard.org>]

On 9/11/25 15:53, Herve Codina wrote:
> Hi Geert,
>
> On Thu, 11 Sep 2025 10:54:02 +0200
> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
>> Hi Hervé,
>>
>> On Thu, 11 Sept 2025 at 10:48, Herve Codina <herve.codina@bootlin.com> wrote:
>>> On Wed, 10 Sep 2025 14:33:45 +1000
>>> David Gibson <david@gibson.dropbear.id.au> wrote:
>>>> On Tue, Sep 09, 2025 at 11:41:26AM +0200, Herve Codina wrote:
>>>>> Suppose a base board with 2 connectors:
>>>>>   - connA
>>>>>   - connB
>>>>>
>>>>> Case 1: Addons are independant
>>>>>                 +--------+
>>>>>    connA <----> | AddonA |
>>>>>                 +--------+
>>>>>                            +--------+
>>>>>    connB <---------------->| AddonB |
>>>>>                            +--------+
>>>>>
>>>>> With addonA and B two addon board each connected at one connector without any
>>>>> relationship between addon A and B
>>>>>
>>>>> Case 2: Only one Addons using ressources from both connector
>>>>>
>>>>>                  +------+
>>>>>    connA <-----> |Addon |
>>>>>                  |      |
>>>>>    connB <-----> |      |
>>>>>                  +------+
>>>> Case 2 is what I'm talking about.  Case 1 is the easy one.
>>>>   
>>>>> The addon is connected to both connector and uses ressources from connA and
>>>>> connB in a dependent manner.
>>>>>
>>>>>
>>>>> The Case 2 can be solved using a connector that described both connA and connB.
>>>>> Having the split connA and connB is a mechanical point of view.
>>>> I don't think that's a good solution, because it means you have to
>>>> make that decision at the board layer.  If I understand his case
>>>> correctly, you have a board where you could do either case 1 or case 2
>>>> at runtime.  We'd want the differences between these cases to only be
>>>> reflected on the addon device tree, not the base board device tree.
>>> Based on my understanding of Geer's use-case, I think decision at base
>>> board level will be needed.
>>>
>>> base board        addon board
>>>    connA +--------+conn1
>>>    connB +--------+conn2
>>>    connC +
>>>
>>> Or
>>>
>>> base board        addon board
>>>    connA +--------+conn1
>>>    connB +    ,---+conn2
>>>    connC +---'
>>>
>>> Or any other combination that would match.
>>>
>>>  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'".
>>>
>>> Also, at base board level, statically defined in the DT
>>> connA is described (type 'foo'), connB and connC are
>>> described (type 'bar').
>> Correct.
>>
>>> 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.
>>>
>>> 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.
>> All these choices and decisions apply to single-connector add-on boards, too.
>>
> Yes, in our use case (me and Luca), each addon has an eeprom, wired exactly the
> same on all supported addon, which allows to known the exact addon. Also addon
> insertions and removals are detected using some gpios wired to the connector.
>
> Based on that our specific driver handling our specific connector perform the
> following operations on addon insertion detection:
>    - load a first addon DT to have access to the eeprom
>    - Read the eeprom to determine the addon type
>    - load the DT matching with the addon type
>
> This part is of course connector type specific. I mean having an eeprom with
> some encoded addon type values and hotplug detection with gpio is a part of
> the contract between the board and the addon (part of connector specification).
>
> Best regards,
> Hervé
>

My usecase is a bit more complicated, since I am trying to model all the 
available headers on BeagleBoard.org sbcs (particularly PocketBeagle 2 
initially) as connectors. However, that still ends up being a single 
connector which can have multiple addon-boards simultaneously instead of 
the other way around.


Best Regards,

Ayush Singh