linux-kernel - Re: [PATCH v6 2/8] Documentation: of: Document graph bindings

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Date:	Mon, 10 Mar 2014 10:28:10 +0100
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	Grant Likely <grant.likely@...aro.org>
Cc:	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Rob Herring <robh+dt@...nel.org>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	devicetree@...r.kernel.org
Subject: Re: [PATCH v6 2/8] Documentation: of: Document graph bindings

Hi Grant,

Am Freitag, den 07.03.2014, 18:27 +0000 schrieb Grant Likely:
> On Wed,  5 Mar 2014 10:20:36 +0100, Philipp Zabel <p.zabel@...gutronix.de> wrote:
> > The device tree graph bindings as used by V4L2 and documented in
> > Documentation/device-tree/bindings/media/video-interfaces.txt contain
> > generic parts that are not media specific but could be useful for any
> > subsystem with data flow between multiple devices. This document
> > describes the generic bindings.
> > 
> > Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> 
> See my comments on the previous version. My concerns are the handling of
> the optional 'ports' node and the usage of reverse links.

would this change address your concern about the reverse links? As the
preexisting video-interfaces.txt bindings mandate the reverse links, I
worry about introducing a second, subtly different binding. It should be
noted somewhere in video-interfaces.txt that the reverse links are
deprecated for the but still supported by the code for backwards
compatibility.

diff --git a/Documentation/devicetree/bindings/graph.txt b/Documentation/devicetree/bindings/graph.txt
index 1a69c07..eb6cae5 100644
--- a/Documentation/devicetree/bindings/graph.txt
+++ b/Documentation/devicetree/bindings/graph.txt
@@ -87,12 +87,13 @@ device {
 Links between endpoints
 -----------------------
 
-Each endpoint should contain a 'remote-endpoint' phandle property that points
-to the corresponding endpoint in the port of the remote device. In turn, the
-remote endpoint should contain a 'remote-endpoint' property. If it has one,
-it must not point to another than the local endpoint. Two endpoints with their
-'remote-endpoint' phandles pointing at each other form a link between the
-containing ports.
+Two endpoint nodes form a link between the two ports they are contained in
+if one contains a 'remote-endpoint' phandle property, pointing to the other
+endpoint. The endpoint pointed to should not contain a 'remote-endpoint'
+property itself. Which direction the phandle should point in depends on the
+device type. In general, links should be pointing outwards from central
+devices that provide DMA memory interfaces, such as display controller,
+video capture interface, or serial digital audio interface cores.
 
 device-1 {
         port {
@@ -104,8 +105,8 @@ device-1 {
 
 device-2 {
         port {
-                device_2_input: endpoint {
-                        remote-endpoint = <&device_1_output>;
+                device_2_input: endpoint { };
+                       /* no remote-endpoint, this endpoint is pointed at */
                 };
         };
 };

regards
Philipp

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