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

Luca Ceresoli
2025-09-19T15:20:54Z

On Fri, 19 Sep 2025 10:47:17 +0530
Ayush Singh <ayush@beagleboard.org> wrote:

> On 9/19/25 10:22, David Gibson wrote:
>
> > On Thu, Sep 18, 2025 at 09:44:09AM +0200, Herve Codina wrote:
> >> Hi David,
> >>
> >> On Thu, 18 Sep 2025 13:16:32 +1000
> >> David Gibson <david@gibson.dropbear.id.au> wrote:
> >>
> >> ...
> >>
> >>>>> Thoughts above suggest a different direction, but here's what I was
> >>>>> thinking before:
> >>>>>
> >>>>> base board:
> >>>>>
> >>>>> connector {
> >>>>> /export/ "i2c" &i2c0;
> >>>>> };
> >>>>>
> >>>>> addon:
> >>>>> eeprom@10 {
> >>>>> compatible = "foo,eeprom";
> >>>>> bus-reg = <&i2c 0x10>;
> >>>>> }
> >>>>>
> >>>>> Or, if the addon had multiple i2c devices, maybe something like:
> >>>>>
> >>>>> board-i2c {
> >>>>> compatible = "i2c-simple-bridge";
> >>>>> bus-ranges = <&i2c 0 0x3ff>; /* Whole addr space */
> >>>>> eeprom@10 {
> >>>>> compatible = "foo,eeprom";
> >>>>> reg = <0x10>;
> >>>>> }
> >>>>> widget@20 {
> >>>>> compatible = "vendor,widget";
> >>>>> reg = <0x20>;
> >>>>> }
> >>>>> }
> >>>>>
> >>>>> Writing that, I realise I2C introduces some complications for this.
> >>>>> Because it has #size-cells = <0>, ranges doesn't really work (without
> >>>>> listing every single address to be translated). Likewise, because we
> >>>>> always need the parent bus phandle, we can't use the trick of an empty
> >>>>> 'ranges' to mean an identity mapping.
> >>>>>
> >>>>> We could invent encodings to address those, but given the addon with
> >>>>> multiple connectors case provides another incentive for a single
> >>>>> connector to allow adding nodes in multiple (but strictly enumerated)
> >>>>> places in the base device tree provides a better approach.
> >>>> and the "place in base device tree" is the goal of the extension bus.
> >>>>
> >>>> The strict enumeration of nodes enumerated is done by two means:
> >>>> - extension busses at connector level
> >>>> Those extensions are described as connector sub-nodes.
> >>>> The addon DT can only add nodes in those sub-nodes to describe devices
> >>>> connected to the relared extension bus.
> >>>> - export symbols
> >>>> An addon DT can only use symbols exported to reference symbols outside
> >>>> the addon DT itself.
> >>>>
> >>>> Can I assume that bus extensions we proposed (i2c-bus-extension and
> >>>> spi-bus-extension) could be a correct solution ?
> >>> Maybe? I prefer the idea of a universal mechanism, not one that's
> >>> defined per-bus-type.
> >>>
> >>>
> >>> Also, IIUC the way bus extension operates is a bit different - nodes
> >>> would be "physically" added under the bus extension node, but treated
> >>> logically as if they go under the main bus. What I'm proposing here
> >>> is something at the actualy overlay application layer that allows
> >>> nodes to be added to different parts of the base device tree - so you
> >>> could add your i2c device under the main i2c bus.
> >> I think we should avoid this kind of node dispatching here and there in
> >> the base DT.
> > Until I saw Geert's multi-connector case, I would have agreed. That
> > case makes me thing differently: in order to support that case we
> > already have to handle adding information in multiple places (under
> > all of the connectors the addon uses). Given we have to handle that
> > anyway, I wonder if it makes more sense to lean into that, and allow
> > updates to multiple (strictly enumerated) places.
>
> Well, I don't love this idea. Here are my main qalms about the approach
> of adding devices directly to the actual i2c/spi etc nodes.
>
> 1. In boards with multiple connectors, they sometimes share the same
> i2c. Now assume that someone decided to connect the same i2c device to
> both the connectors. If we are using something like bus extension, while
> the node would be added, it will fail in the registration since you
> cannot add the same address device a second time. However, if we are
> adding the device directly to the `main_i2c`, the overlay application
> will just end up modifying the exact same device node. There is no
> error, or even a 2nd device node in this case. It is just lost.

Thinking out loud: what about preventing loading any overlay that does
more than just adding nodes? IOW forbidding to create properties in
nodes already in the live tree, and modifying existing properties.

I think being very restrictive in terms of overlays the implementation
can accept is a good idea in general. A requirement can be relaxed in
the future, but forbidding what used to be allowed would be a nightmare.

Best regards,
Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering