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

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Date:	Wed, 6 Nov 2013 13:10:27 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Pantelis Antoniou <panto@...oniou-consulting.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

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

> /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?

>
> 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.

Rob
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