linux-kernel - Re: [Discussion] Global vs Local devicetree overlays for addon board + connector setups

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Message-ID: <CAMuHMdXoNk3cFm10dJNMK19Ym2RWZ_hyRpDXRZ_gjPAPZpAuVQ@mail.gmail.com>
Date: Wed, 30 Apr 2025 16:31:17 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Ayush Singh <ayush@...gleboard.org>, xypron.glpk@....de, 
	Jason Kridner <jkridner@...gleboard.org>, Deepak Khatri <lorforlinux@...gleboard.org>, 
	Dhruva Gole <d-gole@...com>, Robert Nelson <robertcnelson@...gleboard.org>, Andrew Davis <afd@...com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, David Gibson <david@...son.dropbear.id.au>, 
	Luca Ceresoli <luca.ceresoli@...tlin.com>, Pantelis Antoniou <pantelis.antoniou@...il.com>, 
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [Discussion] Global vs Local devicetree overlays for addon board
 + connector setups

Hi Hervé,

On Wed, 30 Apr 2025 at 16:09, Herve Codina <herve.codina@...tlin.com> wrote:
> On Wed, 30 Apr 2025 17:37:33 +0530
> Ayush Singh <ayush@...gleboard.org> wrote:
>
> ...
>
> > 1. __symbols__ based approach [3]
> >
> >
> > This was originally proposed by Andre Davis [3]. It defines an overlay
> > with just special names in `__symbols__`, which is used along with an
> > overlay for the addon-board, which makes use of the node names defined
> > in the connector `__symbols__` overlay. Please take a look at the
> > original patch series since it provides a working example of how it can
> > be used [3].
> >
>
> The __symbols__ based approach needs 2 overlays to handle the case where
> 2 connectors (A and B) are present an you want to connect a board described
> by a single overlay.
>
> The first overlay applied "adapts" the __symbols__ node for the connector
> where the board is connected (for instance connector A) in order to have
> the symbols used by the overlay describing the board resolved to the
> correct symbols.
>
> I think this open race conditions when the overlay is applied by the kernel
> itself. Indeed, we need to perform 2 steps in an atomic way:
>   1) Adapt symbols
>   2) Applied board overlay

I think that can be solved by not applying them in two steps, but by
in-memory merging of the symbol and board overlays first, and applying
the result.

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