MTD: How to get actual image size from MTD partition

[Pintu Agarwal]

Hi,

Our ARM32 Linux embedded system consists of these:
* Linux Kernel: 4.14
* Processor: Qualcomm Arm32 Cortex-A7
* Storage: NAND 512MB
* Platform: Simple busybox
* Filesystem: UBIFS, Squashfs
* Consists of nand raw partitions, squashfs ubi volumes.

My requirement:
To find the checksum of a real image in runtime which is flashed in an
MTD partition.

Problem:
Currently, to find the checksum, we are using:
$ md5sum /dev/mtd14
This returns the proper checksum of the entire partition.
But we wanted to find the checksum only for the actual image data
which will be used by our C utility to validate the image.
Here, we don't know the actual image size.
We only know the "partition-size" and "erasesize".

So, is there a mechanism to somehow find the image size at runtime?

Regards,
Pintu