Day04 - Device Tree Overlay

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Objective

• Understand how to create a Device Tree Overlay
• Compile .dts into .dtbo
• Load overlay dynamically on Raspberry Pi

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Environment

• Board: Raspberry Pi 5
• OS: Debian (Raspberry Pi OS Lite)
• Architecture: arm64

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Step 1 — Create DTS File

Create a file:

nano test-device.dts

Example content:

/dts-v1/;
/plugin/;

/ {
compatible = "brcm,bcm2712";

fragment@0 { target-path = "/";

__overlay__ {
    test_device {
        compatible = "test,device";
        status = "okay";
    };
};

}; };

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Step 2 — Compile Device Tree Overlay

dtc -@ -I dts -O dtb -o test-device.dtbo test-device.dts

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Step 3 — Load Overlay

sudo dtoverlay test-device

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Step 4 — Verify

Check kernel logs:

dmesg | tail

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Observations

• Overlay can be loaded dynamically without reboot
• Kernel accepts new hardware description at runtime
• No actual driver is bound yet (no compatible match)

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Key Learnings

• Device Tree describes hardware, not behavior
• Overlay modifies existing Device Tree
• compatible is critical for driver matching

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Common Issues

1. dtc not found

Install:

sudo apt install device-tree-compiler

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2. Overlay not applied

Check:

dmesg | grep overlay

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Next Step

• Bind Device Tree to a real driver
• Explore platform driver model

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