MTD: How to get actual image size from MTD partition

Pintu Agarwal at
Thu Jul 22 07:10:26 EDT 2021

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:
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

Okay I will discuss this internally and check..

> >> 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.
Thank you so much for your reply!

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 ?

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

> > 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 ?

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