Day 4 — Device Tree & Overlay¶

Objective¶

Understand how Device Tree works in Embedded Linux and learn how to use a Device Tree Overlay to modify hardware configuration without rebuilding the kernel.

1. Device Tree Overview¶

In Embedded Linux, the Device Tree describes the hardware layout of the system.

Instead of hard-coding hardware information inside kernel drivers, Linux uses Device Tree to provide information such as:

hardware address
interrupt lines
bus connections
device compatibility

This allows the same kernel to support multiple boards.

Conceptually:

Driver (kernel)
      ↑
Device Tree (hardware description)

The driver reads the Device Tree and configures the hardware accordingly.

2. Raspberry Pi Device Tree Files¶

On Raspberry Pi OS, the Device Tree files are located in:

/boot/firmware

Example:

ls /boot/firmware

Output includes many .dtb files:

bcm2712-rpi-5-b.dtb
bcm2712-rpi-500.dtb
bcm2712-rpi-cm5-cm5io.dtb
...

These .dtb files represent different Raspberry Pi boards.

For example:

File	Board
bcm2712-rpi-5-b.dtb	Raspberry Pi 5
bcm2711-rpi-4-b.dtb	Raspberry Pi 4
bcm2710-rpi-3-b.dtb	Raspberry Pi 3

During boot, the firmware loads the appropriate Device Tree for the board.

3. Device Tree Overlay¶

A Device Tree Overlay modifies the base Device Tree without rebuilding the kernel.

Conceptually:

Base Device Tree
      +
Device Tree Overlay
      =
Runtime Device Tree

Overlays are commonly used to:

enable hardware modules
configure GPIO
add I2C or SPI devices
support HAT expansion boards

4. Overlay Storage Location¶

Raspberry Pi overlays are stored in:

/boot/firmware/overlays

List available overlays:

ls /boot/firmware/overlays

Each overlay is compiled as a .dtbo file.

5. Overlay Exercise — Create a Fake Device¶

To understand overlays, we created a test device node.

File:

test-device-overlay.dts

Example:

/dts-v1/;
/plugin/;

/ {

    compatible = "brcm,bcm2712";

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

        __overlay__ {

            test_device: test-device {
                compatible = "demo,test-device";
                status = "okay";
            };
        };
    };
};

6. Compile Device Tree Overlay¶

Compile using the Device Tree Compiler:

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

This generates:

test-device.dtbo

7. Install the Overlay¶

Copy the overlay to the Raspberry Pi overlay directory:

sudo cp test-device.dtbo /boot/firmware/overlays/

8. Enable Overlay¶

Edit the boot configuration:

sudo nano /boot/firmware/config.txt

Add:

dtoverlay=test-device

9. Reboot System¶

Overlay is applied during boot:

sudo reboot

10. Verify the Device Tree¶

Linux exposes the runtime Device Tree in:

/proc/device-tree

Check if the device exists:

ls /proc/device-tree

Expected node:

test-device

Check properties:

cat /proc/device-tree/test-device/compatible

11. Dump Runtime Device Tree¶

To view the complete Device Tree:

dtc -I fs -O dts /proc/device-tree > running.dts

Search the test node:

grep test-device running.dts

Example result:

test-device {
    compatible = "demo,test-device";
    status = "okay";
};

12. What We Learned¶

Device Tree allows Linux to separate hardware description from driver implementation.

Key ideas:

hardware configuration is stored in the Device Tree
overlays allow dynamic modification
kernel reads Device Tree during boot
drivers match using the compatible property

Matching flow:

Device Tree node
       ↓
compatible string
       ↓
kernel driver match
       ↓
probe()

13. Summary¶

Today we learned:

what Device Tree is
how Raspberry Pi uses .dtb files
how Device Tree overlays work
how to create and compile an overlay
how to verify the runtime Device Tree

The overlay mechanism allows flexible hardware configuration without modifying the kernel.

Next step:

Linux Driver Model