linux-kernel - Re: MTD: How to get actual image size from MTD partition

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Message-ID: <CAFLxGvywv29u6DJZrJxnJJmUDSQ4xpbT0u5LNKY1uGKyQom+WA@mail.gmail.com>
Date:   Tue, 27 Jul 2021 23:16:25 +0200
From:   Richard Weinberger <richard.weinberger@...il.com>
To:     Pintu Agarwal <pintu.ping@...il.com>
Cc:     Richard Weinberger <richard@....at>,
        Kernelnewbies <kernelnewbies@...nelnewbies.org>,
        Greg KH <greg@...ah.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        Sean Nyekjaer <sean@...nix.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Phillip Lougher <phillip@...ashfs.org.uk>
Subject: Re: MTD: How to get actual image size from MTD partition

On Thu, Jul 22, 2021 at 1:11 PM Pintu Agarwal <pintu.ping@...il.com> wrote:
>
> On Thu, 22 Jul 2021 at 02:24, Richard Weinberger <richard@....at> wrote:
> >
> > ----- 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
> >
> Okay, you mean to say, we should use this ?
> ubi.mtd=5 ubi.block=0,0 root=/dev/ubiblock0_0
> Instead of this:
> root=/dev/mtdblock44 ubi.mtd=40,0,30

Yes. But it is not only about a different command line. It is a
different concept.
You use a emulated block device on top of UBI, and not directly on top
of an MTD part.

> Sorry, I could not get this part. How static volume can give image len ?
> You mean there is some interface available in kernel to get actual image len ?

use the ubinfo tool. Static volumes know exactly how much they are filled.

> > > 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.
> >
> I think this also will give the entire volume size, but we still don't know how
> many pages have real data ?

"ubiinfo /dev/ubiX_Y" will tell you if the volume is of type static.

> For example:
> Suppose, my raw partition/volume is of size 10MB
> But my actual data inside it is of size ~3MB (may be split across?)
> Then, how can we get the actual size of the data content ?

See above.

> You mean to say: /dev/ubiX_Y should contain only data blocks ?

Yes. An UBI volume contains only "user data".

-- 
Thanks,
//richard