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

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Message-Id: <8EBC2D5B-74A4-4A85-8BEE-9EAEBFF045D4@antoniou-consulting.com>
Date:	Wed, 6 Nov 2013 21:24:12 +0200
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	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>,
	Ionut Nicu <ioan.nicu.ext@....com>,
	Michal Simek <monstr@...str.eu>,
	Matt Ranostay <mranostay@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] DT: proc: Add runtime overlay interface in /proc

Hi Rob,

On Nov 6, 2013, at 9:10 PM, Rob Herring wrote:

> On Tue, Nov 5, 2013 at 12:41 PM, Pantelis Antoniou
> <panto@...oniou-consulting.com> 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
> 
> I think we really want all this to live under sysfs. Grant did patches
> to move /proc/device-tree to /sys, but it never went upstream:
> 
> v2: https://lkml.org/lkml/2013/3/21/215
> v1: https://lkml.org/lkml/2013/3/20/311
> 

Yes, I'm aware; the location of this control interface in /proc is
unusual, but had to go somewhere. It should be easy enough to move it to
/sys.

>> /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
> 
> Is the size useful information?
> 

If the overlay doesn't contain part-number/version properties there is nothing
to differentiate each one loaded. No file information, it is just a byte stream
interface.

>> 
>> 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
>> 
>> Removes the overlay.
> 
> This interface seems racy. Could the id change on you between reading
> the status and echoing to remove the overlay?
> 
> I would rather see a file created for each overlay and simply echo 0
> or "remove" to remove the overlay. Or possibly it needs to be a
> directory per overlay with several files for info and control. This
> would be more inline with typical sysfs design.
> 

It was suggested to use a configfs interface. IIRC configfs can do what you
propose.

Something like 

/config/dto/add 	<- load by cat overlay.dtbo >/config/dto/load
/config/dto/0/remove 	<- unload by echo 1 >/config/dto/0/remove
/config/dto/0/${prop}   <- root level properties that are ignore by the overlay
			   mechanism


> Rob

Regards

-- Pantelis

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