Booting a custom device tree file source in buildroot

In the previous article we saw how to setup the default system image for Orange PI Zero 3 using buildroot. We got the basic system up and running. Now, when we do Device Driver Development, we will have to modify the device tree source.

The default dt source for our board is in the kernel sources in:

arch/arm64/boot/dts/allwinner/sun50i-h616-orangepi-zero3.dts

In this exercise we will modify the model name to something different, OrangePi Zero3-custom. To do this we will create a new dts file in the buildroot directory, include in it the original dts file (noted above), and override the model property.

Now create the following file ~/buildroot/sun50i-h616-orangepi-zero3-custom.dts:

// SPDX-License-Identifier: GPL-2.0
#include "/home/ajit/opi/buildroot/output/build/linux-custom/arch/arm64/boot/dts/allwinner/sun50i-h616-orangepi-zero3.dts"
/ {
        model = "OrangePi Zero3-custom";
        compatible = "xunlong,orangepi-zero3", "allwinner,sun50i-h616";

};

In menuconfig set the value of BR2_LINUX_KERNEL_CUSTOM_DTS_PATH to ~/buildroot/sun50i-h616-orangepi-zero3-custom.dts our custom dts file location. And unset the BR2_LINUX_KERNEL_INTREE_DTS_NAME.

Once that's done you're ready to do the build. Here we will be doing an incremental build (since we have already built the system earlier).

make linux-rebuild

This will (in our case) only compile the dtb file and re-install some modules. To do the sdcard image generation run:

make

There is a caveat I noticed with uboot. You now need to set CONFIG_DEFAULT_FDT_FILE in uboot menuconfig to use this custom dtb. But unfortunately our board include files do not honour this setting, and in boot time, uboot proceeds to fetch the default dtb! The work-around is to set the right dtb file on uboot prompt explained below.

Flash the sdcard and boot.

At uboot prompt provide these commands to boot Linux using our custom dtb.

setenv fdtfile sun50i-h616-orangepi-zero3-custom.dtb
boot

This is the workaround that worked for me and in the boot log you can see the custom model that we set: OrangePi Zero3-custom. You may also cat the following file:

cat /sys/firmware/devicetree/base/model

If you get any further issues you can also try to manually boot as follows:

setenv bootargs "root=/dev/mmcblk1p1 rootfstype=ext4"

ext4load mmc 0:1 ${kernel_addr_r} boot/Image
ext4load mmc 0:1 ${fdt_addr_r} boot/sun50i-h616-orangepi-zero3-custom.dtb
booti ${kernel_addr_r} - ${fdt_addr_r}