How to use custom device tree source correctly in buildroot?

Question

I made a copy of the dts from <linux>/board/arch/arm/boot/dts/imx28-evk.dts for using with my custom board. My custom device-tree is named imx28-custom.dts and is pointed in the Out of tree custom DTS menu entry (BR2_LINUX_KERNEL_CUSTOM_DTS_PATH) in the Buildroot config.

However u-boot is configured to use the existing MX28 board config. After building a kernel I have imx28-custom.dtb and zImage in the <buildroot>/output/images folder.

When the system boots, u-boot tells that imx28-evk.dtb is not found. Why was imx28-custom.dtb built but is not found by u-boot? Why u-boot doesn't find its own device-tree (imx28-evk.dtb) for itself and my custom device-tree for the kernel? I assumed that BR2_LINUX_KERNEL_CUSTOM_DTS_PATH relates only to the kernel. How can my custom device-tree be passed to the kernel if u-boot tries to use its own imx28-evk.dtb for this?

Answer 1

u-boot complaining about not finding the device-tree is related to the Linux kernel: when booting, u-boot loads the device-tree and kernel image from the storage to specified addresses in the memory and then passes control to the kernel. The default configuration for the i.MX28-EVK board is to pass the imx28-evk.dtb file.

The BR2_LINUX_KERNEL_CUSTOM_DTS_PATH only serves to include the custom device-tree in the files to build and install but does not configure u-boot to use it.

Which device-tree u-boot passes to the kernel is defined by u-boot's CONFIG_DEFAULT_FDT_FILE option (you can edit this by typing make uboot-menuconfig (see my note at the end) then going under Boot to set the Default fdt file). This option should be set to something like imx28-custom.dtb.

How to configure u-boot from Buildroot is described here and here to make them permanent.

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In the case of the i.MX28-EVK, Buildroot uses the Legacy build system (which is to be used up to u-boot 2015.04, but the config uses 2020.04) which does not allow the use of the make uboot-menuconfig command. You should safely be able to change that. You need to set the Build system to Kconfig and set the Board defconfig to mx28evk.