XELK-AN-006: Modify u-boot environment from userspace

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Info Box
Axel-04.png Applies to Axel Ultra
Axel-02.png Applies to AXEL ESATTA
Axel-lite 02.png Applies to Axel Lite
Warning-icon.png This application note was validated against specific versions of the kit only. It may not work with other versions. Supported versions are listed in the History section. Warning-icon.png

History[edit | edit source]

Version Date Notes
1.0.0 June 2019 First release

Introduction[edit | edit source]

Axel family system-on-modules (SOM for short) are based on u-boot and linux BSP which is configured - at boot time - using the u-boot environment variables configuration.

Many times, it should ve very useful to modify the u-boot environment from linux userspace access. This is possible using the fw_printenv and fw_setenv utilities present on u-boot source codes.

Tools required[edit | edit source]

In order to modify the uboot environment variables from userspace the fw_printenv has to be compiled from the u-boot sources: this is possible using the

make env

command on u-boot source code tree

Once built, the second utility fw_setenv is just a copy of the built one: the tool itself, uses the recognized file name for identifying if accessing in print or set mode the u-boot environment.

Configuration file[edit | edit source]

fw_printenv uses the /etc/fw_env.config configuration file for identifying the correct parameters to access the storage device where the u-boot environment is memorized.

Please find below an example on how to identify the u-boot storage device and its configuration:

MTDs[edit | edit source]

Using the

cat /proc/mtd

command, it is possible to see the memory partitioning and where the environment parts are:

root@imx6qxelk:~# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00800000 00020000 "nand-uboot"
mtd1: 00100000 00020000 "nand-env1"
mtd2: 00100000 00020000 "nand-env2"
mtd3: 00100000 00020000 "nand-fdt"
mtd4: 00100000 00020000 "nand-spare"
mtd5: 00800000 00020000 "nand-kernel"
mtd6: 00400000 00020000 "nand-splash"
mtd7: 3e800000 00020000 "nand-ubi"

So, in this example we can see that the two mtd parts to be used are

mtd1: 00100000 00020000 "nand-env1"
mtd2: 00100000 00020000 "nand-env2"

Sizes[edit | edit source]

The two parameters required for properly configuring the fw_env.config file are:

  • Env. size
  • Flash sector size

Those values are identifyed at compile time into the proper header file used for building u-boot. On AxelLite family this file is include/configs/mx6axel.h and the parameters should be found according to the storage device used.

For example, if the boot storage device is a NAND memory, the config var is:


and the related section (for the environment sizes) is:

#elif defined(CONFIG_ENV_IS_IN_NAND)
#define CONFIG_ENV_OFFSET			(8 * 1024 * 1024)
#define CONFIG_ENV_SECT_SIZE			(1 * 1024 * 1024)
#define CONFIG_ENV_SIZE				(128 * 1024)

So, we found that the proper parameters are:

  • Env. size: 0x20000
  • Flash sector size: 0x100000

fw_env.config[edit | edit source]

Following the configuration information found before, we can set the proper parameters into the fw_env.config file as shown below:

root@imx6qxelk:~# cat /etc/fw_env.config
# Configuration file for fw_(printenv/setenv) utility.
# Up to two entries are valid, in this case the redundant
# environment sector is assumed present.
# Notice, that the "Number of sectors" is not required on NOR and SPI-dataflash.
# Futhermore, if the Flash sector size is ommitted, this value is assumed to
# be the same as the Environment size, which is valid for NOR and SPI-dataflash

# NOR example
# MTD device name       Device offset   Env. size       Flash sector size       Number of sectors
/dev/mtd1               0x0000          0x20000         0x100000
/dev/mtd2               0x0000          0x20000         0x100000


u-boot environment access example[edit | edit source]

  • display the ipaddr and bootfile environment variables:
root@imx6qxelk:~# fw_printenv ipaddr bootfile

Wrong configuration file[edit | edit source]

If some parameters are not correctly configured on the fw_env.config file, the utilities displays an error message:

Warning: Bad CRC, using default environment

It is possible to detect this situation using the return code of fw_printenv:

  • fw_env.config correct file
root@imx6qxelk:~# fw_printenv ipaddr
root@imx6qxelk:~# echo $?
  • wrong fw_env.config file
root@imx6qxelk:~# fw_printenv ipaddr
Warning: Bad CRC, using default environment
## Error: "ipaddr" not defined
root@imx6qxelk:~# echo $?

Command syntax[edit | edit source]

  • print the whole environment
  • print a specific environment variable
fw_printenv <varname>
  • delete environment variable
fw_setenv <varname>
  • set environment variable
fw_setenv <varname> <varvalue>