Input files
There are four types of devicetree input files:
-
sources (
.dts) -
includes (
.dtsi) -
overlays (
.overlay) -
bindings (
.yaml)
The devicetree files inside the zephyr directory look like this:
boards/<ARCH>/<BOARD>/<BOARD>.dts
dts/common/skeleton.dtsi
dts/<ARCH>/.../<SOC>.dtsi
dts/bindings/.../binding.yamlGenerally speaking, every supported board has a BOARD.dts file describing its hardware. For example, the reel_board has boards/phytec/reel_board/reel_board.dts.
BOARD.dts includes one or more .dtsi files. These .dtsi files describe the CPU or system-on-chip Zephyr runs on, perhaps by including other .dtsi files. They can also describe other common hardware features shared by multiple boards. In addition to these includes, BOARD.dts also describes the board's specific hardware.
The dts/common directory contains skeleton.dtsi, a minimal include file for defining a complete devicetree. Architecture-specific subdirectories (dts/<ARCH>) contain .dtsi files for CPUs or SoCs which extend skeleton.dtsi.
The C preprocessor is run on all devicetree files to expand macro references, and includes are generally done with #include <filename> directives, even though DTS has a /include/ "<filename>" syntax.
BOARD.dts can be extended or modified using overlays . Overlays are also DTS files; the .overlay extension is just a convention which makes their purpose clear. Overlays adapt the base devicetree for different purposes:
-
Zephyr applications can use overlays to enable a peripheral that is disabled by default, select a sensor on the board for an application specific purpose, etc. Along with Configuration System (Kconfig), this makes it possible to reconfigure the kernel and device drivers without modifying source code.
-
Overlays are also used when defining Shields.
The build system automatically picks up .overlay files stored in certain locations. It is also possible to explicitly list the overlays to include, via the DTC_OVERLAY_FILE CMake variable. See Set devicetree overlays for details.
The build system combines BOARD.dts and any .overlay files by concatenating them, with the overlays put last. This relies on DTS syntax which allows merging overlapping definitions of nodes in the devicetree. See Example: FRDM-K64F and Hexiwear K64 for an example of how this works (in the context of .dtsi files, but the principle is the same for overlays). Putting the contents of the .overlay files last allows them to override BOARD.dts.
Devicetree bindings (which are YAML files) are essentially glue. They describe the contents of devicetree sources, includes, and overlays in a way that allows the build system to generate C macros usable by device drivers and applications. The dts/bindings directory contains bindings.
Scripts and tools
The following libraries and scripts, located in scripts/dts/, create output files from input files. Their sources have extensive documentation.
A low-level DTS parsing library.
A library layered on top of dtlib that uses bindings to interpret properties and give a higher-level view of the devicetree. Uses dtlib to do the DTS parsing.
A script that uses edtlib to generate C preprocessor macros from the devicetree and bindings.
In addition to these, the standard dtc (devicetree compiler) tool is run on the final devicetree if it is installed on your system. This is just to catch errors or warnings. The output is unused. Boards may need to pass dtc additional flags, e.g. for warning suppression. Board directories can contain a file named pre_dt_board.cmake which configures these extra flags, like this:
list(APPEND EXTRA_DTC_FLAGS "-Wno-simple_bus_reg")Output files
These are created in your application's build directory.
Warning
Don't include the header files directly. Devicetree access from C/C++ explains what to do instead.
<build>/zephyr/zephyr.dts.pre
The preprocessed DTS source. This is an intermediate output file, which is input to gen_defines.py and used to create zephyr.dts and devicetree_generated.h.
<build>/zephyr/include/generated/zephyr/devicetree_generated.h
The generated macros and additional comments describing the devicetree. Included by devicetree.h.
<build>/zephyr/zephyr.dts
The final merged devicetree. This file is output by gen_defines.py. It is useful for debugging any issues. If the devicetree compiler dtc is installed, it is also run on this file, to catch any additional warnings or errors.
A devicetree on its own is only half the story for describing hardware, as it is a relatively unstructured format. Devicetree bindings provide the other half.
A devicetree binding declares requirements on the contents of nodes, and provides semantic information about the contents of valid nodes. Zephyr devicetree bindings are YAML files in a custom format (Zephyr does not use the dt-schema tools used by the Linux kernel).