Device Tree

The bootloader has just copied the Linux kernel into the processor’s SDRAM. It then jumps to the kernel’s entry point. The kernel is now just like any bare-metal application running on a processor. It needs to configure the processor. It needs to set up virtual memory. It needs to print something to the console. But how? All these operations are carried out by writing to registers, but how does the Linux kernel know their addresses? How does it know how many cores it can run on? How much memory it can access?






Before device tree, all these information use to be set in a huge cluster of board files.At this time each kernel compilation use to be for only one specific chip an a specific board.



So there was a long pending wish to compile the kernel for all ARM processors, and let the kernel somehow detect its hardware and apply the right drivers as needed just like your PC.

But how? On a PC, the initial registers are hardcoded, and the rest of the information is supplied by the BIOS. But ARM processors don’t have a BIOS.
The solution chosen was device tree, also referred to as Open Firmware (abbreviated OF) or Flattened Device Tree (FDT). This is essentially a data structure in byte code format which contains information that is helpful to the kernel when booting up.


The bootloader now loads two binaries: the kernel image and the DTB.
DTB is the device tree blob. The bootloader passes the DTB address.


Device tree compiler and its source code  located at scripts/dtc/.
On ARM all device tree source are located at /arch/arm/boot/dts/.
The Device Tree Blob(.dtb) is produced by the compiler, and it is the binary that gets loaded by the bootloader and parsed by the kernel at boot time.


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Each module in device tree is defined by a node and all its properties are defined under that node.


For example a device connected by i2c bus, will have i2c as its parent node, and that device will be one of the child node of i2c node, i2c may have apd bus as its parent and so on.