linux-kernel - Re: Disabling graph endpoints in device trees

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Date:	Mon, 22 Feb 2016 17:12:27 +0100
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	John Keeping <john@...anate.com>
Cc:	devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Frank Rowand <frowand.list@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: Disabling graph endpoints in device trees

Hi John,

Am Montag, den 22.02.2016, 14:14 +0000 schrieb John Keeping:
> Hi,
> 
> Is there a reason why endpoints in a device tree graph can't be
> disabled?

You can always remove them using /delete-node/, which also has the
advantage of reminding you not to leave a single dangling endpoint.

> I would like to be able to force the use of a particular CRTC for
> certain outputs even though the hardware is capable of connecting any
> CRTC to any output.  In this case I need to be able to support a wide
> range of frequencies for external HDMI monitors so I will configure one
> of the CRTCs to be able to generate these while the other will be tied
> into a limited set of clock rates as a result of the overall system
> clock setup.
> 
> Currently this can only be achieved by removing the endpoints from the
> base SoC .dtsi file but it feels like it should be possible to add
> 'status = "disabled"' to the nodes in the board-specific .dts in order
> to disable undesirable configurations.
>
> I tested the change below and it behaves exactly as I want, but I don't
> claim to understand all of the users of these functions to know if it
> will break something else (hence this isn't a formal patch).

I don't know that any driver depends on being able to parse disabled
endpoints, but given the above I'm not sure that keeping disabled
endpoints in the device tree is a useful feature.
Disabling ports makes more sense to me. It should be documented in
Documentation/devicetree/bindings/graph.txt though.

> -- >8 --
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index 017dd94..1e56b91 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -2143,7 +2143,7 @@ struct device_node *of_graph_get_port_by_id(struct device_node *parent, u32 id)
>  	if (node)
>  		parent = node;
>  
> -	for_each_child_of_node(parent, port) {
> +	for_each_available_child_of_node(parent, port) {
>  		u32 port_id = 0;
>  
>  		if (of_node_cmp(port->name, "port") != 0)
> @@ -2209,7 +2209,7 @@ struct device_node *of_graph_get_next_endpoint(const struct device_node *parent,
>  		 * getting the next child. If the previous endpoint is NULL this
>  		 * will return the first child.
>  		 */
> -		endpoint = of_get_next_child(port, prev);
> +		endpoint = of_get_next_available_child(port, prev);
>  		if (endpoint) {
>  			of_node_put(port);
>  			return endpoint;
> @@ -2219,7 +2219,7 @@ struct device_node *of_graph_get_next_endpoint(const struct device_node *parent,
>  		prev = NULL;
>  
>  		do {
> -			port = of_get_next_child(parent, port);
> +			port = of_get_next_available_child(parent, port);
>  			if (!port)
>  				return NULL;
>  		} while (of_node_cmp(port->name, "port"));
> -- 8< --
> 
> 
> Thanks,
> John

best regards
Philipp