# Adding new devices to a device tree

## Introduction

ACPI exposes a platform-independent interface for operating systems to perform power management and other platform-level functions. Some operating systems also use ACPI to enumerate devices that are not immediately discoverable, such as those behind I2C or SPI buses (in contrast to PCI). This document discusses the way that coreboot uses the concept of a “device tree” to generate ACPI tables for usage by the operating system.

## Devicetree and overridetree (if applicable)

For mainboards that are organized around a “reference board” or “baseboard” model (see `src/mainboard/google/octopus` or `hatch` for examples), there is typically a devicetree.cb file that all boards share, and any differences for a specific board (“variant”) are captured in the overridetree.cb file. Any settings changed in the overridetree take precedence over those in the main devicetree. Note, not all mainboards will have the devicetree/overridetree distinction, and may only have a devicetree.cb file. Or you can always just write the ASL (ACPI Source Language) code yourself.

### Naming and referencing devices

When declaring a device, it can optionally be given an alias that can be referred to elsewhere. This is particularly useful to declare a device in one device tree while allowing its configuration to be more easily changed in an overlay. For instance, the AMD Picasso SoC definition (`soc/amd/picasso/chipset.cb`) declares an IOMMU on a PCI bus that is disabled by default:

    chip soc/amd/picasso
    	device domain 0 on
    		...
    		device pci 00.2 alias iommu off end
    		...
    	end
    end

A device based on this SoC can override the configuration for the IOMMU without duplicating addresses, as in `mainboard/google/zork/variants/baseboard/devicetree_trembyle.cb`:

    chip soc/amd/picasso
    	device domain 0
    		...
    		device ref iommu on end
    		...
    	end
    end

In this example the override simply enables the IOMMU, but it could also set additional properties (or even add child devices) inside the IOMMU `device` block.

It is important to note that devices that use `device ref` syntax to override previous definitions of a device by alias must be placed at exactly the same location in the device tree as the original declaration. If not, this will actually create another device rather than overriding the properties of the existing one. For instance, if the above snippet from `devicetree_trembyle.cb` were written as follows:

    chip soc/amd/picasso
    	# NOTE: not inside domain 0!
    	device ref iommu on end
    end

Then this would leave the SoC’s IOMMU disabled, and instead create a new device with no properties as a direct child of the SoC.

## Device drivers

Platform independent device drivers are hooked up via entries in a devicetree. See Driver Devicetree Entries for more info.

## Notes

- All fields that are left unspecified in the devicetree are initialized to zero.