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

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Message-ID: <CAMuHMdWmDwedyPnBERs-tSYEG15nMUuh9u1Q+W_FdquHpUC0-A@mail.gmail.com>
Date: Tue, 23 Sep 2025 12:29:27 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Herve Codina <herve.codina@...tlin.com>
Cc: David Gibson <david@...son.dropbear.id.au>, Ayush Singh <ayush@...gleboard.org>, 
	Krzysztof Kozlowski <krzk@...nel.org>, Rob Herring <robh@...nel.org>, Andrew Davis <afd@...com>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, 
	Luca Ceresoli <luca.ceresoli@...tlin.com>, devicetree@...r.kernel.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>
Subject: Re: Device tree representation of (hotplug) connectors: discussion at ELCE

Hi Hervé,

On Tue, 23 Sept 2025 at 11:49, Herve Codina <herve.codina@...tlin.com> wrote:
> On Tue, 23 Sep 2025 18:09:13 +1000
> David Gibson <david@...son.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?

I was also wondering about endpoints, as they have two sides: one on
the base board, and one on the add-on board. But it seems that typically
both ends are added by the extension, so these fall under rule b.

Thanks!

[1] https://elixir.bootlin.com/linux/v6.16/source/arch/arm64/boot/dts/renesas/white-hawk-ard-audio-da7212.dtso#L165

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