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Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com> Change-Id: I08057576c23cef0343816c3b14c48db77b8dc416 Reviewed-on: https://review.coreboot.org/c/coreboot/+/75695 Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com> Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com> Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com> Tested-by: build bot (Jenkins) <no-reply@coreboot.org> Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
88 lines
2.9 KiB
Markdown
88 lines
2.9 KiB
Markdown
# Adding new devices to a device tree
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## Introduction
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ACPI exposes a platform-independent interface for operating systems to perform
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power management and other platform-level functions. Some operating systems
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also use ACPI to enumerate devices that are not immediately discoverable, such
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as those behind I2C or SPI buses (in contrast to PCI). This document discusses
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the way that coreboot uses the concept of a "device tree" to generate ACPI
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tables for usage by the operating system.
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## Devicetree and overridetree (if applicable)
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For mainboards that are organized around a "reference board" or "baseboard"
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model (see ``src/mainboard/google/octopus`` or ``hatch`` for examples), there is
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typically a devicetree.cb file that all boards share, and any differences for a
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specific board ("variant") are captured in the overridetree.cb file. Any
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settings changed in the overridetree take precedence over those in the main
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devicetree. Note, not all mainboards will have the devicetree/overridetree
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distinction, and may only have a devicetree.cb file. Or you can always just
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write the ASL (ACPI Source Language) code yourself.
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### Naming and referencing devices
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When declaring a device, it can optionally be given an alias that can be
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referred to elsewhere. This is particularly useful to declare a device in one
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device tree while allowing its configuration to be more easily changed in an
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overlay. For instance, the AMD Picasso SoC definition
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(`soc/amd/picasso/chipset.cb`) declares an IOMMU on a PCI bus that is disabled
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by default:
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```
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chip soc/amd/picasso
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device domain 0 on
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...
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device pci 00.2 alias iommu off end
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...
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end
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end
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```
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A device based on this SoC can override the configuration for the IOMMU without
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duplicating addresses, as in
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`mainboard/google/zork/variants/baseboard/devicetree_trembyle.cb`:
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```
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chip soc/amd/picasso
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device domain 0
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...
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device ref iommu on end
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...
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end
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end
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```
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In this example the override simply enables the IOMMU, but it could also
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set additional properties (or even add child devices) inside the IOMMU `device`
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block.
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---
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It is important to note that devices that use `device ref` syntax to override
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previous definitions of a device by alias must be placed at **exactly the same
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location in the device tree** as the original declaration. If not, this will
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actually create another device rather than overriding the properties of the
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existing one. For instance, if the above snippet from `devicetree_trembyle.cb`
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were written as follows:
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```
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chip soc/amd/picasso
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# NOTE: not inside domain 0!
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device ref iommu on end
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end
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```
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Then this would leave the SoC's IOMMU disabled, and instead create a new device
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with no properties as a direct child of the SoC.
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## Device drivers
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Platform independent device drivers are hooked up via entries in a devicetree.
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See [Driver Devicetree Entries](../drivers/dt_entries.md) for more info.
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## Notes
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- **All fields that are left unspecified in the devicetree are initialized to
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zero.**
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