Environment

U-Boot configuration and startup behaviour is defined in its environment and therein held variables. While the environment’s default values are pre-defined in the U-Boot source code, these settings and values can be overwritten by editing the respecitive variables, either for short term till the next reboot or permantly by writing the changed values into area in the permanent memory (NVM: NAND or eMMC). Which in turn will be loaded at boot time.

Comprehensive description of these features is available in the The U-Boot Documentation, there specifically in the Environment Variables section.

Reset of Variables

Users can reset the environment as a whole, or can reset just specific variables. Selectively resetting variables can be used to access variables or environment settings that have been updated or added between releases. To unset an environment variable the same common procedure as setting is used; but instead setting it to an empty string.

Warning

As long as the command saveenv is not executed, changes to the environment will be lost upon reset.

Examples:

• Unset a variable

  unset append_bootargs (set empty string)

  setenv append_bootargs
  saveenv
  

• Reset the whole environment

  env default -a
  saveenv
  

• Reset a specific variable to its compiled-in default value

  env default <variable>
  saveenv
  

  E.g. resetting fdtsave

  env default fdtsave
  saveenv
  

  Note

  Resetting a variable that’s not part of the compiled in default environment will be deleted, accompanied by the following message: WARNING: '<variable>' not in imported env, deleting it!

Automatically assigned variables

U-Boot will automagically assign some environment variables upon boot or after certain actions.

Variable	Purpose	Remarks
ethaddr eth1addr	MAC address(es) assigned to the ethernet interface(s)	Initialized from OTP memory, if unset Once saved, the value won’t be changed by U-Boot
uuid_rootfs uuid_boot uuid_userfs	UUIDs of various mass storage partitions	partition UUID from the GPT
serial#	Globally Unique Identifier from the processor’s OTP memory	will be used e.g. for setting the name of the mass storage device on the host created with the ums command
fileaddr filesize	convey information about the most recent ‘load’ or ‘tftpboot/bootp’ command	will be unset upon boot
fdt_size	provides the size of the in-memory FDT	will be changed by fdt resize
fdtcontroladdr	RAM address of the U-Boot internal FDT	You may inspect the FDT that is used to configure the U-Boot internal devices: fdt addr ${fdtcontroladdr} fdt print