linux-kernel - Re: [PATCH 3/3] DT: proc: Add runtime overlay interface in /proc

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Date:	Wed, 06 Nov 2013 10:51:43 +0100
From:	Ionut Nicu <ioan.nicu.ext@....com>
To:	ext Pantelis Antoniou <panto@...oniou-consulting.com>
CC:	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <robherring2@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Alison Chaiken <Alison_Chaiken@...tor.com>,
	Dinh Nguyen <dinh.linux@...il.com>,
	Jan Lubbe <jluebbe@...net.de>,
	Alexander Sverdlin <alexander.sverdlin@....com>,
	Michael Stickel <ms@...able.de>,
	Guenter Roeck <linux@...ck-us.net>,
	Dirk Behme <dirk.behme@...il.com>,
	Alan Tull <delicious.quinoa@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Michael Bohan <mbohan@...eaurora.org>,
	Michal Simek <monstr@...str.eu>,
	Matt Ranostay <mranostay@...il.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] DT: proc: Add runtime overlay interface in /proc

Hi,

On 05.11.2013 19:41, ext Pantelis Antoniou wrote:
> Add a runtime interface to /proc to enable generic device tree overlay
> usage.
> 
> Two new /proc files are added:
> 
> /proc/device-tree-overlay & /proc/device-tree-overlay-status
> 
> /proc/device-tree-overlay accepts a stream of a device tree objects and
> applies it to the running kernel's device tree.
> 
> 	$ cat ~/BB-UART2-00A0.dtbo >device-tree-overlay
> 	overlay_proc_release: Applied #2 overlay segments @0
> 
> /proc/device-tree-overlay-status displays the the overlays added using
> the /proc interface
> 
> 	$ cat device-tree-overlay-status
> 	0: 861 bytes BB-UART2:00A0
> 
> The format of the status line is
> 	<ID>: <SIZE> bytes <part-number>:<version>
> 
> <ID> is the id of the overlay
> <SIZE> is the size of the overlay in bytes
> <part-number>, <version> are (optional) root level properties of the DTBO
> 
> You can remove an overlay by echoing the <ID> number of the overlay
> precedded with a '-'
> 
> So
> 	$ echo "-0" >device-tree-overlay-status
> 

Wouldn't it be easier if echo "-BB-UART2-00A0" > device-tree-overlay-status was
supported also? That way one doesn't need to know the order in which the
overlays were applied or parse the status file to get the <ID>.

> Removes the overlay.
> 
> Note that this seldom works on most platforms since platform_device
> removal is something that almost never works without extra patches.
> 
> Signed-off-by: Pantelis Antoniou <panto@...oniou-consulting.com>

It would be very helpful to me if I would have a notification mechanism
for overlay add and remove operations based on blocking_notifier_call_chain(),.
This way other drivers can be notified when the dt changes.

But I guess that could be added in the future with another patch.

Thanks,
Ionut
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