linux-kernel - Re: [PATCH v2] of: overlay: user space synchronization

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Message-ID: <294737e8-f94e-5d53-acaf-ecb9a4ef42af@gmail.com>
Date:   Mon, 15 Oct 2018 19:47:16 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     valdis.kletnieks@...edu
Cc:     Rob Herring <robh+dt@...nel.org>, pantelis.antoniou@...sulko.com,
        Pantelis Antoniou <panto@...oniou-consulting.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        geert@...ux-m68k.org, Alan Tull <atull@...nel.org>
Subject: Re: [PATCH v2] of: overlay: user space synchronization

On 10/15/18 17:35, valdis.kletnieks@...edu wrote:
> On Mon, 15 Oct 2018 17:27:01 -0700, frowand.list@...il.com said:
>> From: Frank Rowand <frank.rowand@...y.com>
>>
>> When an overlay is applied or removed, the live devicetree visible in
>> /proc/device-tree/, aka /sys/firmware/devicetree/base/, reflects the
>> changes.  There is no method for user space to determine whether the
>> live devicetree was modified by overlay actions.
>>
>> Provide a sysfs file, /sys/firmware/devicetree/tree_version,  to allow
>> user space to determine if the live devicetree has remained unchanged
>> while a series of one or more accesses of /proc/device-tree/ occur.
>>
>> The use of both dynamic devicetree modifications and overlay apply and
>> removal are not supported during the same boot cycle.  Thus non-overlay
>> dynamic modifications are not reflected in the value of tree_version.
> 
> Is there an easy way from userspace to detect "yes/no dynamic modifications
> have been done since boot"?
> 

Unfortunately not.

I considered adding detecting dynamic modifications in another patch, making
this a two patch series, but that feature will be much more intrusive than
detecting modifications made by overlays.  I may add that feature in the
future, but not anytime soon.

If you want to understand a little bit about why dynamic modifications
occur, please see my reply to Geert in the thread about version 1 of
this patch.