linux-kernel - Re: [PATCH v4 3/3] Documentation: of: Document graph bindings

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Message-ID: <530DFF4C.8080807@ti.com>
Date:	Wed, 26 Feb 2014 16:50:52 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Philipp Zabel <p.zabel@...gutronix.de>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	<linux-kernel@...r.kernel.org>, <linux-media@...r.kernel.org>,
	<devicetree@...r.kernel.org>,
	Guennadi Liakhovetski <g.liakhovetski@....de>
Subject: Re: [PATCH v4 3/3] Documentation: of: Document graph bindings

On 26/02/14 16:57, Philipp Zabel wrote:
> Hi Tomi,
> 
> Am Mittwoch, den 26.02.2014, 15:14 +0200 schrieb Tomi Valkeinen:
>> On 25/02/14 16:58, Philipp Zabel wrote:
>>
>>> +Optional endpoint properties
>>> +----------------------------
>>> +
>>> +- remote-endpoint: phandle to an 'endpoint' subnode of a remote device node.
>>
>> Why is that optional? What use is an endpoint, if it's not connected to
>> something?
> 
> This allows to include the an empty endpoint template in a SoC dtsi for
> the convenience of board dts writers. Also, the same property is
> currently listed as optional in video-interfaces.txt.
> 
>   soc.dtsi:
> 	display-controller {
> 		port {
> 			disp0: endpoint { };
> 		};
> 	};
> 
>   board.dts:
> 	#include "soc.dtsi"
> 	&disp0 {
> 		remote-endpoint = <&panel_input>;
> 	};
> 	panel {
> 		port {
> 			panel_in: endpoint {
> 				remote-endpoint = <&disp0>;
> 			};
> 		};
> 	};
> 
> Any board not using that port can just leave the endpoint disconnected.

Hmm I see. I'm against that.

I think the SoC dtsi should not contain endpoint node, or even port node
(at least usually). It doesn't know how many endpoints, if any, a
particular board has. That part should be up to the board dts.

I've done this with OMAP as (much simplified):

SoC.dtsi:

dss: dss@...00000 {
	status = "disabled";
};

Nothing else (relevant here). The binding documentation states that dss
has one port, and information what data is needed for the port and endpoint.

board.dts:

&dss {
        status = "ok";

        pinctrl-names = "default";
        pinctrl-0 = <&dss_dpi_pins>;

        dpi_out: endpoint {

                remote-endpoint = <&tfp410_in>;
                data-lines = <24>;
        };
};

That's using the shortened version without port node.

Of course, it's up to the developer how his dts looks like. But to me it
makes sense to require the remote-endpoint property, as the endpoint, or
even the port, doesn't make much sense if there's nothing to connect to.

 Tomi



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