You can customize the partition table of a Digi embedded module using a series of U-Boot commands.

On a NAND flash, the partition table is simply a string that gets passed to the kernel during the boot process. The NAND flash itself is not written with any partition table data.

The syntax for describing a NAND partition table is defined at cmd/mtdparts.c file in the U-Boot source code:

 * mtdparts=mtdparts=<mtd-def>[;<mtd-def>...]
 * <mtd-def>  := <mtd-id>:<part-def>[,<part-def>...]
 * <mtd-id>   := unique device tag used by linux kernel to find mtd device (mtd->name)
 * <part-def> := <size>[@<offset>][<name>][<ro-flag>]
 * <size>     := standard linux memsize OR '-' to denote all remaining space
 * <offset>   := partition start offset within the device
 * <name>     := '(' NAME ')'
 * <ro-flag>  := when set to 'ro' makes partition read-only (not used, passed to kernel)
 * <enc-flag> := when set to 'enc' makes partition encrypted (not used, passed to kernel)

In U-Boot, the NAND partition table is stored in variable mtdparts.

File systems have some overhead, so the total partition size won’t be available for use when the file system is mounted. This overhead is significant in UBIFS and NAND flashes since in addition to the metadata of the file system itself, a certain number of blocks must be reserved to account for potential bad blocks.

System partitions

The NAND contains two major MTD partitions to contain the operating system:

  • UBI: This is a relatively small partition that contains UBI volumes for the U-Boot environment and the Linux kernel and device trees.

  • UBI_2: This partition extends to all the remaining space and contains UBI volumes for the Linux rootfs.

It’s unlikely that you want to change the MTD partition layout. Instead, you may want to change the number and size of the UBI volumes inside the UBI or UBI_2 partitions.

Change the system UBI volumes (run-time)

The layout of UBI volumes is defined on variable ubivolscript. You can edit this variable using env edit command in U-Boot.

Although it is possible, it’s not recommended to change the layout of UBI volumes at run-time. Variable ubivolscript is a long script with conditionals, and is difficult to edit on a terminal using env edit command.

Change the system UBI volumes (build-time)

You may want to change the default UBI volumes so that all devices ship with the same layout. To do that:

  1. Open your platform’s header file in U-Boot source code at include/configs/ccmp13-dvk.h.

  2. Locate the UBI volumes layout definition for your module’s NAND capacity:

    • For dual boot system, these are UBIVOLS1_DUALBOOT_xxxMB for UBI partition and UBIVOLS2_DUALBOOT_xxxMB for UBI_2 partition

    • For single system, these are UBIVOLS1_xxxMB for UBI partition and UBIVOLS2_xxxMB for UBI_2 partition

      For instance:

      #define UBIVOLS1_DUALBOOT_256MB	"ubi create " LINUX_A_PARTITION " b00000;" \
      				"ubi create " LINUX_B_PARTITION ";"
      #define UBIVOLS2_DUALBOOT_256MB	"ubi create " ROOTFS_A_PARTITION " 6800000;" \
      				"ubi create " ROOTFS_B_PARTITION " 6800000;" \
      				"ubi create " DATA_PARTITION ";"
  3. Edit the appropriate constant to create the volumes you want. For system updates to work, preserve the original names of the default UBI volumes (you can modify the sizes, though).

  4. Save your changes and recompile U-Boot.

  5. Update the bootloader of your device and reboot it.

  6. Reset the whole environment to default values:

    => env default -a
  7. Run the following script to generate the new UBI volumes layout:

    => run ubivolscript
  8. Save the environment with saveenv.

Adjust Digi Embedded Yocto for partition table changes

After changing the NAND partition table in U-Boot, you must review the MKUBIFS_BOOT_ARGS and MKUBIFS_ARGS variables used in Digi Embedded Yocto. These variables establish the NAND partition parameters to use when building UBIFS images for your system:

  • MKUBIFS_BOOT_ARGS is used for partitions linux and recovery.

  • MKUBIFS_ARGS is used for partition rootfs.

The default parameters in Digi Embedded Yocto are:

# mkfs.ubifs parameters for boot partition (the one holding kernel and device tree files)
MKUBIFS_BOOT_ARGS ?= "-m 2048 -e 126976 -c 127"

# mkfs.ubifs parameters for rootfs partition
MKUBIFS_ARGS ?= "-m 2048 -e 126976 -c 8191"


  • -m 2048 (2 KiB) is the minimum I/O size of the underlying UBI and MTD devices (page size).

  • -e 126976 (124 KiB) is the Logical Erase Block (LEB) size. UBI requires two minimum I/O units out of each Physical Erase Block (PEB) for overhead: one for maintaining erase count information, and another for maintaining the Volume ID information. For a PEB size of 128 KiB and a page size of 2 KiB, the LEB would be 128 - (2 * 2) = 124 KiB.

  • -c 127 is the maximum LEB count, or maximum size, in LEBs, of the UBIFS file system. This can be calculated as the next power of two following the number of erase-blocks in the partition (partition_size / erase_block_size) minus 1. For example for a partition size of 14MiB and an erase block size of 128KiB → 14 * 1024 * 1024 / 128 * 1024 = 112. The next power of two following 112 is 128, so use a max-leb-count of 128 - 1 = 127.

The -m and -e parameters depend on the NAND geometry, so they do not need to be modified. The -c parameter may need to be adjusted depending on the size of your partitions, to better adjust the overhead of the UBIFS image to the real size of the partition.