Devicetree HOWTOs¶
This page has advice for getting things done with Devicetree in Zephyr.
Adding support for a board¶
Devicetree is currently supported on all embedded targets except posix (boards/posix).
Adding devicetree support for a given board requires adding a number of files. These files will contain the DTS information that describes a platform, the bindings in YAML format, and any fixup files required to support the platform.
It is best practice to separate common peripheral information that could be
used across multiple cores, SoC families, or boards in .dtsi files,
reserving the .dts suffix for the primary DTS file for a given board.
Example: FRDM-K64F and Hexiwear K64¶
The FRDM-K64F and Hexiwear K64 board devicetrees are defined in frdm_k64fs.dts and hexiwear_k64.dts respectively. Both boards have NXP SoCs from the same Kinetis SoC family, the K6X.
Common devicetree definitions for K6X are stored in nxp_k6x.dtsi, which is included by both board .dts
files. nxp_k6x.dtsi in turn includes
armv7-m.dtsi, which has common definitions
for Arm v7-M cores.
Since nxp_k6x.dtsi is meant to be
generic across K6X-based boards, it leaves many devices disabled by default
using status properties. For example, there is a CAN controller defined as
follows (with unimportant parts skipped):
can0: can@40024000 {
...
status = "disabled";
...
};
It is up to the board .dts or application overlay files to enable these
devices as desired, by setting status = "okay". The board .dts
files are also responsible for any board-specific configuration of the device,
such as adding nodes for on-board sensors, LEDs, buttons, etc.
For example, FRDM-K64 (but not Hexiwear K64) .dts enables the CAN
controller and sets the bus speed:
&can0 {
status = "okay";
bus-speed = <125000>;
};
The &can0 { ... }; syntax adds/overrides properties on the node with label
can0, i.e. the can@4002400 node defined in the .dtsi file.
Other examples of board-specific customization is pointing properties in
aliases and chosen to the right nodes (see aliases and chosen nodes), and
making GPIO/pinmux assignments.
Devicetree Source File Template¶
A board’s .dts file contains at least a version line, optional
includes, and a root node definition with model and compatible
properties. These property values denote the particular board.
/dts-v1/;
#include <vendor/soc.dtsi>
/ {
model = "Human readable board name";
compatible = "vendor,soc-on-your-board's-mcu";
/* rest of file */
};
You can use other board .dts files as a starting point.
The following is a more precise list of required files:
Base architecture support
Add architecture-specific DTS directory, if not already present. Example: dts/arm for Arm.
Add target specific devicetree files for base SoC. These should be .dtsi files to be included in the board-specific devicetree files.
Add target specific YAML binding files in the dts/bindings/ directory. Create the yaml directory if not present.
SoC family support
Add one or more SoC family .dtsi files that describe the hardware for a set of devices. The file should contain all the relevant nodes and base configuration that would be applicable to all boards utilizing that SoC family.
Add SoC family YAML binding files that describe the nodes present in the .dtsi file.
Board specific support
Add a board level .dts file that includes the SoC family .dtsi files and enables the nodes required for that specific board.
Board .dts file should specify the SRAM and FLASH devices, if present.
Flash device node might specify flash partitions. For more details see Managing flash partitions
Add board-specific YAML binding files, if required. This would occur if the board has additional hardware that is not covered by the SoC family .dtsi/.yaml files.
Fixup files
Fixup files contain mappings from existing Kconfig options to the actual underlying DTS derived configuration #defines. Fixup files are temporary artifacts until additional DTS changes are made to make them unnecessary.
Overlay Files (optional)
Overlay files contain tweaks or changes to the SoC and Board support files described above. They can be used to modify devicetree configurations without having to change the SoC and Board files. See Devicetree Overlays for more information on overlay files and the Zephyr build system.
aliases and chosen nodes¶
Using an alias with a common name for a particular node makes it easier for you
to write board-independent source code. Devicetree aliases nodes are used
for this purpose, by mapping certain generic, commonly used names to specific
hardware resources:
aliases {
led0 = &led0;
sw0 = &button0;
sw1 = &button1;
uart-0 = &uart0;
uart-1 = &uart1;
};
Certain software subsystems require a specific hardware resource to bind to in
order to function properly. Some of those subsystems are used with many
different boards, which makes using the devicetree chosen nodes very
convenient. By doing, so the software subsystem can rely on having the specific
hardware peripheral assigned to it. In the following example we bind the shell
to uart1 in this board:
chosen {
zephyr,shell-uart = &uart1;
};
The table below lists Zephyr-specific chosen properties. The macro
identifiers that start with CONFIG_* are generated from Kconfig symbols
that reference devicetree data via the Kconfig preprocessor.
Note
Since the particular devicetree isn’t known while generating Kconfig documentation, the Kconfig symbol reference pages linked below do not include information derived from devicetree. Instead, you might see e.g. an empty default:
default "" if HAS_DTS
To see how the preprocessor is used for a symbol, look it up directly in the
Kconfig file where it is defined instead. The reference page for the
symbol gives the definition location.
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Adding support for a device driver¶
Zephyr device drivers typically use information from devicetree.h to
statically allocate and initialize struct device
instances. Macros generated from devicetree are usually included via devicetree.h, then
stored in ROM in the value pointed to by a device->config->config_info
field. For example, a struct device corresponding to an I2C peripheral
would store the peripheral address in its reg property there.
Application source code with a pointer to the struct device can then pass
it to driver APIs in include/drivers/. These API functions
usually take a struct device* as their first argument. This allows the
driver API to use information from devicetree to interact with the device
hardware.
Driver writers should allocate a struct device for each enabled instance of a
particular compatible using DT_INST_<instance-number>_<compatible>
Node existence flags.
Managing flash partitions¶
Devicetree can be used to describe a partition layout for any flash device in the system.
Two important uses for this mechanism are:
To force the Zephyr image to be linked into a specific area on Flash.
This is useful, for example, if the Zephyr image must be linked at some offset from the flash device’s start, to be loaded by a bootloader at runtime.
To generate compile-time definitions for the partition layout, which can be shared by Zephyr subsystems and applications to operate on specific areas in flash.
This is useful, for example, to create areas for storing file systems or other persistent state. These defines only describe the boundaries of each partition. They don’t, for example, initialize a partition’s flash contents with a file system.
Partitions are generally managed using device tree overlays. Refer to Devicetree Overlays for details on using overlay files.
Defining Partitions¶
The partition layout for a flash device is described inside the
partitions child node of the flash device’s node in the device
tree.
You can define partitions for any flash device on the system.
Most Zephyr-supported SoCs with flash support in device tree
will define a label flash0. This label refers to the primary
on-die flash programmed to run Zephyr. To generate partitions
for this device, add the following snippet to a device tree overlay
file:
&flash0 {
partitions {
compatible = "fixed-partitions";
#address-cells = <1>;
#size-cells = <1>;
/* Define your partitions here; see below */
};
};
To define partitions for another flash device, modify the above to either use its label or provide a complete path to the flash device node in the device tree.
The content of the partitions node looks like this:
partitions {
compatible = "fixed-partitions";
#address-cells = <1>;
#size-cells = <1>;
partition1_label: partition@START_OFFSET_1 {
label = "partition1_name";
reg = <0xSTART_OFFSET_1 0xSIZE_1>;
};
/* ... */
partitionN_label: partition@START_OFFSET_N {
label = "partitionN_name";
reg = <0xSTART_OFFSET_N 0xSIZE_N>;
};
};
Where:
partitionX_labelare device tree labels that can be used elsewhere in the device tree to refer to the partitionpartitionX_namecontrols how defines generated by the Zephyr build system for this partition will be namedSTART_OFFSET_xis the start offset in hexadecimal notation of the partition from the beginning of the flash deviceSIZE_xis the hexadecimal size, in bytes, of the flash partition
The partitions do not have to cover the entire flash device. The device tree compiler currently does not check if partitions overlap; you must ensure they do not when defining them.
Example Primary Flash Partition Layout¶
Here is a complete (but hypothetical) example device tree overlay snippet illustrating these ideas. Notice how the partitions do not overlap, but also do not cover the entire device.
&flash0 {
partitions {
compatible = "fixed-partitions";
#address-cells = <1>;
#size-cells = <1>;
code_dts_label: partition@8000 {
label = "zephyr-code";
reg = <0x00008000 0x34000>;
};
data_dts_label: partition@70000 {
label = "application-data";
reg = <0x00070000 0xD000>;
};
};
};
Linking Zephyr Within a Partition¶
To force the linker to output a Zephyr image within a given flash partition, add this to a device tree overlay:
/ {
chosen {
zephyr,code-partition = &slot0_partition;
};
};
Then, enable the CONFIG_USE_DT_CODE_PARTITION Kconfig option.
Flash Partition Macros¶
The Zephyr build system generates definitions for each flash device
partition. These definitions are available to any files which
include <zephyr.h>.
Consider this flash partition:
dts_label: partition@START_OFFSET {
label = "def-name";
reg = <0xSTART_OFFSET 0xSIZE>;
};
The build system will generate the following corresponding defines:
#define FLASH_AREA_DEF_NAME_LABEL "def-name"
#define FLASH_AREA_DEF_NAME_OFFSET_0 0xSTART_OFFSET
#define FLASH_AREA_DEF_NAME_SIZE_0 0xSIZE
#define FLASH_AREA_DEF_NAME_OFFSET FLASH_AREA_MCUBOOT_OFFSET_0
#define FLASH_AREA_DEF_NAME_SIZE FLASH_AREA_MCUBOOT_SIZE_0
As you can see, the label property is capitalized when forming the
macro names. Other simple conversions to ensure it is a valid C
identifier, such as converting “-” to “_”, are also performed. The
offsets and sizes are available as well.
MCUboot Partitions¶
MCUboot is a secure bootloader for 32-bit microcontrollers.
Some Zephyr boards provide definitions for the flash partitions which are required to build MCUboot itself, as well as any applications which must be chain-loaded by MCUboot.
The device tree labels for these partitions are:
- boot_partition
This is the partition where the bootloader is expected to be placed. MCUboot’s build system will attempt to link the MCUboot image into this partition.
- slot0_partition
MCUboot loads the executable application image from this partition. Any application bootable by MCUboot must be linked to run from this partition.
- slot1_partition
This is the partition which stores firmware upgrade images. Zephyr applications which receive firmware updates must ensure the upgrade images are placed in this partition (the Zephyr DFU subsystem can be used for this purpose). MCUboot checks for upgrade images in this partition, and can move them to
slot0_partitionfor execution. Theslot0_partitionandslot1_partitionmust be the same size.- scratch_partition
This partition is used as temporary storage while swapping the contents of
slot0_partitionandslot1_partition.
Important
Upgrade images are only temporarily stored in slot1_partition.
They must be linked to execute of out of slot0_partition.
See the MCUboot documentation for more details on these partitions.
File System Partitions¶
- storage_partition
This is the area where e.g. LittleFS or NVS or FCB expects its partition.