Notes on Yocto Linux bringup on iMX8mn router board

rolandash

This note is memo for adaptation process of Yocto Linux on imx8mn router board, by creating a new layer and new recipe for custom image.

rolandash

1 - Create new layer and build directory
The whole building process for the new layer can be identified by a tuple of 3 variables, and path of new layer.
DISTRO
MACHINE
KARO_BASEBOARD
These 3 variables are persistent in file bld-dir/conf/bblayers.conf and local.conf.
In belayers.conf the path of new layer (meta-xxxx) must be added to variable BBLAYERS.
In local.conf the 3 variables must be defined.

rolandash

2 - Create new recipes for u-boot and kernel
if any changes need to be made on u-boot, we should create a recipe under path of new layer
recipes-bsp/u-boot
by creating a .bbappend file with name exactly the same as recipe name.
Usually the change is a modification to u-boot env file, or specify a different device tree to load.
So include the path of the new env file to SRC_URI variable.

u-boot can be built separately using bitbake command:
bitbake u-boot

Any changes made to kernel should be in new recipe under
recipes-kernel

rolandash

3 - Make changes to device tree
A better practice is organizing device tree into one base file and more overlays. Create the new base dtb file by include the dts/dtsi file from previous layer.
For new hardware, the new base dtb file name to be loaded needs to be defined in .env file using 'baseboard=' and 'fdt_file=' operation. The env file name is specified in u-boot recipe bbappend file by variable UBOOT_ENV_FILE.

Note the dts files under u-boot/dts are only used for u-boot running. The dts files for kernel are under kernel recipes. The new dtb file name is used for u-boot to pass as argument when loading kernel.

the list of overlays to be loaded is specified in layer/conf/machine/include/imx8m-overlays.inc.

Add the new dtb file name to variable KERNEL_DEVICETREE in kernel recipe .bbappend file.
The new dts files (base and overlays) for kernel to use should be created under linux/linux-karo-5.15/mx8-nxp-bsp/dts/freescale/ directory.

rolandash

4 - Post-build changes to rootfs

In case of need a different /etc/network/interfaces config file on boot, a different interfaces file need to be provided and copied to the rootfs path after the kernel built, and before the final image is generated. This can be done by create new recipes under recipes-core/images to customize the image building process.

Create a recipe .bbappend file, then add call to the 'install' utility as a task inserting after do_rootfs and before do_flush_pesudodb to copy the different interfaces file to the rootfs path.