The U-Boot environment is a block of memory in persistent storage.
It is used to store variables in the form name=value
.
Where is the U-Boot environment?
The ConnectCore 8M Nano reads the U-Boot environment (including the MAC addresses) from the boot media, which means:
-
If the device boots from the eMMC, it reads the environment from the eMMC.
-
If the device boots from microSD card, it reads the environment from the microSD card.
-
If it does not find a valid environment on the boot media, it uses a hard-coded default environment.
When the ConnectCore 8M Nano boots from the eMMC, it reads U-Boot from eMMC boot partition 1 (device /dev/mmcblk0boot0
in Linux) and reads the U-Boot environment (including the MAC addresses) from eMMC boot partition 2 (device /dev/mmcblk0boot1
in Linux).
-
The environment is located at the offset determined by constant
CONFIG_ENV_OFFSET
in U-Boot source code atinclude/configs/ccimx8m_common.h
. -
The size of the environment is determined by constant
CONFIG_ENV_SIZE
in U-Boot source code atinclude/configs/digi_common.h
. -
There is a back-up copy of the environment in the same partition (where the original copy is) at the offset determined by constant
CONFIG_ENV_OFFSET_REDUND
in U-Boot source code atinclude/configs/ccimx8m_common.h
.
Access U-Boot environment from Linux
You can access the U-Boot environment from a Linux application.
In the U-Boot source tree, you can find the environment tools in the directory tools/env
.
Digi Embedded Yocto compiles this tool by default and installs it on your target’s root file system as two executable files:
-
fw_printenv
: to print the value of variables. -
fw_setenv
: to set the value of variables.
The tools work with the configuration file /etc/fw_env.config
, which must contain two entries in the form: Device name, offset, size.
The first entry must point to the U-Boot environment location.
The second must point to its redundant copy.
For example:
# Configuration file for fw_(printenv/setenv) utility.
# Up to two entries are valid, in this case the redundant
# environment sector is assumed present.
# Device name Offset Size
/dev/mmcblk0boot1 0x1C0000 0x4000
/dev/mmcblk0boot1 0x1E0000 0x4000
Append boot arguments to the kernel command line
You can use the extra_bootargs
variables to append commands to the default kernel command line.
Important U-Boot environment variables on the ConnectCore 8M Nano
The following U-Boot environment variables are worth mentioning:
MAC addresses
Variable | Description | Flags |
---|---|---|
|
MAC address of the first wired Ethernet interface |
change-default |
|
MAC address of the second wired Ethernet interface if there is one |
change-default |
|
MAC address of the Wi-Fi interface |
change-default |
|
MAC address of the Bluetooth interface |
change-default |
About flags
|
Digi programs the MAC addresses of the ConnectCore 8M Nano during manufacturing and saves them in the U-Boot environment on the eMMC.
If the device boots from an external media different from the eMMC (for example microSD card), the system reads the U-Boot environment from it, so MAC addresses will not match the ones assigned by Digi during manufacturing. You must program the MAC addresses of the available interfaces and save the environment on your boot storage media.
You can find the Digi-assigned MAC address on the ConnectCore 8M Nano module label. See Determine Digi MAC addresses for more information.
Wireless virtual MAC addresses
The wireless interface on the ConnectCore 8M Nano module allows you to define up to three optional virtual interfaces so that the platform can run concurrently as station, p2p, and SoftAP. Digi neither reserves nor programs unique MAC addresses for these virtual interfaces. A user who wishes to assign unique MAC addresses to such virtual interfaces can use the following U-Boot environment variables:
Variable | Description | Flags |
---|---|---|
|
MAC address of virtual wireless interface 1 |
change-default |
|
MAC address of virtual wireless interface 2 |
change-default |
|
MAC address of virtual wireless interface 3 |
change-default |
MMC boot device
Variable | Description | Flags |
---|---|---|
|
MMC device index where U-Boot booted from |
write-once |
During start-up, U-Boot initially sets this variable to the MMC device index where the device booted from:
-
0 if booting from internal eMMC (USDHC3)
-
1 if booting from microSD card (USDHC2)
You can use this variable to determine which MMC device the ConnectCore 8M Nano booted from.
MMC device (update target)
Variable | Description | Flags |
---|---|---|
|
MMC device index to update firmware to |
- |
The update
command uses this variable to determine the MMC device index to write the firmware to:
-
0 for the internal eMMC (USDHC3)
-
1 for the microSD card (USDHC2)
You can modify this variable as needed.
Module variant
Variable | Description | Flags |
---|---|---|
|
Variant ID code for the System-On-Chip |
write-once |
During start-up, U-Boot automatically sets this variable to the ConnectCore 8M Nano System-On-Chip variant ID number (a hexadecimal code programmed in the SOM one-time programmable bits).
Carrier board version and ID
Variable | Description | Flags |
---|---|---|
|
Version number of the carrier board |
write-once |
|
ID number of the carrier board |
write-once |
These optional write-once variables can be set to the carrier board’s version and ID numbers during manufacturing. See Carrier board version and ID.
Determine Digi MAC addresses
Sequential MAC address scheme
Digi assigns MAC addresses according to a sequential scheme. The order of assignment depends on the available interfaces, but the scheme always respects the order Ethernet 1, Ethernet 2, Wi-Fi, Bluetooth and skips non-available interfaces.
Determine MAC addresses
You can determine the MAC addresses for your device either by reading the Ethernet MAC off the label and using the scheme to calculate subsequent addresses, or by reading them from your device.
To read the stock environment variables from the device using the printenv
command executed at the U-Boot prompt:
To get first Ethernet MAC address:
=> printenv ethaddr
If your SOM has a second Ethernet MAC address:
=> printenv eth1addr
To get Wi-Fi MAC address:
=> printenv wlanaddr
To get Bluetooth MAC address:
=> printenv btaddr