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From:   Pintu Agarwal <pintu.ping@gmail.com>
Date:   Thu, 24 Jun 2021 21:39:06 +0530
Message-ID: <CAOuPNLhoLBwksNt+tJ2=hZr-TGHZgEiu7dgk66BUsraGA16Juw@mail.gmail.com>
Subject: Re: Query: UBIFS: How to detect empty volumes
To:     open list <linux-kernel@vger.kernel.org>,
        linux-mtd <linux-mtd@lists.infradead.org>,
        Richard Weinberger <richard@nod.at>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Phillip Lougher <phillip@squashfs.org.uk>,
        Sean Nyekjaer <sean@geanix.com>,
        Kernelnewbies <kernelnewbies@kernelnewbies.org>
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Hi,

I have one basic query related to UBIFS volumes on a system with NAND partition.
In short, how to detect a particular empty volume inside a system
partition while flashing the partition image?

Suppose I have one big system partition that consists of 4-5 ubi
volumes inside it with varying sizes.
Lets say:
-- System Partition (ubi image)
    - rootfs volume (ro, squashfs)
    - data volume (rw, ubifs)
    - firmware volume (ro, ubifs)
    - some-other volume (ro, squashfs)

Consider that all these could be flashed together as part of
system.ubi image in a single shot from the bootloader.
Now, suppose, one of the volume image (say firmware) is missing or
remains empty (as you know we can have empty volumes).

So, during system image flashing, we wanted to detect if one of the
volume (firmware) is empty.
Since this is an important volume, so we wanted to detect if this
volume is empty/missing we will abort flashing the system partition.
As there is no point in booting the system without this partition.

So, I am exploring options, how can this be detected ?
I mean is there any kind of magic number or header information which
we can read to detect a particular empty volume ?
Can we get any information from just "system.ubi" image to indicate
about the volume information ?
Also it could be possible that 2 or more volumes are empty, but we are
only concerned about one particular volume (firmware), so how to
detect particular volume ?

If anyone has any thoughts about this requirement, please share your opinion.


Thanks,
Pintu