Hi,
On Tue, Nov 18, 2025 at 2:43 PM Doug Anderson dianders@chromium.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