linux-kernel - Re: Proposal: Officially allow "incomplete" trees as a base

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Message-ID: <CAD=FV=UbejKdD2q2Z3RuYH0Ooc6XRb0oynchDsqnq7GzM6ah0w@mail.gmail.com>
Date: Mon, 1 Dec 2025 09:42:40 -0800
From: Doug Anderson <dianders@...omium.org>
To: devicetree-spec@...r.kernel.org, boot-architecture@...ts.linaro.org, 
	Chen-Yu Tsai <wenst@...omium.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk@...nel.org>
Cc: "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Linux ARM <linux-arm-kernel@...ts.infradead.org>, 
	William McVicker <willmcvicker@...gle.com>, Julius Werner <jwerner@...omium.org>, 
	Conor Dooley <conor+dt@...nel.org>, Peter Griffin <peter.griffin@...aro.org>, 
	Tudor Ambarus <tudor.ambarus@...aro.org>, André Draszik <andre.draszik@...aro.org>
Subject: Re: Proposal: Officially allow "incomplete" trees as a base

Hi,

On Tue, Nov 18, 2025 at 2:43 PM Doug Anderson <dianders@...omium.org> wrote:
>
> This is a continuation of the discussion that started in reply to my
> patch adding basic device trees for Pixel 10 phones [1].
>
>
> Problem statement:
> ------------------
>
> We would like an officially accepted scheme that lets us more
> efficiently ship compiled device trees for a handful of related
> products by breaking the device trees up into a common "base" device
> tree and then applying "overlay" device trees atop the base to make a
> full and complete device tree.
>
> To make it more concrete, we'd like to build a "base" device tree that
> describes a SoC and then have the overlays be enough to make a full
> description of a board. In theory, one could also imagine wanting to
> expand this to 3 or more levels (perhaps SoC, baseboard, derived
> boards), though this is not planned at this time.
>
> The primary reason for wanting to break device trees like this is
> efficiency of the shipped binary device trees. A large portion of a
> final device tree just describes the SoC. We save space in the final
> compiled device trees if they don't need to contain as much duplicated
> information.
>
> A secondary reason for wanting to break device trees like this is to
> more nicely handle when a board has a socketed SoC that can be
> replaced with a finite (and small) number of different SoCs (usually
> revisions of the same SoC). Even if this secondary reason is
> considered invalid or too difficult, the primary reason still
> describes a compelling need.
>
> In order to make this proposal work, it's expected that a bootloader
> will understand the scheme and will know how to combine the overlay
> atop the base before passing a complete device tree to the main OS.

It's been roughly two weeks since I sent out this proposal. Do DT
folks have any comments? Are the goals I have stated understood? Do
people agree that these goals are reasonable? Is there any question
that there is a need to solve these problems not just for Google, but
for the community as a whole? I'm happy to reach out to people and
have them reply "yes, I have this problem too" if it would somehow
help. I don't doubt that there are still people at Qualcomm who would
like a solution even if I think Elliot isn't driving it there
anymore...

How do we make forward progress? Does anyone have any comments on
Julius's reply? At the moment, I think there are some conflicts with
what Julius would like to see (no changes to the rules for how
overlays are applied) and what Rob said previously (we need to find
some way to combine the compatible strings). Did I misunderstand? Can
we find a common ground?

Thanks!

-Doug