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

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Message-ID: <CAMuHMdUgnw4Q90a5yzOcK30iBUAQTgUDgAauzSifnngQf7bwqQ@mail.gmail.com>
Date: Tue, 16 Sep 2025 15:34:52 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Ayush Singh <ayush@...gleboard.org>
Cc: Herve Codina <herve.codina@...tlin.com>, David Gibson <david@...son.dropbear.id.au>, 
	Luca Ceresoli <luca.ceresoli@...tlin.com>, Krzysztof Kozlowski <krzk@...nel.org>, devicetree@...r.kernel.org, 
	Rob Herring <robh@...nel.org>, Jason Kridner <jkridner@...il.com>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	devicetree-compiler@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>, Andrew Davis <afd@...com>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>
Subject: Re: Device tree representation of (hotplug) connectors: discussion at ELCE

Hi Ayush,

On Tue, 16 Sept 2025 at 14:22, Ayush Singh <ayush@...gleboard.org> wrote:
> On 9/16/25 15:44, Geert Uytterhoeven wrote:
> > On Tue, 16 Sept 2025 at 08:46, Herve Codina <herve.codina@...tlin.com> wrote:
> >> On Mon, 15 Sep 2025 14:51:41 +1000
> >> David Gibson <david@...son.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?

> 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

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds