Skip to main content.

14.3.28. How can I access U-Boot environment variables in Linux?

Question:
I would like to access U-Boot's environment variables from my Linux application. Is this possible?

Answer:
Yes, you can. The environment variables must be stored in flash memory, and your Linux kernel must support flash access through the MTD layer. In the U-Boot source tree you can find the environment tools in the directory tools/env, which can be built with command:

make env

For building against older versions of the MTD headers (meaning before v2.6.8-rc1) it is required to pass the argument "MTD_VERSION=old" to make.

The resulting binary is called fw_printenv, but actually includes support for setting environment variables too. To achieve this, the binary behaves according to the name it is invoked as, so you will have to create a link called fw_setenv to fw_printenv.

These tools work exactly like the U-Boot commands printenv resp. setenv You can either build these tools with a fixed configuration selected at compile time, or you can configure the tools using the /etc/fw_env.config configuration file in your target root filesystem. Here is an example configuration file:

# Configuration file for fw_(printenv/saveenv) utility.
# Up to two entries are valid, in this case the redundand
# environment sector is assumed present.

#########################################################################
# For TQM8xxL modules:
#########################################################################
# MTD device name       Device offset   Env. size       Flash sector size
/dev/mtd0            0x8000          0x4000          0x4000
/dev/mtd0            0xC000          0x4000          0x4000

#########################################################################
# For NSCU:
#########################################################################
# MTD device name       Device offset   Env. size       Flash sector size
#/dev/mtd1              0x0000          0x8000          0x20000
#/dev/mtd2              0x0000          0x8000          0x20000


#########################################################################
# For LWMON
#########################################################################
# MTD device name       Device offset   Env. size       Flash sector size
#/dev/mtd1               0x0000          0x2000          0x40000
14.3.27. Use SCC UART with Hardware Handshake 1. Abstract 14.3.29. The appWeb server hangs OR /dev/random hangs
Prev Home Next