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

Ayush Singh
2025-09-24T17:03:56Z

On 9/24/25 09:41, David Gibson wrote:
> On Tue, Sep 23, 2025 at 12:29:27PM +0200, Geert Uytterhoeven wrote:
>> Hi Hervé,
>>
>> On Tue, 23 Sept 2025 at 11:49, Herve Codina <herve.codina@bootlin.com> wrote:
>>> On Tue, 23 Sep 2025 18:09:13 +1000
>>> David Gibson <david@gibson.dropbear.id.au> wrote:
>>>> Ah, right. To be clear: we absolutely don't want multiple addons
>>>> altering the same nodes. But I think we could do that in ways other
>>>> than putting everything under a connector. This is exactly why I
>>>> think we should think this through as an end-to-end problem, rather
>>>> trying to do it as a tweak to the existing (crap) overlay system.
>>>>
>>>> So, if we're thinking of this as an entirely new way of updating the
>>>> base dt - not "an overlay" - we can decide on the rules to ensure that
>>>> addition and removal is sane. Two obvious ones I think we should
>>>> definitely have are:
>>>>
>>>> a) Addons can only add completely new nodes, never modify existing
>>>> ones. This means that whatever addons are present at runtime,
>>>> every node has a single well defined owner (either base board or
>>>> addon).
>>> In this rule I suppose that "never modify existing ones" should be understood
>>> as "never modify, add or remove properties in existing ones". Because, of course
>>> adding a full node in a existing one is allowed (rule b).
>> What if the add-on board contains a provider for the base board.
>> E.g. the connector has a clock input, fed by an optional clock generator
>> on the add-on board. Hooking that into the system requires modifying
>> a clocks property in the base board, cfr. [1].
>> Or is there some other solution?
> Hmm. My first inclination would be that this case is not in scope for
> the protocol we're trying to design now. If the widget provides
> things to the base board as well as the other way around, it's no
> longer an "addon" for the purposes of this spec.
>
> But it's possible I've underestimated how common / useful such a case
> is.
>
> Note that I'd expect the existing overlay mechanism to still be
> around. It may be ugly and not very well thought out, but its
> drawbacks are much less severe if you're not dealing with hot unplug.
>

Well, while that was not an initial use-case in my mind, external clock
inputs are a valid use-case when talking about connectors for board
headers specifically (e.g. pocketbeagle connector).

Best Regards,

Ayush Singh