linux-kernel - Re: Portable Device Tree Connector -- conceptual

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Date:	Thu, 7 Jul 2016 14:51:35 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
Cc:	Frank Rowand <frowand.list@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Stephen Boyd <stephen.boyd@...aro.org>,
	Mark Brown <broonie@...nel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Mark Rutland <mark.rutland@....com>,
	Matt Porter <mporter@...sulko.com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Guenter Roeck <linux@...ck-us.net>,
	Marek Vasut <marex@...x.de>, Wolfram Sang <wsa@...-dreams.de>,
	devicetree <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-i2c@...r.kernel.org
Subject: Re: Portable Device Tree Connector -- conceptual

On Fri, Jul 01, 2016 at 01:59:58PM +0300, Pantelis Antoniou wrote:
> Hi Frank,
> 
> Comments inline.
> 
> > On Jul 1, 2016, at 03:02 , Frank Rowand <frowand.list@...il.com> wrote:
> > 
> > Hi All,
> > 
> > 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.
> > 
> > To start with, assume that the device that will eventually be on a daughter
> > board is first soldered onto the main board.  Then the device tree 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";
> > 	};
> > };
> > 
> 
> The correct form now for the /plugin/ declaration should be like
> 
> /dts-v1/ /plugin/;
> 
> The old method still works for backward compatibility.
> 
> In fact with the new patches you don’t even /plugin/ since when
> compiling an overlay we can turn on the plugin flag by looking
> at the output type (dtbo).

I'd prefer to see the dtbo option go away however, in favour of the
/plugin/ flag.

-- 
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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