Date:	Thu, 7 Jul 2016 17:15:48 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	frowand.list@...il.com
Cc:	robh+dt@...nel.org, pantelis.antoniou@...sulko.com,
	stephen.boyd@...aro.org, broonie@...nel.org,
	grant.likely@...retlab.ca, mark.rutland@....com,
	mporter@...sulko.com, koen@...inion.thruhere.net,
	linux@...ck-us.net, marex@...x.de, wsa@...-dreams.de,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-i2c@...r.kernel.org, panto@...oniou-consulting.com
Subject: Re: [RFC PATCH 0/1] Portable Device Tree Connector -- conceptual

On Sat, Jul 02, 2016 at 04:55:49PM -0700, frowand.list@...il.com wrote:
> From: Frank Rowand <frank.rowand@...sony.com>
> 
> Hi All,
> 
> This is version 2 of this email.
> 
> Changes from version 1:
> 
>   - some rewording of the text
>   - removed new (theoretical) dtc directive "/connector/"
>   - added compatibility between mother board and daughter board
>   - added info on applying a single .dtbo to different connectors
>   - attached an RFC patch showing the required kernel changes
>   - changes to mother board .dts connector node:
>      - removed target_path property
>      - added connector-socket property
>   - changes to daughter board .dts connector node:
>      - added connector-plug property
> 
> 
> I've been trying to wrap my head around what Pantelis and Rob have written
> on the subject of a device tree representation of a connector for a
> daughter board to connect to (eg a cape or a shield) and the representation
> of the daughter board.  (Or any other physically pluggable object.)
> 
> After trying to make sense of what had been written (or presented via slides
> at a conference - thanks Pantelis!), I decided to go back to first principals
> of what we are trying to accomplish.  I came up with some really simple bogus
> examples to try to explain what my thought process is.
> 
> This is an extremely simple example to illustrate the concepts.  It is not
> meant to represent the complexity of a real board.
> 
> To start with, assume that the device that will eventually be on a daughter
> board is first soldered onto the mother board.  The mother board contains
> two devices connected via bus spi_1.  One device is described in the .dts
> file, the other is described in an included .dtsi file.
> Then the device tree files will look like:
> 
> $ cat board.dts
> /dts-v1/;
> 
> / {
>         #address-cells = < 1 >;
>         #size-cells = < 1 >;
> 
>         tree_1: soc@0 {
>                 reg = <0x0 0x0>;
> 
>                 spi_1: spi1 {
>                 };
>         };
> 
> };
> 
> &spi_1 {
>         ethernet-switch@0 {
>                 compatible = "micrel,ks8995m";
>         };
> };
> 
> #include "spi_codec.dtsi"
> 
> 
> $ cat spi_codec.dtsi
> &spi_1 {
> 	codec@1 {
> 		compatible = "ti,tlv320aic26";
> 	};
> };
> 
> 
> #----- codec chip on cape
> 
> Then suppose I move the codec chip to a cape.  Then I will have the same
> exact .dts and .dtsi and everything still works.
> 
> 
> @----- codec chip on cape, overlay
> 
> If I want to use overlays, I only have to add the version and "/plugin/",
> then use the '-@' flag for dtc (both for the previous board.dts and
> this spi_codec_overlay.dts):
> 
> $ cat spi_codec_overlay.dts
> /dts-v1/;
> 
> /plugin/;
> 
> &spi_1 {
> 	codec@1 {
> 		compatible = "ti,tlv320aic26";
> 	};
> };
> 
> 
> Pantelis pointed out that the syntax has changed to be:
>    /dts-v1/ /plugin/;
> 
> 
> #----- codec chip on cape, overlay, connector
> 
> Now we move into the realm of connectors.  My mental model of what the
> hardware and driver look like has not changed.  The only thing that has
> changed is that I want to be able to specify that the connector that
> the cape is plugged into has some pins that are the spi bus /soc/spi1.
> 
> The following _almost_ but not quite gets me what I want.  Note that
> the only thing the connector node does is provide some kind of
> pointer or reference to what node(s) are physically routed through
> the connector.  The connector node does not need to describe the pins;
> it only has to point to the node that describes the pins.
> 
> This example will turn out to be not sufficient.  It is a stepping
> stone in building my mental model.
> 
> $ cat board_with_connector.dts
> /dts-v1/;
> 
> / {
> 	#address-cells = < 1 >;
> 	#size-cells = < 1 >;
> 
> 	tree_1: soc@0 {
> 		reg = <0x0 0x0>;
> 
> 		spi_1: spi1 {
> 		};
> 	};
> 
> 	connector_1: connector_1 {
> 		spi1 {
> 			target_phandle = <&spi_1>;
> 		};
> 	};
> 
> };
> 
> &spi_1 {
> 	ethernet-switch@0 {
> 		compatible = "micrel,ks8995m";
> 	};
> };
> 
> 
> $ cat spi_codec_overlay_with_connector.dts
> /dts-v1/;
> 
> /plugin/;
> 
> &connector_1 {
> 	spi1 {
> 		codec@1 {
> 			compatible = "ti,tlv320aic26";
> 		};
> 	};
> };
> 
> 
> The result is that the overlay fixup for spi1 on the cape will
> relocate the spi1 node to /connector_1 in the host tree, so
> this does not solve the connector linkage yet:
> 
> -- chunk from the decompiled board_with_connector.dtb:
> 
> 	__symbols__ {
> 		connector_1 = "/connector_1";
> 	};
> 
> -- chunk from the decompiled spi_codec_overlay_with_connector.dtb:
> 
> 	fragment@0 {
> 		target = <0xffffffff>;
> 		__overlay__ {
> 			spi1 {
> 				codec@1 {
> 					compatible = "ti,tlv320aic26";
> 				};
> 			};
> 		};
> 	};
> 	__fixups__ {
> 		connector_1 = "/fragment@0:target:0";
> 	};
> 
> 
> After applying the overlay, the codec@1 node will be at
> /connector_1/spi1/codec@1.  What I want is for that node
> to be at /spi1/codec@1.
> 
> 
> 
> #----- magic new syntax
> 
> What I really want is some way to tell dtc that I want to do one
> level of dereferencing when resolving the path of device nodes
> contained by the connector node in the overlay dts.
> 
> Version 1 of this email suggested using dtc magic to do this extra
> level of dereferencing.  This version of the email has changed to
> have the kernel code that applies the overlay do the extra level
> of dereferencing.
> 
> The property "connector-socket" tells the kernel overlay code
> that this is a socket.  The overlay code does not actually
> do anything special as a result of this property; it is simply
> used as a sanity check that this node really is a socket.  The
> person writing the mother board .dts must provide the
> target_phandle property, which points to a node responsible for
> some of the pins on the connector.
> 
> The property "connector-plug" tells the kernel overlay code
> that each child node in the overlay corresponds to a node in the
> socket, and the socket will contain one property that is
> a phandle pointing to the node that is the target of that child
> node in the overlay node.
> 
> 
> $ cat board_with_connector_v2.dts
> 
> /dts-v1/;
> 
> / {
> 	#address-cells = < 1 >;
> 	#size-cells = < 1 >;
> 
> 	tree_1: soc@0 {
> 		reg = <0x0 0x0>;
> 
> 		spi_1: spi1 {
> 		};
> 	};
> 
> 	connector_1: connector_1 {
> 		compatible = "11-pin-accessory";
> 		connector-socket;

I don't see any advantage to allowing connectors anywhere in the tree:
pretty much by definition a connector is a "whole board" concept.  So
I think instead they should all go in a new special node under the
root, say /connectors.  With that done, you don't need the
connector-socket tag any more.

> 		spi1 {
> 			target_phandle = <&spi_1>;
> 		};
> 	};
> 
> };
> 
> &spi_1 {
> 	ethernet-switch@0 {
> 		compatible = "micrel,ks8995m";
> 	};
> };
> 
> 
> $ cat spi_codec_overlay_with_connector_v2.dts
> 
> /dts-v1/;
> 
> /plugin/;
> 
> &connector_1 {
> 	connector-plug;
> 	compatible = "11-pin-accessory";
> 
> 	spi1 {
> 		codec@1 {
> 			compatible = "ti,tlv320aic26";
> 		};
> 	};
> };
> 
> 
> The spi_codec_overlay_with_connector_v2.dtb __fixups__ information
> is unchanged from the previous example, but the kernel overlay
> code will do the correct extra level of dereferencing when it
> detects the connector-plug property in the overlay.
> 
> The one remaining piece that this patch does not provide is how
> the overlay manager (which does not yet exist in the mainline
> tree) can apply an overlay to two different targets.  That
> final step should be a trivial change to of_overlay_create(),
> adding a parameter that is a mapping of the target (or maybe
> even targets) in the overlay to different targets in the
> active device tree.
> 
> This seems like a more straight forward way to handle connectors.
> 
> First, ignoring pinctrl and pinmux, what does everyone think?
> 
> Then, the next step is whether pinctrl and pinmux work with this method.
> Pantelis, can you point me to a good example for
> 
>   1) an in-tree board dts file
>   2) an overlay file (I am assuming out of tree) that applies to the board
>   3) an in-tree .dtsi file that would provide the same features as
>      the overlay file if it was included by the board dts file
> 
> It should be easier to discuss pinctrl and pinmux with an example.

Hrm.. so I think you're trying to stick too close to the existing
overlay model.  Something I've always disliked about that model is
that the plugin can overlay *anywhere* in the master tree, meaning it
must have intimate knowledge of that tree.  Instead of using the
global __symbols__, there should be a set of "symbols" local to the
specific connector (socket), which are the *only* points which the
plugin is allowed to overlay or reference.

Given that we're going to need new code to support this new connector
model, I think we should also fix some of the uglies in the current
overlay format while we're at it.

I have to run now, but I'll try to send out a counter-proposal
shortly.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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