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

[Ayush Singh]

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@xxxxxxxxxxx> wrote:
> > > On Tue, 23 Sep 2025 18:09:13 +1000
> > > David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> 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