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Date:	Thu, 20 Mar 2014 17:36:09 +0000
From:	Grant Likely <grant.likely@...aro.org>
To:	Philipp Zabel <p.zabel@...gutronix.de>
Cc:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	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 Tue, 11 Mar 2014 16:07:00 +0100, Philipp Zabel <p.zabel@...gutronix.de> wrote:
> Hi Grant,
> 
> Am Montag, den 10.03.2014, 14:58 +0000 schrieb Grant Likely:
> > On Mon, 10 Mar 2014 14:52:53 +0100, Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:
> > > On Monday 10 March 2014 12:18:20 Tomi Valkeinen wrote:
> > > > 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.
> > > 
> > > 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. And if you want a better 
> > > understanding of how complex media graphs can become, have a look at 
> > > http://ideasonboard.org/media/vsp1.0.pdf (that's a real world example, albeit 
> > > all connections are internal to the SoC in that particular case, and don't 
> > > need to be described in DT).
> > > 
> > > - There's also no such thing as a master device that can just point to slave 
> > > devices. Once again simple cases exist where that model could work, but real 
> > > world examples exist of complex pipelines with dozens of elements all 
> > > implemented by a separate IP core and handled by separate drivers, forming a 
> > > graph with long chains and branches. We thus need real graph bindings.
> > > 
> > > - Finally, having no backlinks in DT would make the software implementation 
> > > very complex. We need to be able to walk the graph in a generic way without 
> > > having any of the IP core drivers loaded, and without any specific starting 
> > > point. We would thus need to parse the complete DT tree, looking at all nodes 
> > > and trying to find out whether they're part of the graph we're trying to walk. 
> > > The complexity of the operation would be at best quadratic to the number of 
> > > nodes in the whole DT and to the number of nodes in the graph.
> > 
> > Not really. To being with, you cannot determine any meaning of a node
> > across the tree (aside from it being an endpoint) without also
> > understanding the binding that the node is a part of. That means you
> > need to have something matching against the compatible string on both
> > ends of the linkage. For instance:
> > 
> > panel {
> > 	compatible = "acme,lvds-panel";
> > 	lvds-port: port {
> > 	};
> > };
> > 
> > display-controller {
> > 	compatible = "encom,video";
> > 	port {
> > 		remote-endpoint = <&lvds-port>;
> > 	};
> > };
> > 
> > In the above example, the encom,video driver has absolutely zero
> > information about what the acme,lvds-panel binding actually implements.
> > There needs to be both a driver for the "acme,lvds-panel" binding and
> > one for the "encom,video" binding (even if the acme,lvds-panel binding
> > is very thin and defers the functionality to the video controller).
> > 
> > What you want here is the drivers to register each side of the
> > connection. That could be modeled with something like the following
> > (pseudocode):
> > 
> > struct of_endpoint {
> > 	struct list_head list;
> > 	struct device_node *ep_node;
> > 	void *context;
> > 	void (*cb)(struct of_endpoint *ep, void *data);
> > }
> > 
> > int of_register_port(struct device *node, void (*cb)(struct of_endpoint *ep, void *data), void *data)
> > {
> > 	struct of_endpoint *ep = kzalloc(sizeof(*ep), GFP_KERNEL);
> > 
> > 	ep->ep_node = node;
> > 	ep->data = data;
> > 	ep->callback = cb;
> > 
> > 	/* store the endpoint to a list */
> > 	/* check if the endpoint has a remote-endpoint link */
> > 		/* If so, then link the two together and call the
> > 		 * callbacks */
> > }
> > 
> > That's neither expensive or complicated.
> > 
> > Originally I suggested walking the whole tree multiple times, but as
> > mentioned that doesn't scale, and as I thought about the above it isn't
> > even a valid thing to do. Everything has to be driven by drivers, so
> > even if the backlinks are there, nothing can be done with the link until
> > the other side goes through enumeration independently.
> 
> I have implemented your suggestion as follows. Basically, this allows
> either endpoint to contain the remote-endpoint link, as long as all
> drivers register their endpoints in the probe function and return
> -EPROBE_DEFER from their component framework bind callback until all
> their endpoints are connected.

This looks reasonable.

g.

> 
> From fdda1fb2bd133200d4620adcbb28697cb360e1cb Mon Sep 17 00:00:00 2001
> From: Philipp Zabel <p.zabel@...gutronix.de>
> Date: Tue, 11 Mar 2014 15:56:18 +0100
> Subject: [PATCH] of: Implement of_graph_register_endpoint
> 
> This patch adds a function that lets drivers register their endpoints in a
> global list. Newly registered endpoints are compared against known endpoints
> to check if a connection should be made. If so, the driver is notified via
> a simple callback.
> 
> Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> ---
>  drivers/of/base.c        | 69 +++++++++++++++++++++++++++++++++++++++++++++---
>  include/linux/of_graph.h | 20 +++++++++++++-
>  2 files changed, 84 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index ebb001a..77ae54a 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -29,6 +29,7 @@
>  #include "of_private.h"
>  
>  LIST_HEAD(aliases_lookup);
> +LIST_HEAD(endpoint_list);
>  
>  struct device_node *of_allnodes;
>  EXPORT_SYMBOL(of_allnodes);
> @@ -2002,6 +2003,7 @@ int of_graph_parse_endpoint(const struct device_node *node,
>  	memset(endpoint, 0, sizeof(*endpoint));
>  
>  	endpoint->local_node = node;
> +	endpoint->remote_node = of_parse_phandle(node, "remote-endpoint", 0);
>  	/*
>  	 * It doesn't matter whether the two calls below succeed.
>  	 * If they don't then the default value 0 is used.
> @@ -2126,6 +2128,19 @@ struct device_node *of_graph_get_next_endpoint(const struct device_node *parent,
>  }
>  EXPORT_SYMBOL(of_graph_get_next_endpoint);
>  
> +static struct of_endpoint *__of_graph_lookup_endpoint(
> +				const struct device_node *node)
> +{
> +	struct of_endpoint *ep;
> +
> +	list_for_each_entry(ep, &endpoint_list, list) {
> +		if (ep->local_node == node)
> +			return ep;
> +	}
> +
> +	return NULL;
> +}
> +
>  /**
>   * of_graph_get_remote_port_parent() - get remote port's parent node
>   * @node: pointer to a local endpoint device_node
> @@ -2136,11 +2151,15 @@ EXPORT_SYMBOL(of_graph_get_next_endpoint);
>  struct device_node *of_graph_get_remote_port_parent(
>  			       const struct device_node *node)
>  {
> +	struct of_endpoint *ep;
>  	struct device_node *np;
>  	unsigned int depth;
>  
>  	/* Get remote endpoint node. */
> -	np = of_parse_phandle(node, "remote-endpoint", 0);
> +	ep = __of_graph_lookup_endpoint(node);
> +	if (!ep || !ep->remote_node)
> +		return NULL;
> +	np = ep->remote_node;
>  
>  	/* Walk 3 levels up only if there is 'ports' node */
>  	for (depth = 3; depth && np; depth--) {
> @@ -2163,13 +2182,14 @@ EXPORT_SYMBOL(of_graph_get_remote_port_parent);
>   */
>  struct device_node *of_graph_get_remote_port(const struct device_node *node)
>  {
> +	struct of_endpoint *ep;
>  	struct device_node *np;
>  
>  	/* Get remote endpoint node. */
> -	np = of_parse_phandle(node, "remote-endpoint", 0);
> -	if (!np)
> +	ep = __of_graph_lookup_endpoint(node);
> +	if (!ep || !ep->remote_node)
>  		return NULL;
> -	np = of_get_next_parent(np);
> +	np = of_get_next_parent(ep->remote_node);
>  	if (of_node_cmp(np->name, "port")) {
>  		of_node_put(np);
>  		return NULL;
> @@ -2177,3 +2197,44 @@ struct device_node *of_graph_get_remote_port(const struct device_node *node)
>  	return np;
>  }
>  EXPORT_SYMBOL(of_graph_get_remote_port);
> +
> +int of_graph_register_endpoint(const struct device_node *node,
> +		void (*cb)(struct of_endpoint *ep, void *data), void *data)
> +{
> +	struct of_endpoint *remote_ep, *ep = kmalloc(sizeof(*ep), GFP_KERNEL);
> +	if (!ep)
> +		return -ENOMEM;
> +
> +	of_graph_parse_endpoint(node, ep);
> +	ep->callback = cb;
> +	ep->data = data;
> +
> +	list_add(&ep->list, &endpoint_list);
> +
> +	list_for_each_entry(remote_ep, &endpoint_list, list) {
> +		struct of_endpoint *from, *to;
> +		if (ep->remote_node) {
> +			from = ep;
> +			to = remote_ep;
> +		} else {
> +			from = remote_ep;
> +			to = ep;
> +		}
> +		if (from->remote_node &&
> +		    from->remote_node == to->local_node) {
> +			WARN_ON(to->remote_node &&
> +				to->remote_node != from->local_node);
> +			to->remote_node = from->local_node;
> +			to->remote_ep = from;
> +			from->remote_ep = to;
> +			if (from->callback)
> +				from->callback(from, from->data);
> +			if (to->callback)
> +				to->callback(to, to->data);
> +			return 0;
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(of_graph_register_endpoint);
> diff --git a/include/linux/of_graph.h b/include/linux/of_graph.h
> index 3a3c5a9..f00ac4e 100644
> --- a/include/linux/of_graph.h
> +++ b/include/linux/of_graph.h
> @@ -23,7 +23,14 @@
>  struct of_endpoint {
>  	unsigned int port;
>  	unsigned int id;
> -	const struct device_node *local_node;
> +	struct device_node *local_node;
> +	struct device_node *remote_node;
> +	struct of_endpoint *remote_ep;
> +
> +	/* Internal use only */
> +	struct list_head list;
> +	void (*callback)(struct of_endpoint *ep, void *data);
> +	void *data;
>  };
>  
>  #ifdef CONFIG_OF
> @@ -35,6 +42,10 @@ struct device_node *of_graph_get_next_endpoint(const struct device_node *parent,
>  struct device_node *of_graph_get_remote_port_parent(
>  					const struct device_node *node);
>  struct device_node *of_graph_get_remote_port(const struct device_node *node);
> +
> +int of_graph_register_endpoint(const struct device_node *ep_node,
> +				void (*cb)(struct of_endpoint *ep, void *data),
> +				void *data);
>  #else
>  
>  static inline int of_graph_parse_endpoint(const struct device_node *node,
> @@ -68,6 +79,13 @@ static inline struct device_node *of_graph_get_remote_port(
>  	return NULL;
>  }
>  
> +static inline int of_graph_register_endpoint(const struct device_node *ep_node,
> +				void (*cb)(struct of_endpoint *ep, void *data),
> +				void *data);
> +{
> +	return -ENOSYS;
> +}
> +
>  #endif /* CONFIG_OF */
>  
>  #endif /* __LINUX_OF_GRAPH_H */
> -- 
> 1.9.0
> 
> 

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