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Date:	Mon, 10 Mar 2014 12:18:20 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Grant Likely <grant.likely@...aro.org>,
	Philipp Zabel <p.zabel@...gutronix.de>
CC:	Sascha Hauer <s.hauer@...gutronix.de>,
	Rob Herring <robherring2@...il.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	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-kernel@...r.kernel.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Philipp Zabel <philipp.zabel@...il.com>
Subject: Re: [RFC PATCH] [media]: of: move graph helpers from drivers/media/v4l2-core
 to drivers/of

On 08/03/14 13:41, Grant Likely wrote:

>> Ok. If we go for single directional link, the question is then: which
>> way? And is the direction different for display and camera, which are
>> kind of reflections of each other?
> 
> In general I would recommend choosing whichever device you would
> sensibly think of as a master. In the camera case I would choose the
> camera controller node instead of the camera itself, and in the display
> case I would choose the display controller instead of the panel. The
> binding author needs to choose what she things makes the most sense, but
> drivers can still use if it it turns out to be 'backwards'

I would perhaps choose the same approach, but at the same time I think
it's all but clear. The display controller doesn't control the panel any
more than a DMA controller controls, say, the display controller.

In fact, in earlier versions of OMAP DSS DT support I had a simpler port
description, and in that I had the panel as the master (i.e. link from
panel to dispc) because the panel driver uses the display controller's
features to provide the panel device a data stream.

And even with the current OMAP DSS DT version, which uses the v4l2 style
ports/endpoints, the driver model is still the same, and only links
towards upstream are used.

So one reason I'm happy with the dual-linking is that I can easily
follow the links from the downstream entities to upstream entities, and
other people, who have different driver model, can easily do the opposite.

But I agree that single-linking is enough and this can be handled at
runtime, even if it makes the code more complex. And perhaps requires
extra data in the dts, to give the start points for the graph.

 Tomi



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