linux-kernel - Re: [RFC PATCH] [media]: of: move graph helpers from drivers/media/v4l2-core to drivers/of

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Date:	Tue, 11 Mar 2014 15:27:43 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:	Philipp Zabel <p.zabel@...gutronix.de>,
	Grant Likely <grant.likely@...aro.org>,
	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>,
	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 11/03/14 15:16, Laurent Pinchart wrote:

>> And if I gathered Grant's opinion correctly (correct me if I'm wrong),
>> he thinks things should be explicit, i.e. the bindings for, say, an
>> encoder should state that the encoder's output endpoint _must_ contain a
>> remote-endpoint property, whereas the encoder's input endpoint _must
>> not_ contain a remote-endpoint property.
> 
> Actually my understand was that DT links would have the same direction as the 
> data flow. There would be no ambiguity in that case as the direction of the 

Ok. At least at some point in the discussion the rule of thumb was to
have the "master" point at the "slave", which are a bit vague terms, but
I think both display and camera controllers were given as examples of
"master".

> data flow is known. What happens for bidirectional data flows still need to be 
> discussed though. And if we want to use the of-graph bindings to describe 
> graphs without a data flow, a decision will need to be taken there too.

Yep. My worry is that if the link direction is defined in the bindings
for the device, somebody will at some point have a complex board which
happens to use two devices in such a way, that either neither of them
point to each other, or both point to each other.

Maybe we can make sure that never happens, but I feel a bit nervous
about it especially for bi-directional cases. If the link has no clear
main-dataflow direction, it may be a bit difficult to say which way to link.

With double-linking all those concerns just disappear.

 Tomi

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