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

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Message-ID: <20250918094409.0d5f92ec@bootlin.com>
Date: Thu, 18 Sep 2025 09:44:09 +0200
From: Herve Codina <herve.codina@...tlin.com>
To: David Gibson <david@...son.dropbear.id.au>, Krzysztof Kozlowski
 <krzk@...nel.org>, Rob Herring <robh@...nel.org>, Andrew Davis
 <afd@...com>, Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc: Ayush Singh <ayush@...gleboard.org>, Luca Ceresoli
 <luca.ceresoli@...tlin.com>, devicetree@...r.kernel.org, Jason Kridner
 <jkridner@...il.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, Krzysztof
 Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
 devicetree-compiler@...r.kernel.org, linux-kernel@...r.kernel.org, Thomas
 Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: Device tree representation of (hotplug) connectors: discussion
 at ELCE

Hi David,

On Thu, 18 Sep 2025 13:16:32 +1000
David Gibson <david@...son.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.

We work on decoupling busses wired to a connector and dispatching nodes
looks like this decoupling is ignored.

IMHO, keeping devices available on an addon board as nodes under the
connector is a real hardware representation.

Also, at runtime, once an addon board DT is applied, when you look at
your current DT either using /proc/device-tree or some links such as
/sys/bus/devices/.../of_node, the connector and extension bus appear
and clearly identify devices behind the connector.

> 
> That approach does complicate removal, but its not as bad as overlays
> at the moment, because a) it could be limited to adding new nodes, not
> modifying existing ones and b) the connector would specify exactly the
> places that additions are allowed.
> 

I think bus extensions comply with a) and b).

Yes, bus extensions need to be handled per-bus types but they have the
advantage of keeping the hardware reality well described and visible at
runtime in term of "wiring" topology.

Whatever the solution, this will already be handled per-bus types.
Only busses that support runtime DT node addition/removal (OF_RECONFIG_*
notifications in the kernel implementation) will support adding or
removing nodes.

Your approach is more complex, dispatch node here and there and actually
is also a per-bus types solution.

I think, in order to choose between both solutions, the main question is:
Do we want to dispatch nodes provided by an addon DT everywhere in the base
DT ?

IMHO, the answer is no.

Rob, others, any opinion ?

Best regards,
Hervé