linux-kernel - Re: [PATCH v4 1/3] [media] of: move graph helpers from drivers/media/v4l2-core to drivers/of

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Date:	Sat, 8 Mar 2014 12:46:05 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Grant Likely <grant.likely@...aro.org>,
	Philipp Zabel <p.zabel@...gutronix.de>
CC:	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-media@...r.kernel.org>,
	<devicetree@...r.kernel.org>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Philipp Zabel <philipp.zabel@...il.com>
Subject: Re: [PATCH v4 1/3] [media] of: move graph helpers from drivers/media/v4l2-core
 to drivers/of

On 07/03/14 19:18, Grant Likely wrote:

> From a pattern perspective I have no problem with that.... From an
> individual driver binding perspective that is just dumb! It's fine for
> the ports node to be optional, but an individual driver using the
> binding should be explicit about which it will accept. Please use either
> a flag or a separate wrapper so that the driver can select the
> behaviour.

Why is that? The meaning of the DT data stays the same, regardless of
the existence of the 'ports' node. The driver uses the graph helpers to
parse the port/endpoint data, so individual drivers don't even have to
care about the format used.

As I see it, the graph helpers should allow the drivers to iterate the
ports and the endpoints for a port. These should work the same way, no
matter which abbreviated format is used in the dts.

>> The helper should find the two endpoints in both cases.
>> Tomi suggests an even more compact form for devices with just one port:
>>
>> 	device {
>> 		endpoint { ... };
>>
>> 		some-other-child { ... };
>> 	};
> 
> That's fine. In that case the driver would specifically require the
> endpoint to be that one node.... although the above looks a little weird

The driver can't require that. It's up to the board designer to decide
how many endpoints are used. A driver may say that it has a single input
port. But the number of endpoints for that port is up to the use case.

> to me. I would recommend that if there are other non-port child nodes
> then the ports should still be encapsulated by a ports node.  The device
> binding should not be ambiguous about which nodes are ports.

Hmm, ambiguous in what way?

If the dts uses 'ports' node, all the ports and endpoints are inside
that 'ports' node. If there is no 'ports' node, there may be one or more
'port' nodes, which then contain endpoints. If there are no 'port'
nodes, there may be a single 'endpoint' node.

True, there are many "if"s there. But I don't think it's ambiguous. The
reason we have these abbreviations is that the full 'ports' node is not
needed that often, and it is rather verbose. In almost all the use
cases, panels and connectors can use the single endpoint format.

 Tomi

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