Hi Frank,
Sorry for taking a bit to reply, had to grok it well first.
> On Jul 3, 2016, at 02:55 , frowand.list@xxxxxxxxx wrote:
>
> From: Frank Rowand <frank.rowand@xxxxxxxxxxx>
>
> 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";
> };
> };
> };
>
You target connector_1. In theory multiples of the same connector
may be available.
There are complications about how they are applied. A method that’s not
referring to a single phandle/path is going to be needed.
Thinking about it a bit more maybe we can sugar it with DTC with something like
this:
$ cat arduino_connector.dts
/dts-v1/ /plugin/ /portable/;
&arduino_connector {
spi1 {
codec@1 {
compatible = “ti,tlv320aic26”;
};
};
};
$ cat board_with_arduino_connectors.dts
/dts-v1/;
/ {
#address-cells = < 1 >;
#size-cells = < 1 >;
tree_1: soc@0 {
reg = <0x0 0x0>;
spi_1: spi1 {
};
};
connector_1 {
connector-socket;
compatible = “arduino_connector”;
status = “okay”;
spi1 {
target_phandle = <&spi_1>;
};
};
connector_2 {
connector-socket;
compatible = “arduino_connector”;
spi2 {
target_phandle = <&spi_2>;
};
};
};
&spi_1 {
ethernet-switch@0 {
compatible = "micrel,ks8995m";
};
};
&spi_2 {
ethernet-switch@0 {
compatible = "micrel,ks8995m";
};
};
The &arduino_connector construct at a portable overlay can be resolved
as follows:
fragment0 {
target-compatible = “arduino_connector“;
….
};
The new thing here is the ‘target-compatible’ option which the
loader will use to find the target node.
>
> 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";
> };
>
^ See above. Not going to work cause we need to support multiple
identical connectors on the same board.
> -- 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;
^ we won’t need this, nor the compatible string with the
version I mentioned earlier
> 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
>
Looks good for a starting point. We need to figure out pinmux and
gpio/irq references for starter.
It is imperative that references do not leak out of the connector
node.
> It should be easier to discuss pinctrl and pinmux with an example.
>
> -Frank
> —
Regards
— Pantelis
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