Yocto add custom UBoot Environment Variables

I am trying to add two new u-boot environment variables through my Yocto build process.

My file u-boot-imx_2021.04.bbappend contains

FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
SRC_URI += " file://uboot.patch"

My file uboot.patch contains

--- a/configs/mx6ull_14x14_evk_emmc_defconfig	2023-02-23 10:49:03.969189476 -0600
+++ a/configs/mx6ull_14x14_evk_emmc_defconfig	2023-02-23 10:50:06.401233950 -0600
@@ -91,3 +91,14 @@
 CONFIG_FASTBOOT_BUF_SIZE=0x40000000
 CONFIG_FASTBOOT_FLASH=y
 CONFIG_EFI_PARTITION=y
+
+CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
+CONFIG_ENV_OFFSET_REDUND=0xE2000
+CONFIG_BOOTCOUNT_BOOTLIMIT=3
+CONFIG_SYS_MALLOC_F_LEN=0xF000
+CONFIG_CMD_SAVEENV=y
+CONFIG_CMD_LOADENV=y
+
+CONFIG_SWUPDATE_BOOTCMD="setenv bootargs console=ttymxc0,115200 root=/dev/ram0 rootfstype=ext4 rw;load mmc 1:3 0x83000000 /swupdate-image-imx6ull14x14evk.ext4.gz.u-boot;load mmc 1 0x82a00000 imx6ull-14x14-evk.dtb;load mmc 1 0x80800000 zImage;load mmc 1 0x83000000 300000 100000;setenv root /dev/ram0;setenv rootfstype ext4;setenv fw_env_config /etc/fw_env.config;bootz 0x80800000 0x83000000 0x82a00000;"
+CONFIG_SWUPDATE_KERNEL=0
+

This patch is applied, for example I can see CONFIG_SYS_REDUNDAND_ENVIRONMENT, and CONFIG_SYS_MALLOC_F_LEN values are set.

When I look into build-fb/tmp/work/imx6ull14x14evk-poky-linux-gnueabi/u-boot-imx/2021.04-r0/build/mx6ull_14x14_evk_emmc_config/include/autoconf.mk I see the following:

CONFIG_SWUPDATE_BOOTCMD="setenv swupdate_bootcmd; setenv bootargs console=ttymxc0,115200 root=/dev/ram0 rootfstype=ext4 rw;load mmc 1:3 0x83000000 /swupdate-image-imx6ull14x14evk.ext4.gz.u-boot;load mmc 1 0x82a00000 imx6ull-14x14-evk.dtb;load mmc 1 0x80800000 zImage;load mmc 1 0x83000000 300000 100000;setenv root /dev/ram0;setenv rootfstype ext4;setenv fw_env_config /etc/fw_env.config;bootz 0x80800000 0x83000000 0x82a00000;"
CONFIG_SWUPDATE_KERNEL="setenv swupdate_kernel 0"

When I boot my device and enter U-Boot, I run printenv and my two new variables are not present. What small step am I missing?

> I am trying to add two new u-boot environment variables
> ...
> When I boot my device and enter U-Boot, I run printenv and my two new variables are not present. What small step am I missing?

The relationship between configuration options (e.g. CONFIG_XXX) and environment variables is not as correlated as you assume. Only a known set of configuration options (after they have been translated into preprocessor macros) are used to compose the default environment. (Since the configuration options (in .config) and macros (in include/generated/autoconf.h) appear identical, some developers forget or are unaware that there is a conversion process.) So you simply cannot create your own environment variable through a new configuration option (or macro).

To see which configuration options are used to compose the default environment, refer to include/env_default.h.

U-Boot actually has several sets of environment variables, specifically the default, active, and saved.

• The default environment is the set of environment variables that are
  defined at build time, and retained in the binary image.

• The saved environment is the set of environment variables that are
  retained in persistent storage. They have to be written using the
  saveenv command, and are validated using a CRC32 checkword.

• The active environment is the set of environment variables that are
  held in RAM while U-Boot executes.

On startup U-Boot tries to populate the (empty) active environment with the saved environment, but only if the CRC32 checkword confirms the integrity of the saved environment.
When validation is successful, the environment variables from the persistent storage (silently) become the active environment.
When validation fails, a notice is displayed, "Warning: Bad CRC, using default environment", and the environment variables from the default environment become the active environment.

The printenv command (only) displays the set of active environment variables.
When building U-Boot, the set of default environment variables is defined. Optionally the set of saved environment variables can also be created (e.g. a pre-built u-boot.env file that can be installed with the new u-boot.bin executable) using the mkenvimage tool.

One method of adding new environment variables to the default environment is to use CONFIG_EXTRA_ENV_SETTINGS. From the README file in U-Boot source:

CONFIG_EXTRA_ENV_SETTINGS

Define this to contain any number of null terminated
strings (variable = value pairs) that will be part of
the default environment compiled into the boot image.

For example, place something like this in your
board's config file:

	#define CONFIG_EXTRA_ENV_SETTINGS \
		"myvar1=value1
CONFIG_EXTRA_ENV_SETTINGS
Define this to contain any number of null terminated
strings (variable = value pairs) that will be part of
the default environment compiled into the boot image.
For example, place something like this in your
board's config file:
#define CONFIG_EXTRA_ENV_SETTINGS \
"myvar1=value1\0" \
"myvar2=value2\0"
Warning: This method is based on knowledge about the
internal format how the environment is stored by the
U-Boot code. This is NOT an official, exported
interface! Although it is unlikely that this format
will change soon, there is no guarantee either.
You better know what you are doing here.
Note: overly (ab)use of the default environment is
discouraged. Make sure to check other ways to preset
the environment like the "source" command or the
boot command first.
" \
		"myvar2=value2
CONFIG_EXTRA_ENV_SETTINGS
Define this to contain any number of null terminated
strings (variable = value pairs) that will be part of
the default environment compiled into the boot image.
For example, place something like this in your
board's config file:
#define CONFIG_EXTRA_ENV_SETTINGS \
"myvar1=value1\0" \
"myvar2=value2\0"
Warning: This method is based on knowledge about the
internal format how the environment is stored by the
U-Boot code. This is NOT an official, exported
interface! Although it is unlikely that this format
will change soon, there is no guarantee either.
You better know what you are doing here.
Note: overly (ab)use of the default environment is
discouraged. Make sure to check other ways to preset
the environment like the "source" command or the
boot command first.
"

Warning: This method is based on knowledge about the
internal format how the environment is stored by the
U-Boot code. This is NOT an official, exported
interface! Although it is unlikely that this format
will change soon, there is no guarantee either.
You better know what you are doing here.

Note: overly (ab)use of the default environment is
discouraged. Make sure to check other ways to preset
the environment like the "source" command or the
boot command first.

Note that CONFIG_EXTRA_ENV_SETTINGS is typically only defined as a macro, rather than used as a configuration option. For example refer to include/configs/mx6ullevk.h.

Another method of adding new environment variables to the default environment is to use CONFIG_USE_DEFAULT_ENV_FILE (which is a configuration option). Its Kconfig description is:

 CONFIG_USE_DEFAULT_ENV_FILE:
 
 Normally, the default environment is automatically generated
 based on the settings of various CONFIG_* options, as well
 as the CONFIG_EXTRA_ENV_SETTINGS. By selecting this option,
 you can instead define the entire default environment in an 
 external file.