From: Jassi Brar jassisinghbrar@gmail.com Date: Fri, 8 Sep 2023 09:37:12 -0500
Hi Simon,
On Thu, Sep 7, 2023 at 3:08 PM Simon Glass sjg@chromium.org wrote:
On Wed, 6 Sept 2023 at 23:20, Ilias Apalodimas
I beg to differ. Devicetree is more than just hardware and always has been. See, for example the /chosen and /options nodes.
There are exceptions...
We've been this over and over again and frankly it gets a bit annoying. It's called *DEVICE* tree for a reason. As Rob pointed out there are exceptions, but those made a lot of sense. Having arbitrary internal ABI stuff of various projects in the schema just defeats the definition of a spec.
Our efforts should not just be about internal ABI, but working to provide a consistent configuration system for all firmware elements.
Sure, programmatically we can pass any data/info via DT, however it is only meant to map hardware features onto data structures.
devicetree.org landing page "The devicetree is a data structure for describing hardware."
devicetree-specification-v0.3.pdf Chapter-2 Line-1 "DTSpec specifies a construct called a devicetree to describe system hardware."
But it doesn't say that it describes *only* hardware. And it does describe more than just hardware. There is /chosen to specify firmware configuration and there are several examples of device tree bindings that describe non-discoverable firmware interfaces in the Linux tree. And it has been that way since the days of IEEE 1275. There are also bindings describing things like the firmware partition layout.
If we want to digress from the spec, we need the majority of developers to switch sides :) which is unlikely to happen and rightly so, imho.
It isn't even clear what the spec is. There is the document you reference above, there are the yaml files at https://github.com/devicetree-org/dt-schema and then there are additional yaml files in the Linux tree. As far as I know it isn't written down anywhere that those are the only places where device tree bindings can live.
Anyway, let's face it, there is some consensus among developers that what Simon has done in U-Boot is pushing the use of devicetree beyond the point where a significant fraction of developers thinks it makes sense. And I think I agree with that. But you can't beat him with the spec to make your point.
Now the devicetree is cleverly constructed such that it is possible to define additional bindings without the risk of conflicting with bindings developed by other parties. In particular if U-Boot is augmenting a device tree with properties that are prefixed with "u-boot," this isn't going to hurt an operating system that uses such an augmented device tree.
The real problem is that some folks developed a certification program that allegedly requires schema verification and now propose adding code to U-Boot that doesn't really solve any problem. My proposed solution would be to change said certification program to allow firmware to augment the device tree with properties and nodes with compatibles that are in the namespace controlled by the firmware.
Cheers,
Mark