Phytec i.MX 93 - Page 47

i.MX 93 BSP Manual PD24.1.1 Documentation Rev.: imx8mp-pd22.1.2-51-ga548be7d
We use the ${overlays} variable for overlays describing expansion boards that can not be detected during
run time. To prevent applying overlays listed in the ${overlays} variable during boot, the ${no_overlays}
variable can be set to 1 in the bootloader environment.
u-boot=> setenv no_overlays 1
u-boot=> boot
6.2.3 U-boot External Environment
During the start of the Linux Kernel the external environment bootenv.txt text le will be loaded from the
boot partition of an MMC device or via TFTP. The main intention of this le is to store the ${overlays}
variable. This makes it easy to pre-dene the overlays in Yocto depending on the used machine. The
content from the le is dened in the Yocto recipe bootenv found in meta-phytec: https://git.phytec.de/
meta-phytec/tree/recipes-bsp/bootenv?h=mickledore
Other variables can be set in this le, too. They will overwrite the existing settings in the environment.
But only variables evaluated after issuing the boot command can be overwritten, such as ${nfsroot} or
${mmcargs}. Changing other variables in that le will not have an eect. See the usage during netboot as
an example.
If the external environment can not be loaded the boot process will be anyway continued with the values of
the existing environment settings.
6.2.4 Change U-boot Environment from Linux on Target
Libubootenv is a tool included in our images to modify the U-Boot environment of Linux on the target
machine.
Print the U-Boot environment using the following command:
target:~$ fw_printenv
Modify a U-Boot environment variable using the following command:
target:~$ fw_setenv <variable> <value>
Caution
Libubootenv takes the environment selected in a conguration le. The environment to use is inserted
there, and by default it is congured to use the eMMC environment (known as the default used environ-
ment).
If the eMMC is not ashed or the eMMC environment is deleted, libubootenv will not work. You should
modify the /etc/fw_env.config le to match the environment source that you want to use.
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