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

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Message-Id: <FE390E17-5AFB-41AE-9EB3-7EBA3B5D92FF@antoniou-consulting.com>
Date:	Wed, 6 Nov 2013 11:57:50 +0200
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	Ionut Nicu <ioan.nicu.ext@....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 Ionut,

On Nov 6, 2013, at 11:51 AM, Ionut Nicu wrote:

> 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>.
> 

Unfortunately no since this is a raw bytestream interface; there is no file
information. The patchset does display any root level part-number & version properties
but that's completely options.

We could standardize in a named root property to use on each overlay, and the part-number &
version are as good as any.



>> 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.
> 

Yes. Some people expressed interest in something similar.

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

Yep.

> Thanks,
> Ionut

Let's get the basic support in now, and we can fix all of that on later patches.

Regards

-- Pantelis

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