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Date:   Wed, 21 Jul 2021 22:54:28 +0200 (CEST)
From:   Richard Weinberger <richard@nod.at>
To:     Pintu Agarwal <pintu.ping@gmail.com>
Cc:     Greg KH <greg@kroah.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mtd <linux-mtd@lists.infradead.org>,
        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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Subject: Re: MTD: How to get actual image size from MTD partition
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----- Ursprüngliche Mail -----
>> But let me advertise ubiblock a second time.
> Sorry, I could not understand about the ubiblock request. Is it
> possible to elaborate little more ?
> We are already using squashfs on top of our UBI volumes (including
> rootfs mounting).
> This is the kernel command line we pass:
> rootfstype=squashfs root=/dev/mtdblock44 ubi.mtd=40,0,30
> And CONFIG_MTD_UBI_BLOCK=y is already enabled in our kernel.
> Do we need to do something different for ubiblock ?

From that command line I understand that you are *not* using squashfs on top of UBI.
You use mtdblock. ubiblock is a mechanism to turn an UBI volume into a read-only
block device.
See: http://www.linux-mtd.infradead.org/doc/ubi.html#L_ubiblock

>> If you place your squashfs on a UBI static volume, UBI knows the exact length
>> and you can checksum it
>> more easily.
> Yes, we use squashfs on UBI volumes, but our volume type is still dynamic.
> Also, you said, UBI knows the exact length, you mean the whole image length ?
> How can we get this length at runtime ?

You need a static volume for that. If you update a static volume the length is
known by UBI.

> Also, how can we get the checksum of the entire UBI volume content
> (ignoring the erased/empty/bad block content) ?

Just read from the volume. /dev/ubiX_Y.

> Or, you mean to say, the whole checksum logic is in-built inside the
> UBI layer and users don't need to worry about the integrity at all ?

Static volumes have a crc32 checksum over the whole content.
Of course this offers no cryptographic integrity.
See: http://www.linux-mtd.infradead.org/doc/ubi.html#L_overview

Thanks,
//richard