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Date:	Thu, 20 Mar 2014 17:54:31 +0000
From:	Grant Likely <grant.likely@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Rob Herring <robherring2@...il.com>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Rob Herring <robh+dt@...nel.org>,
	Sylwester Nawrocki <s.nawrocki@...sung.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 Wed, 12 Mar 2014 10:25:56 +0000, Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> On Mon, Mar 10, 2014 at 02:52:53PM +0100, Laurent Pinchart wrote:
> > In theory unidirectional links in DT are indeed enough. However, let's not 
> > forget the following.
> > 
> > - There's no such thing as single start points for graphs. Sure, in some 
> > simple cases the graph will have a single start point, but that's not a 
> > generic rule. For instance the camera graphs 
> > http://ideasonboard.org/media/omap3isp.ps and 
> > http://ideasonboard.org/media/eyecam.ps have two camera sensors, and thus two 
> > starting points from a data flow point of view.
> 
> I think we need to stop thinking of a graph linked in terms of data
> flow - that's really not useful.
> 
> Consider a display subsystem.  The CRTC is the primary interface for
> the CPU - this is the "most interesting" interface, it's the interface
> which provides access to the picture to be displayed for the CPU.  Other
> interfaces are secondary to that purpose - reading the I2C DDC bus for
> the display information is all secondary to the primary purpose of
> displaying a picture.
> 
> For a capture subsystem, the primary interface for the CPU is the frame
> grabber (whether it be an already encoded frame or not.)  The sensor
> devices are all secondary to that.
> 
> So, the primary software interface in each case is where the data for
> the primary purpose is transferred.  This is the point at which these
> graphs should commence since this is where we would normally start
> enumeration of the secondary interfaces.
> 
> V4L2 even provides interfaces for this: you open the capture device,
> which then allows you to enumerate the capture device's inputs, and
> this in turn allows you to enumerate their properties.  You don't open
> a particular sensor and work back up the tree.
> 
> I believe trying to do this according to the flow of data is just wrong.
> You should always describe things from the primary device for the CPU
> towards the peripheral devices and never the opposite direction.

Agreed.

g.
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