Raw flash


This chapter introduces the technical details of raw flash support in Mender.


Support for raw flash memory under Linux is generally more complicated than working with block devices. It is advised to have a fully working bootloader, kernel and rootfs before introducing Mender.

The Mender Yocto layer comes with support for the following raw flash boards in the main tree:

  • vexpress-a9 emulated by QEMU meta-mender-qemu

Both layers provide a set of patches based on their respective upstream U-Boot sources and can be used as a reference when implementing your raw flash based machine.

As an example, to illustrate potential pain points we will use a Versatile Express CortexA9x4 board, emulated under QEMU (vexpress-a9 target). See [the next example section]().

Raw flash storage

The Mender Yocto layer comes with support for auto-configuring most aspects of the Flash specific components, including partitioning and the U-Boot bootloader. Some configuration values need manual intervention, however, and they are described below.

For more information about these or other variables that affect a Flash build, see the Variables section.


The MTDID string is required, and describes the Flash type and the location it has in the device's memory. For example:

MENDER_MTDIDS = "nor0=40000000.flash"

Having one entry will allow the Mender Yocto layer to deduce the proper storage parameters and partition layout. If the device has more than one unit of Flash storage, it is possible to add more than one entry, separated by comma; in this case you will also need to set MENDER_IS_ON_MTDID to the Flash device that you want Mender to reside on and update. For example:

MENDER_MTDIDS = "nand0=40000000.flash,nand1=60000000.flash"
MENDER_IS_ON_MTDID = "60000000.flash"

Note that currently Mender only supports running on, and updating, one Flash storage device, but you can have more Flash storage devices inside one system outside of Mender's control.

Please refer to the documentation for your device to find out what value to put in MENDER_MTDIDS. If in doubt, a good place to look is the U-Boot source code for the board, specifically the value of CONFIG_MTDIDS_DEFAULT.


The MTDPARTS string expresses the MTD partitions that are present on the device. If it is not provided, Mender will provide a default one which contains only a u-boot entry and the ubi section, but the string often needs to be manually specified to match the layout expectations of the device.

# Example of string that Mender itself provides:
MENDER_MTDPARTS = "40000000.flash:512k(u-boot),-(ubi)"

# Example of a custom string (this one is for the Toradex Colibri board):
MENDER_MTDPARTS = "gpmi-nand:512k(mx7-bcb),1536k(u-boot1)ro,1536k(u-boot2)ro,512k(u-boot-env),-(ubi)"

The MENDER_MTDPARTS variable has an impact on the mtdimg image type that Mender produces. Normally it tries to produce an image which contains the U-Boot boot code inside the u-boot partition (actually the file specified in the MENDER_IMAGE_BOOTLOADER_FILE variable), and the ubimg inside the ubi partition. For custom strings it may not be able to put things in the right places, ant it may be preferable to turn this image type off, and use the ubimg instead:

IMAGE_FSTYPES:remove = "mtdimg"

See also image types below for more information about the different image types for Flash.

If you don't know what value the MENDER_MTDPARTS string should have, like CONFIG_MTDIDS a good place to look is the U-Boot source code, particularly the value of CONFIG_MTDPARTS_DEFAULT for the particular board.

Physical erase block (PEB) size

Mender needs to know what the physical erase block size of the Flash storage is by setting MENDER_STORAGE_PEB_SIZE. For example:

# PEB size of 128KiB, a common size.

If you do not know what the PEB size is, you can run this command on the board.

mtdinfo -a | grep -i 'eraseblock size:' |sort -u

This assumes that you have a working image installed on the board, and that the mtdinfo tool is available.


This is only relevant if you are using the mtdimg image type. If you need to flash a bootloader into the u-boot MTD partition, provide the bootloader filename, like this:


Image types

There are three relevant image types when building for raw flash devices. Append the image types to the IMAGE_FSTYPES Yocto variable to enable them.


A UBI filesystem that contains the root filesystem. The data filesystem is also built using the same filesystem type.


A UBI image that contains multiple UBI volumes, at minimum including two root filesystem volumes and one data volume. It will also normally contain two volumes to store two redundant copies of U-Boot's environment data.

This image can be flashed to an MTD device into its "ubi" partition, as identified from its "mtdparts" string (MENDER_MTDPARTS).


A raw flash image for MTD devices that contains the ubimg as well as other components specified in the MENDER_MTDPARTS variable.

This image can be flashed directly to a raw Flash device.

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