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Subject[RFC PATCH 0/1] Portable Device Tree Connector -- conceptual
Date
From: Frank Rowand <frank.rowand@am.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;
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.

-Frank

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