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

Ayush Singh
2025-09-16T15:35:29Z

On 9/16/25 19:04, Geert Uytterhoeven wrote:
> Hi Ayush,
>
> On Tue, 16 Sept 2025 at 14:22, Ayush Singh <ayush@beagleboard.org> wrote:
>> On 9/16/25 15:44, Geert Uytterhoeven wrote:
>>> On Tue, 16 Sept 2025 at 08:46, Herve Codina <herve.codina@bootlin.com> wrote:
>>>> On Mon, 15 Sep 2025 14:51:41 +1000
>>>> David Gibson <david@gibson.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.
>> 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?
> Why would you want to disable a connector?

So a lot of embedded SoCs (eg. TI AM6254 [0]) have co-processors (M4F in
case of AM6254). These co-processors can run full blow RTOS (most
BeagleBoard boards like PocketBeagle 2 [1] have Zephyr support). When
using a peripheral from the co-processor running such RTOS, it needs to
be disabled on Linux side.

>
>> Also, I would assume that most such connectors would want to provide
>> some kind of configfs based API to add/remove addon boards.
> Yes, we need some way to configure add-on board add/remove,
> and on which connector(s).
>
> Gr{oetje,eeting}s,
>
> Geert
>

Best Regards

Ayush Singh

[0]: https://www.ti.com/product/AM625

[1]: https://www.ti.com/product/AM625