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

[Ayush Singh]

On 9/11/25 15:53, Herve Codina wrote:
> Hi Geert,
>
> On Thu, 11 Sep 2025 10:54:02 +0200
> Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>
> > Hi Hervé,
> >
> > On Thu, 11 Sept 2025 at 10:48, Herve Codina <herve.codina@xxxxxxxxxxx> wrote:
> > > On Wed, 10 Sep 2025 14:33:45 +1000
> > > David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> 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