Modifying Device Tree Blob (DTB)

What is a Device Tree?

A Device Tree (DT) is a data structure used in embedded Linux systems to describe hardware components and their configurations to the Linux kernel. It eliminates the need for hardcoded board-specific information in the kernel source code.

What is a Device Tree Blob (DTB)?

The Device Tree Blob (DTB) is a compiled binary representation of the Device Tree Source (DTS) file. The Linux kernel reads the DTB during boot to configure hardware settings properly.

Why is the Device Tree Important?

• It provides flexibility to configure hardware without modifying kernel source code.
• It defines how peripherals (I2C, SPI, UART, etc.) interact with the SoC.
• It allows enabling/disabling specific hardware components dynamically.

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Here is a step by step process to modify DTB for mecha comet device.

Prerequisites for the Host Machine

Before modifying the DTB, ensure your host machine has the necessary tools:

1. Linux Kernel Source Code

   You need the Linux source tree for your Mecha Comet device. Clone the repository if you haven’t already:

   $ git clone https://github.com/chiragp-mecha/linux-imx
   $ cd linux-imx
   

2. Device Tree Compiler (DTC)

   Required to compile and decompile device trees. Install with:

   $ sudo apt install device-tree-compiler
   

3. Cross-Compiler (if building for an embedded system)

   Install an ARM64 cross-compiler:

   $ sudo apt install gcc-aarch64-linux-gnu
   

   Ensure the cross-compiler is set in the environment:

   $ export CROSS_COMPILE=aarch64-linux-gnu-
   

4. Build Essentials

   Install necessary build dependencies:

   $ sudo apt install build-essential flex bison libssl-dev bc
   

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Steps on the Host Machine

These tasks are performed on your development machine where you modify and compile the DTS.

1. Locate the Device Tree Source File

Find the DTS file for the Mecha Comet device:

$ cd linux
$ nano arch/arm64/boot/dts/freescale/imx8mm-mecha-comet-m-gen1.dts

2. Modify the Device Tree

Make changes, such as enabling a hardware interface:

&i2c2 {
    status = "okay";
};

Save and exit the file.

3. Compile the Device Tree

Convert the DTS file into a DTB:

$ make DTBS

This generates the updated DTB:

arch/arm64/boot/dts/freescale/imx8mm-mecha-comet-m-gen1.dtb

4. Transfer the DTB to the Target Device

Use scp to copy the new DTB to the target device:

$ scp arch/arm64/boot/dts/freescale/imx8mm-mecha-comet-m-gen1.dtb user@target:/home/mecha

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Steps on the Target Device

These steps are executed on the Mecha Comet device.

1. Backup the Existing DTB

Before replacing the DTB, create a backup:

$ sudo cp /usr/lib/linux-image-6.6.36+mecha+/freescale/imx8mm-mecha-comet-m-gen1.dtb /usr/lib/linux-image-6.6.36+mecha+/freescale/imx8mm-mecha-comet-m-gen1.dtb.bak

2. Replace the DTB

Move the new DTB to the correct location:

$ mv /home/mecha/imx8mm-mecha-comet-m-gen1.dtb /usr/lib/linux-image-6.6.36+mecha+/freescale/imx8mm-mecha-comet-m-gen1.dtb

3. Reboot the Device

Apply the changes by rebooting:

$ sudo reboot

4. Verify the Changes

After the device boots, check if the modifications were applied:

$ dtc -I dtb -O dts /boot/imx8mm-mecha-comet-m-gen1.dtb -o modified.dts

Inspect modified.dts to confirm the changes.

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Troubleshooting

Device fails to boot? Restore the backup:

mv /usr/lib/linux-image-6.6.36+mecha+/freescale/imx8mm-mecha-comet-m-gen1.dtb.bak /usr/lib/linux-image-6.6.36+mecha+/freescale/imx8mm-mecha-comet-m-gen1.dtb
sudo reboot

Compilation fails? Check if device-tree-compiler is installed:

sudo apt install device-tree-compiler