Device Tree¶

Device Tree is a data structure used by the Linux kernel to describe the hardware of a system.

Instead of hardcoding hardware details in kernel drivers, the system provides hardware information through a Device Tree Blob (DTB) that is loaded during boot.

This mechanism allows the same kernel to support multiple hardware platforms.

1. Why Device Tree Exists¶

Embedded systems often have many different hardware configurations.

Examples:

different CPUs
different GPIO layouts
different sensors
different memory maps

Without Device Tree, drivers would need board-specific code.

Device Tree separates:

Hardware description
Driver implementation

2. Device Tree Files¶

Device Tree is written in Device Tree Source (DTS) format.

Example:

example.dts

It is compiled into a binary format:

example.dtb

Compilation tool:

dtc (Device Tree Compiler)

Example command:

dtc -I dts -O dtb -o example.dtb example.dts

3. Boot Process and Device Tree¶

During system boot:

Boot ROM
   ↓
Bootloader / Firmware
   ↓
Load Kernel
Load Device Tree Blob (DTB)
   ↓
Kernel parses Device Tree
   ↓
Kernel creates devices

The Device Tree tells the kernel:

what hardware exists
how devices are connected
what drivers should handle them

4. Device Tree Structure¶

Device Tree is organized as a hierarchical tree.

Example:

/ {
    cpus {
        cpu@0 {
            compatible = "arm,cortex-a76";
        };
    };

    memory@0 {
        device_type = "memory";
    };

    soc {
        i2c@7e804000 {
            compatible = "brcm,bcm2835-i2c";
        };
    };
};

Key characteristics:

hierarchical nodes
properties inside nodes
hardware address in node names

5. Nodes¶

A node represents a hardware component.

Example:

i2c@7e804000 {
};

Node name format:

device-name@address

Example:

uart@7e201000
spi@7e204000
i2c@7e804000

6. Properties¶

Nodes contain properties.

Example:

compatible = "brcm,bcm2835-i2c";
reg = <0x7e804000 0x1000>;
status = "okay";

Common properties:

Property	Purpose
compatible	driver matching
reg	register address
interrupts	interrupt lines
status	enable/disable device

7. The "compatible" Property¶

The compatible string connects Device Tree nodes to kernel drivers.

Example:

compatible = "brcm,bcm2835-i2c";

Kernel driver defines a matching table:

static const struct of_device_id match_table[] = {
    { .compatible = "brcm,bcm2835-i2c" },
};

Matching flow:

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

8. Device Tree in Linux Runtime¶

Linux exposes the runtime Device Tree through:

/proc/device-tree

Example:

ls /proc/device-tree

To dump the full tree:

dtc -I fs -O dts /proc/device-tree

This shows the final Device Tree after overlays are applied.

9. Device Tree in Raspberry Pi¶

On Raspberry Pi systems:

Device Tree files are located in:

/boot/firmware

Example files:

bcm2712-rpi-5-b.dtb
bcm2711-rpi-4-b.dtb

Overlays are located in:

/boot/firmware/overlays

Enabled in:

/boot/firmware/config.txt

Example:

dtoverlay=i2c1
dtoverlay=spi0

10. Summary¶

Device Tree provides a standardized way to describe hardware to the Linux kernel.

Key ideas:

hardware description separate from drivers
hierarchical tree structure
nodes represent hardware devices
properties describe configuration
drivers match using the compatible property

Device Tree is essential for Embedded Linux systems.

11. From Device Tree to Platform Device¶

In embedded Linux, many devices are not dynamically discovered.

Instead, they are defined in Device Tree and converted into platform devices by the kernel.

Flow:

@@@ Device Tree node ↓ Kernel parses DT ↓ platform_device created ↓ Matched with platform_driver ↓ probe() is called @@@

Example Observation¶

In the system:

@@@ /proc/device-tree/test-device@0 /sys/bus/platform/devices/test-device@0 @@@

This shows:

Device Tree node exists
Kernel created a platform device
Device is visible in sysfs

Important Insight¶

Even without a driver:

@@@ Device exists (platform device) Driver may NOT exist yet @@@

This separation is fundamental in Linux.

12. /dev vs /sys vs Device Tree¶

Linux uses different layers to represent devices.

/dev¶

User-space interface

Examples:

@@@ /dev/ttyAMA0 /dev/i2c-1 /dev/spidev0.0 @@@

Used by applications.

/sys¶

Kernel device model representation

Examples:

@@@ /sys/bus/ /sys/devices/ /sys/class/ @@@

Shows:

device hierarchy
driver binding
bus structure

/proc/device-tree¶

Hardware description

@@@ /proc/device-tree @@@

Represents:

hardware layout
DT nodes
compatible strings

Key Relationship¶

@@@ Device Tree → creates device /sys → shows device model /dev → user access interface @@@

Example¶

@@@ /dev/ttyAMA10 → /sys/class/tty/ttyAMA10 → /sys/devices/platform/.../ttyAMA10 @@@

This shows how layers are connected.

13. Notes on Device Tree Warnings¶

When dumping Device Tree using:

@@@ dtc -I fs -O dts /proc/device-tree @@@

You may see many warnings.

Common Reasons¶

Live device tree is not identical to original DTS
Missing metadata (phandle, reg format)
Flattened structure after boot

Example Warning¶

@@@ Warning (unit_address_vs_reg): node has a unit name, but no reg property @@@

Cause:

@@@ test-device@0 // has @0 but no reg property @@@

Recommended Fix¶

If no register mapping exists:

@@@ test-device { compatible = "my,test-device"; }; @@@

Key Takeaway¶

Many dtc warnings are normal
Focus on real hardware definition issues
Naming conventions still matter