Devicetree: Difference between revisions
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The '''devicetree''' (device tree, DT | The '''devicetree''' (device tree, DT) is a data structure which holds information about all components present on a device. This data is structured in nested nodes with key/value property pairs for configuration. | ||
In simpler terms - a devicetree tells the kernel (or another DT-compatible piece of software/firmware like a bootloader) where each component is in register space/on an I2C or similar bus, and what settings to use to set it up. It is the basic mechanism for discovering components on embedded platforms, including ARM. | In simpler terms - a devicetree tells the kernel (or another DT-compatible piece of software/firmware like a bootloader) where each component is in register space/on an I2C or similar bus, and what settings to use to set it up. It is the basic mechanism for discovering components on embedded platforms, including ARM. | ||
Devicetrees are written in a plaintext format known as the '''Device Tree Source (DTS)''' format. The DTS is later compiled into a '''Device Tree Blob (DTB)'''; in this form, it can be loaded by software/firmware. | |||
Devicetrees are validated using '''devicetree schema''', which is described by bindings; see [[/Bindings]] for more information. | Devicetrees are validated using '''devicetree schema''', which is described by bindings; see [[/Bindings]] for more information. | ||
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Before the introduction of devicetrees, ARM kernels used '''board files'''. These were C files stored in <code>arch/arm/mach-*</code> which served a similar purpose to devicetrees - they contained structures for defining component configuration ("platform data"). Unlike device trees however, they could also define C functions, since they were regular C sources compiled into the kernel. Board files technically still exist (citation needed?), but are no longer in wide use. | Before the introduction of devicetrees, ARM kernels used '''board files'''. These were C files stored in <code>arch/arm/mach-*</code> which served a similar purpose to devicetrees - they contained structures for defining component configuration ("platform data"). Unlike device trees however, they could also define C functions, since they were regular C sources compiled into the kernel. Board files technically still exist (citation needed?), but are no longer in wide use. | ||
== Device Tree Source (DTS) | == Device Tree Source (DTS) basics == | ||
Here is a very simple DTS file to explain the basics of what you might see in a device tree source file: | Here is a very simple DTS file to explain the basics of what you might see in a device tree source file: | ||
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$ make CHECK_DTBS=y DT_SCHEMA_FILES=trivial-devices.yaml qcom/sm8450-hdk.dtb | $ make CHECK_DTBS=y DT_SCHEMA_FILES=trivial-devices.yaml qcom/sm8450-hdk.dtb | ||
</syntaxhighlight> | </syntaxhighlight> | ||
== Working with DTC == | |||
The <code>dtc</code> tool handles compiling DTS files into DTBs, as well as decompiling DTBs back into DTS. (TODO: add instructions) | |||
== See also == | == See also == | ||