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Practical Zephyr – Devicetree semantics (Part 4)

By February 14, 2024No Comments

Having covered the Devicetree basics in the previous article, we now add semantics to our Devicetree using so-called bindings: For each supported type, we’ll create a corresponding binding and look at the generated output to understand how it can be used with Zephyr’s Devicetree API.

Notice that we’ll only look at Zephyr’s basic Devicetree API and won’t analyze specific subsystems such as gpio in detail. We’re saving this for a practice round in the next article of this Practical Zephyr series.

Table of Contents

Prerequisites

This article is part of the Practical Zephyr article series. In case you haven’t read the previous articles, please go ahead and have a look. In this article we’re building up on what we’ve seen in the Devicetree basics: We’ll add bindings to the same nodes that we’ve created in the previous article. If you have some experience with Devicetree and Zephyr, you should be able to follow along just fine without reading the previous articles.

Note: A full example application including all files that we’ll see throughout this article is available in the 03_devicetree_semantics folder of the accompanying GitHub repository.

We’ll again be using the development kit for the nRF52840 as a reference and exploring its files in Zephyr, but you can follow along with any target – real or virtual.

Warm-up

Before we get started, let’s quickly review what we’ve seen when we had a look at the UART nodes of the nRF52840 Development Kit from Nordic. We’re using the same old freestanding application with an empty prj.conf and the following file tree:

$ tree --charset=utf-8 --dirsfirst
.
├── src
│   └── main.c
├── CMakeLists.txt
└── prj.conf

The CMakeLists.txt only includes the necessary boilerplate to create a freestanding Zephyr application. As an application, we’ll again use the same old main function that outputs the string “Message in a bottle.” each time it is called, and thus each time the device starts.

#include 
#define SLEEP_TIME_MS 100U

void main(void)
{
    printk("Message in a bottle.n");
    while (1)
    {
        k_msleep(SLEEP_TIME_MS);
    }
}

The following command builds this application for the nRF52840 Development Kit from Nordic that we’re using as a reference. As usual, you can follow along with any board (or emulation target) and you should see a similar output.

$ west build --board nrf52840dk_nrf52840 --build-dir ../build

Using the --board parameter, Zephyr selects the matching Devicetree source file zephyr/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts, which specifies the properties status and current-speed for the node with the label uart0, as follows:

zephyr/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts

&uart0 {
  compatible = "nordic,nrf-uarte";
  status = "okay";
  current-speed = ;
  /* other properties */
};

Zephyr’s DTS generator script produces the matching output in devicetree_generated.h for the specified properties:

build/zephyr/include/generated/devicetree_generated.h

#define DT_N_S_soc_S_uart_40002000_P_current_speed 115200
#define DT_N_S_soc_S_uart_40002000_P_status "okay"

Can we get a similar output for Devicetree nodes that we define from scratch? In the previous article, we’ve been using overlay files to define custom nodes with their own properties to showcase Zephyr’s Devicetree types. We’ll see if we can get some output for the nodes and properties in these overlays, but before we go ahead and make blind use of overlays yet again, let’s finally have a look and learn how they work.

Devicetree overlays

Overlays are used to extend or modify the board’s Devicetree source file. Even though by convention overlay files use the .overlay file extension, they’re just plain old Devicetree source (DTS) files. The build system combines the board’s .dts file and any .overlay files by concatenating them, with the overlays put last. Thus, the contents of the .overlay file have priority over any definitions in the board’s .dts file or its includes.

Automatic overlays

The Zephyr build system automatically picks up additional overlays based on their location and file name. The following repeats the steps in Zephyr’s official documentation used by the build system to detect overlay files.

Before we have a look at the steps, there’s one detail that we haven’t seen yet. So far, we’ve specified the board as a command-line argument in the format --board . Zephyr also supports building for a board revision in case you have multiple revisions or versions of a specific board. In such a case, you can use the format --board @.

Note: As we’ve seen in previous articles, there are several ways to specify the board, but for the sake of simplicity we’re only using west and its --board argument here. You can specify the board in your path using the variable BOARD, you can provide it directly in your CMakeLists.txt file, or even pass it to an explicit cmake call.

With this detail out of the way, here’s the search performed by the CMake module zephyr/cmake/modules/dts.cmake:

  • In case the CMake variable DTC_OVERLAY_FILE is set, the build system uses the specified file(s) and stops the search.
  • If the file boards/.overlay exists in the application’s root directory, the build system selects the provided file as overlay and proceeds with the following step.
  • If a specific revision has been specified for the BOARD in the format @ and boards/_.overlay exists in the application’s root directory, this file is used in addition to boards/.overlay, if both exist.
  • If overlays have been encountered in any of the previous steps, the search stops.
  • If no files have been found and .overlay exists in the application’s root directory, the build system uses the overlay and stops the search.
  • Finally, if none of the above overlay files exist but app.overlay exists in the application’s root directory, the build system uses the overlay.

On top of the overlay files that have or haven’t been discovered by the build process, the CMake variable EXTRA_DTC_OVERLAY_FILE allows to specify additional overlay files that are added regardless of the outcome of the overlay search.

The important thing to remember is, that the Devicetree overlay files that were detected last have the highest precedence, since they may overwrite anything in the previously added overlay files. The precedence is always visible in the build output, where Zephr lists all overlay files using the output Found devicetree overlay: .overlay in the order that they are detected and thus added. The precedence increases with the given list.

Overlays by example

Let’s try and visualize this list using an imaginary file tree and board “dummy_board”. I’ve annotated the files with precedence numbers, even though – as we’ve learned before – not all files will be used by the build:

$ tree --charset=utf-8 --dirsfirst.
.
├── boards
│   ├── dummy_board_123.overlay -- #3
│   └── dummy_board.overlay     -- #4
├── dts
│   └── extra
│       ├── extra_0.overlay -- #2
│       └── extra_1.overlay -- #1
├── src
│   └── main.c
├── CMakeLists.txt
├── app.overlay         -- #6
├── dummy_board.overlay -- #5
└── prj.conf

Let’s assume we would use the following command that doesn’t include the CMake variable DTC_OVERLAY_FILE:

$ west build --board dummy_board@123 -- 
  -DEXTRA_DTC_OVERLAY_FILE="dts/extra/extra_0.overlay;dts/extra/extra_1.overlay"

The overlay files would be detected and added as follows, depending on whether or not they exist. As mentioned, the precedence increases and thus the files listed last have the highest precedence:

  • boards/dummy_board.overlay
  • boards/dummy_board_123.overlay
  • if none of the previous files exist, dummy_board.overlay
  • if none of the previous files exist, app.overlay
  • dts/extra/extra_0.overlay
  • dts/extra/extra_1.overlay

Note: It is recommended to use the boards directory for board overlay files. You should no longer place your board’s overlay files in the application’s root directory.

If, instead, we specify the CMake variable DTC_OVERLAY_FILE as app.overlay in our command as follows, the automatic detection is skipped and the build process only picks the selected DTC overlay files:

$ west build --board dummy_board@123 -- 
  -DTC_OVERLAY_FILE="app.overlay" 
  -DEXTRA_DTC_OVERLAY_FILE="dts/extra/extra_0.overlay;dts/extra/extra_1.overlay"

The overlay files would thus be added as follows and with increasing precedence:

  • app.overlay
  • dts/extra/extra_0.overlay
  • dts/extra/extra_1.overlay

What does that mean in practice? If you provide the CMake variable DTC_OVERLAY_FILE in your build, the board overlays will no longer be picked up automatically. For quick builds that might be helpful, but in case you’re building an application that should run on multiple boards, you should not use DTC_OVERLAY_FILE but maybe rather list additional overlays using EXTRA_DTC_OVERLAY_FILE.

Towards bindings

Now that we finally know what overlays are and how we can use them in our build, let’s find out what Zephyr produces for our own overlays in its devicetree_generated.h file.

Extending the example application

We start by creating our own overlay file dts/playground/props-basics.overlay:

$ tree --charset=utf-8 --dirsfirst .
├── dts
│   └── playground
│       └── props-basics.overlay
├── src
│   └── main.c
├── CMakeLists.txt
└── prj.conf

In the development kit’s Devicetree source file nrf52840dk_nrf52840.dts, the &uart0 node has two properties: A property current-speed of type int, and the property status of type string. We’ve seen that Zephyr creates some output in devicetree_generated.h for the UART node, so let’s try the same with a custom node:

dts/playground/props-basics.overlay

/ {
  node_with_props {
    int = ;
    string = "foo";
  };
};

When running a build whilst specifying the path to the overlay using the CMake variable EXTRA_DTC_OVERLAY_FILE, we can verify that the overlay is indeed picked up by the build system, since it informs us about the overlays it found using the output Found devicetree overlay:

$ west build --board nrf52840dk_nrf52840 --build-dir ../build -- 
  -DEXTRA_DTC_OVERLAY_FILE="dts/playground/props-basics.overlay"
-- Found Dtc: /opt/nordic/ncs/toolchains/4ef6631da0/bin/dtc (found suitable version "1.6.1", minimum required is "1.4.6")
-- Found BOARD.dts: /opt/nordic/ncs/v2.4.0/zephyr/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts
-- Found devicetree overlay: dts/playground/props-basics.overlay
-- Generated zephyr.dts: /path/to/build/zephyr/zephyr.dts
-- Generated devicetree_generated.h: /path/to/build/zephyr/include/generated/devicetree_generated.h
-- Including generated dts.cmake file: /path/to/build/zephyr/dts.cmake

Checking the output file build/zephyr/zephyr.dts we also see that our node_with_props can be found in the Devicetree that Zephyr is using as input for its generator script, that in turn produces devicetree_generated.h:

build/zephyr/zephyr.dts

/ {
  /* ... */
  node_with_props {
    int = ;
    string = "foo";
  };
};

Let’s see if we can find anything in devicetree_generated.h by searching for the quite unique value foo of our string property:

$ grep foo ../build/zephyr/include/generated/devicetree_generated.h
$

The search yields no results! Did we miss something? One thing that we can easily verify, is that the devicetree_generated.h at least contains our node: We can search using the node’s full path /node_with_props. You should find a large comment, separating the macros generated for our node, containing a list of definitions:

build/zephyr/include/generated/devicetree_generated.h

/*
 * Devicetree node: /node_with_props
 *
 * Node identifier: DT_N_S_node_with_props
 */

/* Node's full path: */
#define DT_N_S_node_with_props_PATH "/node_with_props"
// ---snip ---
/* (No generic property macros) */

The omitted lines contain lots of generic macros for our node. Don’t worry, we’ll skim through these soon enough. However, we won’t find anything that is even remotely related to the two properties that we defined for our node. In fact, the comment “(No generic property macros)” seems to hint that the generator did not encounter the properties int and string in our node /node_with_props.

Understanding Devicetree macro names

Before we dig deeper, let’s try to gain a better understanding of the macro names in devicetree_generated.h. Once we understand those, we should be able to know which macro (or macros) Zephyr should generate for our node’s properties.

In Zephyr’s doc folder, you can find the “RFC 7405 ABNF grammar for Devicetree macros” zephyr/doc/build/dts/macros.bnf. This RFC describes the macros that are directly generated out of the Devicetree sources. In simple words, the following rules apply (we’ve seen some of those already in the previous article, but it really can’t hurt to repeat them):

  • DT is the common prefix for Devicetree macros,
  • S is a forward slash /,
  • N refers to a node,
  • P is a property,
  • all letters are converted to lowercase,
  • and non-alphanumerics characters are converted to underscores “_

Let’s look at our favorite /soc/uart@40002000 node, specified in the nRF52840’s DTS file, and modified by the nRF52840 development kit’s DTS file:

zephyr/dts/arm/nordic/nrf52840.dtsi

/ {
  soc {
    uart0: uart@40002000 {
      compatible = "nordic,nrf-uarte";
      reg = ;
    };
  };
};

zephyr/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts

&uart0 {
  compatible = "nordic,nrf-uarte";
  status = "okay";
  current-speed = ;
  /* other properties */
};

Following the previous rules, we can transform the paths and property names into tokens:

  • The node path /soc/uart@40002000 is transformed to _S_soc_S_uart_40002000.
  • The property name current-speed is transformed to current_speed.
  • The property name status stays the same.

Since node paths are unique, by combining the node’s path with its property names we can create a unique macro for each property – and that’s exactly what Zephyr’s Devicetree generator does. The leading _N_ is used to indicate that it is followed by a node’s path, _P_ separates the node’s path from its property. For uart@40002000, we get the following:

  • /soc/uart@40002000, property current-speed
    becomes N_S_soc_S_uart_40002000_P_current_speed
  • /soc/uart@40002000, property status
    becomes N_S_soc_S_uart_40002000_P_status

Adding the DT_ prefix, we can indeed find those macros in devicetree_generated:

$ grep -sw DT_N_S_soc_S_uart_40002000_P_current_speed 
  ../build/zephyr/include/generated/devicetree_generated.h
#define DT_N_S_soc_S_uart_40002000_P_current_speed 115200

$ grep -sw DT_N_S_soc_S_uart_40002000_P_status 
  ../build/zephyr/include/generated/devicetree_generated.h
#define DT_N_S_soc_S_uart_40002000_P_status "okay"

For our little demo overlay we can do the same:

dts/playground/props-basics.overlay

/ {
  node_with_props {
    int = ;
    string = "foo";
  };
};

For /node_with_props’ properties, the generator should create the following macros:

  • DT_N_S_node_with_props_P_int for the property int,
  • DT_N_S_node_with_props_P_string for the property string.

Thus, in our devicetree_generated.h we should be able to find the above macros – but we don’t:

$ grep -sw DT_N_S_node_with_props_P_int 
  ../build/zephyr/include/generated/devicetree_generated.h

$ grep -sw DT_N_S_node_with_props_P_string 
  ../build/zephyr/include/generated/devicetree_generated.h

Something’s still missing.

Matching compatible bindings

What’s the difference between /soc/uart@40002000 and our /node_with_props? Admittedly, there are lots of differences, but the significant difference is that /soc/uart@40002000 has a compatible property. compatible is a standard property defined in the DTSpec, so let’s look it up:

Property name compatible, value type .

The compatible property value consists of one or more strings that define the specific programming model for the device. This list of strings should be used by a client program for device driver selection. The property value consists of a concatenated list of null-terminated strings, from most specific to most general. They allow a device to express its compatibility with a family of similar devices […].

The recommended format is "manufacturer,model". […] The compatible string should consist only of lowercase letters, digits and dashes, and should start with a letter. […]

Let’s rephrase this: compatible is a list of strings, where each string is essentially a reference to some model. The DTSpec uses a specific term for such models: They are called bindings.

The DTSpec defines bindings as “requirements […] for how specific types and classes of devices are represented in the Devicetree.” In very simple words, a binding defines the properties that a node (or its children) can or even must have, their exact type, and their meaning.

How is this any different from what we’ve been doing until now in our Devicetree source files? Well, without a binding, we can give a node any number of properties and can assign any property any value of any type, as long as all nodes, properties, and values are syntactically sound. Also, any property name must be considered random. Let’s take our own node_with_props:

dts/playground/props-basics.overlay

/ {
  node_with_props {
    int = ;
    string = "foo";
  };
};

The Devicetree compiler won’t complain if we assign a string value to the property int. In fact, it doesn’t even know whether or not node_with_props should have the int property at all. We could delete it and the compiler won’t complain. We also don’t know what the purpose of int or string is, what they mean, or what they’re used for.

While the property names int or string are not very revealing as such, the same is also true for the current-speed property of the /soc/uart@40002000 node in the nRF52840 Devicetree: Without any additional information, we can only assume that this is the baud rate in bits/s, but we can’t know for sure.

By providing compatible bindings, we’re telling the Devicetree compiler to check whether the given node really matches the properties and types defined in a binding, and we’re telling it what to do with the information provided in the Devicetree. Bindings also add semantics to a Devicetree by giving properties a meaning.

The DTSpec includes some standard bindings, e.g., bindings for serial devices such as our UART device. It thus defines how serial devices need to look like in a Devicetree. This binding includes the current-speed property:

Field Details
Property current-speed
Value type
Description Specifies the current speed of a serial device in bits per second. A boot program should set this property if it has initialized the serial device.
Example 115,200 Baud: current-speed = ;

Note: In case you’re wondering why current-speed is not listed in the “Standard Properties” section in the DTSpec, but all of a sudden appears in the section about “Serial devices” in the “Device Bindings”, there’s a very short answer for that: current-speed is only relevant for certain node types. It isn’t a property that is globally defined and can be applied to any kind of node.

Finally, we’re getting somewhere! We now know what the current-speed property is all about: We know its type, its physical unit, and its meaning. But this is just some table in the DTSpec, how is this information represented in Zephyr?

Bindings live outside the Devicetree. The DTSpec doesn’t specify any file format or syntax that is used for bindings. Zephyr, like Linux, uses .yaml files for its bindings. I’m assuming you’re familiar with YAML – in case you’re not, have a quick look at its online documentation.

Bindings in Zephyr

Zephyr’s official documentation provides a comprehensive explanation of the syntax used for bindings. Unless you’re adding your own devices and drivers, you’ll hardly ever need to create bindings yourself. Therefore, in this section, we’ll walk through some existing bindings in Zephyr to gain a general understanding, and then create our own example bindings in the next section.

Let’s have a look at what we can find out about /soc/uart@40002000’s current-speed and status properties in Zephyr: Via its compatible property, the node /soc/uart@40002000 claims compatibility with a binding nordic,nrf-uarte. The comma is used to specify a vendor prefix (we’ll see cover this when looking at binding names), so we’re looking for nordic’s model (binding) for nrf-uarte devices:

zephyr/dts/arm/nordic/nrf52840.dtsi

/ {
  soc {
    uart0: uart@40002000 {
      compatible = "nordic,nrf-uarte";
      /* ... */
    };
  };
};

Zephyr recursively looks for Devicetree bindings (.yaml files) in zephyr/dts/bindings. Bindings are matched against the strings provided in the compatible property of a node. Thus, for our UART node, Zephyr looks for a binding that matches nordic,nrf-uarte. Conveniently, binding files in Zephyr use the same basename as the compatible string, and thus we can find the correct binding by searching for a file called nordic,nrf-uarte.yaml, which can be found in the serial bindings subfolder:

zephyr/dts/bindings/serial/nordic,nrf-uarte.yaml

description: Nordic nRF family UARTE (UART with EasyDMA)
compatible: "nordic,nrf-uarte"
include: ["nordic,nrf-uart-common.yaml", "memory-region.yaml"]

Checking the compatible key, we see that it indeed matches the node’s compatible property. This compatible key is what Zephyr really matches the Node’s compatible property against: In theory, the file could have a different name, but by convention, the filename matches what’s in its compatible key. Zephyr doesn’t care about the convention.

Apart from compatible, the binding also has a textual description and some includes. As the name suggests, the include key allows to include the content of other bindings. Files are included by filename without specifying any paths or directories – Zephyr determines the search paths, one of which includes all subfolders of zephyr/dts/bindings. For the exact syntax, have a look at the official documentation.

The contents of included files are essentially merged using a recursive dictionary merge. In short: Everything that’s declared in included bindings is available in the including file.

Note: In case you’re wondering what happens with duplicated keys and/or values, try it out based on what we’ll learn in the next section, where we’ll create our own bindings.

Since nordic,nrf-uarte.yaml doesn’t seem to define anything related to our properties – just like in the article about Kconfig – we once again have to follow the include tree. Let’s try nordic,nrf-uart-common.yaml:

zephyr/dts/bindings/serial/nordic,nrf-uart-common.yaml

include: [uart-controller.yaml, pinctrl-device.yaml]
properties:
  # --snip--
  current-speed:
    description: |
      Initial baud rate setting for UART. Only a fixed set of baud
      rates are selectable on these devices.
    enum:
      - 1200
      - 2400
      # --snip--
      - 921600
      - 1000000

This must be it! There’s now a key properties, with a child element that matches our current-speed property. And sure enough, the properties key is used to define all properties for nodes with the matching binding may or must contain.

Here, Nordic seems to restrict the allowed baud rates using an enumeration: We can only specify values from the given list. But how does the Devicetree compiler know the type of the property? Couldn’t we also use an enumeration to pre-define strings? Yes, we could, and there’s indeed something missing: A key for the property’s type.

To find out the property’s type, we need to step further down the include tree, into uart-controller.yaml. This is Zephyr’s base model for UART controllers, which is used regardless of the actual vendor:

zephyr/dts/bindings/serial/uart-controller.yaml

include: base.yaml

bus: uart

properties:
  # --snip--
  current-speed:
    type: int
    description: Initial baud rate setting for UART
  # --snip--

Now, we finally know that current-speed is of type int and is used to configure the initial baud rate setting for UART (though the description fails to mention that the baud rate is specified in bits per second).

Combining this with the information in nordic,nrf-uart-common.yaml, we know that we can only select from a pre-defined list of baud rates and cannot specify our own custom baud rate – at least not in the Devicetree. Given this binding, the Devicetree compiler now rejects any but the allowed values, and it is therefore not possible to specify a syntactically correct value that is not an integer of the given list.

Note: bus: uart is a special key/value pair that allows you to associate devices with a bus system, e.g., I2C, SPI or UART. This feature is especially useful if a device supports multiple bus types, e.g., a sensor that can be connected either via SPI or I2C. This is out of the scope of this article, though, but is explained nicely in the official documentation.

We now know about current-speed, but what about status? As you might have guessed, we need to take yet another step down the include tree and have a look at the base.yml binding. This binding contains common fields used by all devices in Zephyr. Here, we do not only encounter the status property but also compatible:

zephyr/dts/bindings/base/base.yaml

include: [pm.yaml]

properties:
  status:
    type: string
    description: indicates the operational status of a device
    enum:
      - "ok" # Deprecated form
      - "okay"
      - "disabled"
      - "reserved"
      - "fail"
      - "fail-sss"

  compatible:
    type: string-array
    required: true
    description: compatible strings
  # --snip--

status therefore is simply a property of type string with pre-defined values that can be assigned in the Devicetree. It indicates the operational status of a device, which we’ll see in action the practical example in the next article. You can, however, imagine that a device with the status “disabled” won’t be functioning.

While going through binding files may seem tedious, the include tree depth for bindings is usually quite limited, and once you know about the base bindings such as base.yaml, it typically comes down to a handful of files. There is, however, no conveniently “flattened” output file like build/zephyr/zephyr.dts, and therefore it is always necessary to walk through the bindings. We’ll have a quick look at Nordic’s plugin for Visual Studio Code in the next article, but thus far you’ll always need to look at the source files. The following shows the include tree of nordic,nrf-uarte.yaml at the time of writing:

nordic,nrf-uarte.yaml
├── nordic,nrf-uart-common.yaml
│   ├── uart-controller.yaml
│   │   └── base.yaml
│   │       └── pm.yaml
│   └── pinctrl-device.yaml
└── memory-region.yaml

There’s one more important thing about compatible and matching bindings: If a node has more than one string in its compatible property, the build system looks for compatible bindings in the listed order and uses the first match.

Bindings directory

Before we can go ahead and experiment using our own bindings, we need to know where to place them. Just like the dts/bindings in Zephyr’s root directory, the build process also picks up any bindings in dts/bindings in the application’s root directory.

In contrast to overlays, however, detected bindings are not listed in the build output.

Bindings by example

This is it, we’re finally there! We’ll now add bindings for our extended example application to get some generated output for our node’s properties. We’ve just seen that we can place our bindings in the dts/bindings directory.

Note: A full example application including all files that we’ll see throughout this article is available in the 03_devicetree_semantics folder of the accompanying GitHub repository.

Naming

Before we start, we need to solve one of the hardest problems in engineering: Finding a good name for our bindings. The Devicetree specification contains a guideline on the value for the compatible property of a node – and therefore the name of the binding:

“The compatible string should consist only of lowercase letters, digits, and dashes, and should start with a letter. A single comma is typically only used following a vendor prefix. Underscores should not be used.” DTSpec

Let’s try this with the binding name custom,props-basic, and thus vendor prefix custom. We’ll follow the convention and use the binding’s name as filename and create a new file custom,props-basics.yaml in the application’s dts/bindings directory:

$ tree --charset=utf-8 --dirsfirst.
├── dts
│   ├── bindings
│   │   └── custom,props-basics.yaml
│   └── playground
│       └── props-basics.overlay
├── src
│   └── main.c
├── CMakeLists.txt
└── prj.conf

In our binding, we set the compatible key as "custom,props-basics", and add the two properties int and string of the matching type, without providing any description:

dts/bindings/custom,props-basics.yaml

description: Custom properties
compatible: "custom,props-basics"

properties:
  int:
    type: int
  string:
    type: string

As we’ve seen before, bindings define a node’s properties under the key properties. The template given below is the simplest form for a property in a binding:

properties:
  :
    type: 
    # required: false -> omitted by convention if false

Properties have a name and are therefore unique within the node, and each property is assigned a type using the corresponding key. Other keys such as required are optional.

There are several other keys and “features”, e.g., it is possible to define the properties for children of a node with the matching compatible property, but we’ll only have a look at the very basics. Definitely dive into Zephyr’s official documentation once you’re through with this article!

Finally, we create a new property compatible = "custom,props-basic" for our existing node_with_props

dts/playground/props-basics.overlay

/ {
  node_with_props {
    compatible = "custom,props-basic"
    int = ;
    string = "foo bar baz";
  };
};

… and recompile:

$ rm -rf ../build
$ west build --board nrf52840dk_nrf52840 --build-dir ../build -- 
  -DEXTRA_DTC_OVERLAY_FILE="dts/playground/props-basics.overlay"
-- Found devicetree overlay: dts/playground/props-basics.overlay
node '/node_with_props' compatible 'dummy,props-basics' has unknown vendor prefix 'dummy'
-- Generated zephyr.dts: /path/to/build/zephyr/zephyr.dts
-- Generated devicetree_generated.h: /path/to/build/zephyr/include/generated/devicetree_generated.h

Note: As mentioned, at the time of writing and in contrast to overlay files, bindings are not listed in the build output, not even for bindings in the application’s dts/bindings directory.

Even though the build passes, it seems that the Devicetree compiler is not too happy about our “dummy” vendor prefix. Zephyr warns us here since it maintains a “Devicetree binding vendor prefix registry” zephyr/dts/bindings/vendor-prefixes.txt to avoid name-space collisions for properties and bindings. If you’re a vendor, you can of course add your name upstream, but for this article, we’ll simply skip the vendor prefix and use the binding name custom-props-basics instead.

$ mv dts/bindings/dummy,props-basics.yaml dts/bindings/custom-props-basics.yaml
$ sed -i .bak 's/dummy,/custom-/g' dts/bindings/custom-props-basics.yaml
$ sed -i .bak 's/dummy,/custom-/g' dts/playground/props-basics.overlay
$ rm dts/**/*.bak

$ tree --charset=utf-8 --dirsfirst.
├── dts
│   ├── bindings
│   │   └── custom-props-basics.yaml
│   └── playground
│       └── props-basics.overlay
├── src
│   └── main.c
├── CMakeLists.txt
└── prj.conf

Without recompiling, we can check whether the generator script has added our properties to devicetree_generated.h. The value foo is unique enough for a quick grep; for our property int we’ll use what we’ve learned and expect to find some macro containing node_with_props_P_int. And indeed, we finally have our generated output!

$ grep foo ../build/zephyr/include/generated/devicetree_generated.h
#define DT_N_S_node_with_props_P_string "foo bar baz"
#define DT_N_S_node_with_props_P_string_STRING_UNQUOTED foo bar baz
#define DT_N_S_node_with_props_P_string_STRING_TOKEN foo_bar_baz
$ grep node_with_props_P_int ../build/zephyr/include/generated/devicetree_generated.h
#define DT_N_S_node_with_props_P_int 1
#define DT_N_S_node_with_props_P_int_EXISTS 1

We’ll learn how to use those macros in the section about Zephyr’s Devicetree API. In this section, we just make sure that our bindings lead to some generated output for all supported types. In case you can’t wait and want to have a more detailed look instead, I suggest you have a look at Zephyr’s great introduction to Devicetree bindings.

Note: If you’re experimenting and don’t see any output, make sure that the compatible property is set correctly. The Devicetree compiler does not complain in case it doesn’t find a matching binding. E.g., in case you have a typo in your compatible property, the application builds without warnings, but devicetree_generated.h won’t have any content for the desired properties.

Basic types

In the previous article, we’ve seen all of the types supported by devicetrees in Zephyr. We’ll now use the same node, but extend it by two properties called enum-int and enum-string, since enumerations are represented differently in bindings. In the previous article, we’ve seen that Zephyr’s Devicetree generator ignores value labels. We’ll throw in two of those, named second_value and string_value, for good practice, and also add the label_with_props for /node_with_props:

dts/playground/props-basics.overlay

/ {
  label_with_props: node_with_props {
    compatible = "custom-props-basics";
    existent-boolean;
    int = ;
    array = ;
    uint8-array = [ 12 34 ];
    string = string_value: "foo bar baz";
    string-array = "foo", "bar", "baz";
    enum-int = ;
    enum-string = "whatever";
  };
};

Now we need to extend our binding by adding an entry for each of the node’s properties under the properties key:

dts/bindings/custom-props-basics.yaml

description: Custom properties
compatible: "custom-props-basics"

properties:
  existent-boolean:
    type: boolean
  int:
    type: int
    required: true
  array:
    type: array
  uint8-array:
    type: uint8-array
  string:
    type: string
  string-array:
    type: string-array
  enum-int:
    type: int
    enum:
      - 100
      - 200
      - 300
  enum-string:
    type: string
    enum:
      - "whatever"
      - "works"

After recompiling …

$ rm -rf ../build
$ west build --board nrf52840dk_nrf52840 --build-dir ../build -- 
  -DEXTRA_DTC_OVERLAY_FILE="dts/playground/props-basics.overlay"

… we can now find macros for all of our properties in devicetree_generated.h for the node /node_with_props, and the comment block also indicates which binding was selected to generate the macros.

build/zephyr/include/generated/devicetree_generated.h

/*
 * Devicetree node: /node_with_props
 *
 * Node identifier: DT_N_S_node_with_props
 *
 * Binding (compatible = custom-props-basics):
 *   /path/to/dts/bindings/custom-props-basics.yaml
 */

/* Node's full path: */
#define DT_N_S_node_with_props_PATH "/node_with_props"
/* --snip-- */
/* Generic property macros: */
/* --snip-- */

Let’s have a look at the generated macros listed beyond the marker Generic property macros.

boolean

For the property existent-boolean of type boolean, the Devicetree generator produces the following macros:

#define DT_N_S_node_with_props_P_existent_boolean 1
#define DT_N_S_node_with_props_P_existent_boolean_EXISTS 1

Using what we’ve learned in the section about understanding Devicetree macro names, we can easily understand the basename DT_N_S_node_with_props_P_existent_boolean of the generated macros:

DT is the Devicetree prefix, N indicates that what follows is a node’s path, S is a forward slash /, and finally P indicates the start of a property. Thus DT_N_S_node_with_props_P_existent_boolean essentially translates to node=/node_with_props, property=existent_boolean.

Since the existent-boolean property is present in the node in our overlay, its value translates to 1. If we’d remove the property from our node, we’d end up with the following:

#define DT_N_S_node_with_props_P_existent_boolean 0
#define DT_N_S_node_with_props_P_existent_boolean_EXISTS 1

Thus, the value of booleans is 0 if the property is false, or 1 if it is true.

What about _EXISTS? Remember that any path or property name is transformed to its lowercase form in the Devicetree macros. _EXIST is all uppercase, which indicates that it isn’t something that we defined, but a macro that is generated for use with the Devicetree API.

For properties of any type but booleans, Zephyr’s Devicetree generator creates a matching _EXISTS macro only if the property exists in the Devicetree. If a property is not present, no macros are generated. Booleans are an exception where this macro is always generated since a missing property value means that the property is set to false.

Note: In case you’re wondering if it is possible to unset or delete a boolean that is defined somewhere else in the Devicetree – yes it is, and we’ll try it out in the section “deleting properties”.

int

For the property int of type int of our node /node_with_props, Zephyr’s Devicetree generator produces the following macros:

#define DT_N_S_node_with_props_P_int 1
#define DT_N_S_node_with_props_P_int_EXISTS 1

Unsurprisingly, for properties of type int the value of the macro is an integer literal (in decimal format). As mentioned in the previous section, the macro _EXISTS is created for every property that exists in the Devicetree. If we tried to remove the property from our Devicetree node, however, we’d get the following error when rebuilding since we specified int as a required property in our binding:

...
-- Found devicetree overlay: dts/playground/props-basics.overlay
devicetree error: 'int' is marked as required in 'properties:' in /path/to/dts/bindings/custom-props-basics.yaml, but does not appear in 

If we’d remove required: true from the binding file and delete the node’s property in the overlay, both macros would indeed be removed from devicetree_generated.h; any search for N_S_node_with_props_P_int would fail.

Note: Knowing how macros work in C, you might be curious why Zephyr removes the _EXISTS macro instead of defining its value to 0. After all, without using the compile time switches #ifdef or #if defined() you can’t check a macro’s value if the macro is not defined – or can you? Turns out there is a neat trick that allows to do this, and Zephyr makes use of it, but we won’t go into detail about this trick in this article. If you still want to know how this works, have a look at the documentation of the IS_ENABLED macro in zephyr/include/zephyr/sys/util_macro.h and the macros it expands to. It is explained nicely in the macros’ documentation!

Note: In case you specify the value of a node’s property of type int in the Devicetree in the hexadecimal format, at the time of writing the integer literal is converted to its decimal value in devicetree_generated.h.

array and uint8-array

The following macros are produced for our array property of type array:

#define DT_N_S_node_with_props_P_array {10 /* 0xa */, 11 /* 0xb */, 12 /* 0xc */}
#define DT_N_S_node_with_props_P_array_IDX_0 10
#define DT_N_S_node_with_props_P_array_IDX_0_EXISTS 1
#define DT_N_S_node_with_props_P_array_IDX_1 11
#define DT_N_S_node_with_props_P_array_IDX_1_EXISTS 1
#define DT_N_S_node_with_props_P_array_IDX_2 12
#define DT_N_S_node_with_props_P_array_IDX_2_EXISTS 1
/* array_FOREACH_ ... */
#define DT_N_S_node_with_props_P_array_LEN 3
#define DT_N_S_node_with_props_P_array_EXISTS 1

For properties of the type array and uint8-array, the Devicetree generator produces initializer expressions in braces, whose elements are integer literals. In the Devicetree API section, we’ll use those expressions as, well, initialization values for our variables or constants. The macro with the suffix _LEN defines the number of elements in the array.

For each element and its position n within the array, the generator also produces the macros _IDX_n and _IDX_n_EXISTS. In addition, several _FOREACH macros (hidden in the snippet) are generated that expand an expression for each element in the array at compile time.

For the node’s uint8-array property with the corresponding type, a similar set of macros is generated. Only the _FOREACH macros are slightly different, but that doesn’t concern us right now (in case you’re still curious, check out the documentation of DT_FOREACH_PROP_ELEM in the Zephyr’s Devicetree API documentation).

#define DT_N_S_node_with_props_P_uint8_array {18 /* 0x12 */, 52 /* 0x34 */}
#define DT_N_S_node_with_props_P_uint8_array_IDX_0 18
#define DT_N_S_node_with_props_P_uint8_array_IDX_0_EXISTS 1
#define DT_N_S_node_with_props_P_uint8_array_IDX_1 52
#define DT_N_S_node_with_props_P_uint8_array_IDX_1_EXISTS 1
/* --snip-- uint8_array_FOREACH_ ... */
#define DT_N_S_node_with_props_P_uint8_array_LEN 2
#define DT_N_S_node_with_props_P_uint8_array_EXISTS 1

Just like for our int property, if we’d remove the array or uint8-array property from our node in the Devicetree, no macros (not even the _EXISTS macros) are generated for the property.

string

The following is a snipped of our devicetree_generated.h for the property string:

#define DT_N_S_node_with_props_P_string "foo bar baz"
#define DT_N_S_node_with_props_P_string_STRING_UNQUOTED foo bar baz
#define DT_N_S_node_with_props_P_string_STRING_TOKEN foo_bar_baz
#define DT_N_S_node_with_props_P_string_STRING_UPPER_TOKEN FOO_BAR_BAZ
/* --snip-- string_FOREACH_ ... */
#define DT_N_S_node_with_props_P_string_EXISTS 1

For properties of the type string, the Devicetree generator produces string literals. The generator also produces macros with a special suffix:

  • _STRING_UNQUOTED contains the string literals without quotes and thus all values as tokens.
  • _STRING_TOKEN produces a single token out of the string literals. Special characters and spaces are replaced by underscores.
  • _STRING_UPPER_TOKEN produces the same token as _STRING_TOKEN, but in uppercase letters.

In addition, the generator also produces _FOREACH macros, which expand for each character in the string. E.g., for our value “foo bar baz” with the string length 11, the _FOREACH macro would expand 11 times.

Note: The characters used by the token in _STRING_TOKEN are not converted to lowercase. If we use the value “Foo Bar Baz” for string, the generated token would be Foo_Bar_Baz. Only the _STRING_UPPER_TOKEN is always all uppercase.

A string’s value as a token can be useful, e.g., when using the token to form a C variable or code. It is very rarely used, though (in case you’re curious, try looking for DT_INST_STRING_TOKEN in the Zephyr repository).

Again, no macros are generated in case the property does not exist in the Devicetree.

string-array

The following macros are produced for our string-array property of the same-named type:

#define DT_N_S_node_with_props_P_string_array {"foo", "bar", "baz"}
#define DT_N_S_node_with_props_P_string_array_IDX_0 "foo"
#define DT_N_S_node_with_props_P_string_array_IDX_0_STRING_UNQUOTED foo
#define DT_N_S_node_with_props_P_string_array_IDX_0_STRING_TOKEN foo
#define DT_N_S_node_with_props_P_string_array_IDX_0_STRING_UPPER_TOKEN FOO
#define DT_N_S_node_with_props_P_string_array_IDX_0_EXISTS 1
/* --snip-- the same IDX_n_ macros are generated for "bar" and "baz" */
/* --snip-- string_array_FOREACH_ ... */
#define DT_N_S_node_with_props_P_string_array_LEN 3
#define DT_N_S_node_with_props_P_string_array_EXISTS 1

string-arrays are handled similarly to array and uint8-array: Instead of integer literals, the Devicetree generator produces initializer expressions in braces whose elements are string literals. The macro with the suffix _LEN defines the number of elements in the array.

For each element and its position n within the array, the generator also produces the macros _IDX_n like we’ve seen for properties of type string – except that the generator won’t produce _FOREACH macros for the characters within each string literal. Instead, several _FOREACH macros (hidden in the snippet) are generated that expand an expression for each element in the array at compile time.

No macros are generated in case the property does not exist in the Devicetree.

enum

For enumerations, the generator produces the same macros as it would for the corresponding base type, e.g., int or string, and in addition it generates _ENUM macros that indicate the position within the enumeration (and some more for strings).

For our enumeration enum-int with the allowed values 100, 200, and 300, the generator produced the following macros for the selected value 200:

#define DT_N_S_node_with_props_P_enum_int 200
#define DT_N_S_node_with_props_P_enum_int_ENUM_IDX 1
#define DT_N_S_node_with_props_P_enum_int_EXISTS 1

Thus, in addition to the macros that would also have been generated if the property was a plain int, the generator produced an additional _ENUM macro indicating the index of the selected value within the enumeration.

The following is produced for enum-string with the allowed values “whatever” and “works”, for the selected value “whatever”:

#define DT_N_S_node_with_props_P_enum_string "whatever"
#define DT_N_S_node_with_props_P_enum_string_STRING_UNQUOTED whatever
#define DT_N_S_node_with_props_P_enum_string_STRING_TOKEN whatever
#define DT_N_S_node_with_props_P_enum_string_STRING_UPPER_TOKEN WHATEVER
#define DT_N_S_node_with_props_P_enum_string_ENUM_IDX 0
#define DT_N_S_node_with_props_P_enum_string_ENUM_TOKEN whatever
#define DT_N_S_node_with_props_P_enum_string_ENUM_UPPER_TOKEN WHATEVER
/* --snip-- enum_string_FOREACH_ ... */
#define DT_N_S_node_with_props_P_enum_string_EXISTS 1

Here, the generator produced the same macros that we’d get if enum-string was a plain string (including the character-based _FOREACH macros), and again we get the index of the chosen value within the enumeration. In addition, the generator produces TOKEN and TOKEN_UPPER macros with an ENUM prefix – to make the tokens accessible using the Devicetree API macros for enumerations.

Labels and paths

Before we move on to properties using phandle types, let’s have a look at labels and paths. In the previous article, we’ve seen that the Devicetree specification allows labels not only to refer to nodes but also to other positions in the Devicetree, e.g., in front of property values. In our example, we have three labels:

  • One label label_with_props for our node /node_with_props,
  • Two labels second_value and string_value for some property values.

dts/playground/props-basics.overlay

/ {
  label_with_props: node_with_props {
    compatible = "custom-props-basics";
    /* ... */
    array = ;
    /* ... */
    string = string_value: "foo bar baz";
    string-array = "foo", "bar", "baz";
    /* ... */
  };
};

Zephyr’s DTS generator accepts this input and – as we’ve seen – won’t complain about the labels second_value and string_value referring to the property values. However, Zephyr simply ignores any label that doesn’t refer to a node and therefore doesn’t generate anything usable for the property labels. Only the node label label_with_props has a macro:

$ grep second_value ../build/zephyr/include/generated/devicetree_generated.h
$ grep string_value ../build/zephyr/include/generated/devicetree_generated.h
$ grep label_with_props ../build/zephyr/include/generated/devicetree_generated.h
#define DT_N_NODELABEL_label_with_props DT_N_S_node_with_props

Note: In case you want some practice, look at the Devicetree specification and try to use references to value labels in your Devicetree. I promise it won’t work either.

The following macros are generated to represent a node and its labels:

build/zephyr/include/generated/devicetree_generated.h

/*
 * Devicetree node: /node_with_props
 *
 * Node identifier: DT_N_S_node_with_props
 * --snip--
 */
/* Node's full path: */
#define DT_N_S_node_with_props_PATH "/node_with_props"
/* Node's name with unit-address: */
#define DT_N_S_node_with_props_FULL_NAME "node_with_props"
// --snip--
/* Existence and alternate IDs: */
#define DT_N_S_node_with_props_EXISTS 1
#define DT_N_INST_0_custom_props_basics DT_N_S_node_with_props
#define DT_N_NODELABEL_label_with_props DT_N_S_node_with_props

The node identifier that is used for our /node_with_props is DT_N_S_node_with_props, which is exactly what we’d expect knowing the node’s path and from what we’ve learned about understanding Devicetree macro names. This node identifier is itself just a token: No macro with the name DT_N_S_node_with_props exists!

Identifiers are used by the macros of the Devicetree API. As we’ll see, these macros essentially do nothing but paste together different tokens. E.g., accessing a node’s property’s value, all we need to do is paste together tokens:

  • Using the node identifier DT_N_S_node_with_props,
  • and knowing that _P_ is used for property names,
  • we end up with DT_N_S_node_with_props_P_int for the property int.

In turn, a label is nothing else than a different name for our node’s identifier, as we can see in the above snippet of devicetree_generated.h. Labels use the format DT_N_NODELABEL_ as the macro name and the node’s identifier as the value.

In addition, the generator also produces macros with the suffix _PATH and _FULL_NAME that contain the node’s full path and name including the unit address as string literals.

/aliases and /chosen

In the previous article about Devicetree basics, we’ve also seen the standard nodes /aliases and /chosen, defined by the Devicetree specification. These nodes allow referring to a node using a phandle (either using a reference to its label or the ${/full/path} syntax), or the node’s full path as a plain string. Let’s try this out in our overlay file:

dts/playground/props-basics.overlay

/ {
  aliases {
    alias-by-label = &label_with_props;
    alias-by-path = &{/node_with_props};
    alias-as-string = "/node_with_props";
  };

  chosen {
    chosen-by-label = &label_with_props;
    chosen-by-path = &{/node_with_props};
    chosen-as-string = "/node_with_props";
  };
  /* ... previous content ... */
};

The node’s aliases are translated to macros with the format DT_N_ALIAS_ in the node’s section “Existence and alternate IDs” that we’ve seen just before. Looking at the generated ouput it is now also clear why Zephyr makes no clear distinction between the terms “references” and “phandle”:

/* Existence and alternate IDs: */
#define DT_N_S_node_with_props_EXISTS 1
#define DT_N_ALIAS_alias_by_label       DT_N_S_node_with_props
#define DT_N_ALIAS_alias_by_path        DT_N_S_node_with_props
#define DT_N_ALIAS_alias_as_string      DT_N_S_node_with_props
#define DT_N_INST_0_custom_props_basics DT_N_S_node_with_props
#define DT_N_NODELABEL_label_with_props DT_N_S_node_with_props

Whether we reference a node using its label, a path reference, or its path as a string, it all translates to the node’s identifier token. The only difference is the Devicetree API macro that we use to obtain this token (or node identifier), as we’ll see later.

Finally, /chosen nodes are listed in their own output section in devicetree_generated.h. The only difference to /aliases is, that each macro also gets its own _EXISTS pair:

/*
 * Chosen nodes
 */
// --snip--
#define DT_CHOSEN_chosen_by_label         DT_N_S_node_with_props
#define DT_CHOSEN_chosen_by_label_EXISTS  1
#define DT_CHOSEN_chosen_by_path          DT_N_S_node_with_props
#define DT_CHOSEN_chosen_by_path_EXISTS   1
#define DT_CHOSEN_chosen_as_string        DT_N_S_node_with_props
#define DT_CHOSEN_chosen_as_string_EXISTS 1

Phandles

Having seen labels and the /chosen and /aliases nodes, we can guess what Zephyr’s output for the types phandle and phandles is – maybe even path and phandle-array is. Nevertheless, we’ll walk through the types using a similar example to the one we’ve used in the previous article.

path, phandle, and phandles

Let’s get started by creating two new files custom-props-phandles.yaml and props-phandles.overlay:

$ tree --charset=utf-8 --dirsfirst.
├── dts
│   ├── bindings
│   │   ├── custom-props-basics.yaml
│   │   └── custom-props-phandles.yaml
│   └── playground
│       ├── props-basics.overlay
│       └── props-phandles.overlay
├── src
│   └── main.c
├── CMakeLists.txt
└── prj.conf

Our Devicetree overlay is pretty much identical to what we’ve used in the previous article, but for now, we’re skipping phandle-array properties. Thus, we’re creating properties of type path, phandle, and phandles:

/ {
  label_a: node_a { /* Empty. */ };
  label_b: node_b { /* Empty. */ };

  node_refs {
    compatible = "custom-props-phandles";

    path-by-path = &{/node_a};
    path-by-label = &label_a;

    phandle-by-path = ;
    phandle-by-label = ;

    // Simple array of phandles
    phandles = ;
  };
};

The compatible binding has the following contents:

description: Custom properties
compatible: "custom-props-phandles"

properties:
  path-by-path:
    type: path
  path-by-label:
    type: path
  phandle-by-path:
    type: phandle
  phandle-by-label:
    type: phandle
  phandles:
    type: phandles

In the build, we could just switch out our overlay and ignore props-basics.overlay, but let’s include both of them instead, remembering that we need to separate the paths by semicolon (or a space):

$ rm -rf ../build
$ west build --board nrf52840dk_nrf52840 --build-dir ../build -- 
  -DDTC_OVERLAY_FILE="dts/playground/props-basics.overlay;dts/playground/props-phandles.overlay"

The build succeeds without any problems and both overlays are detected. In the devicetree_generated.h we now find all of our new nodes, but we’re especially interested in node_refs which is using our new binding custom-props-phandles.yaml:

/*
 * Devicetree node: /node_refs
 *
 * Node identifier: DT_N_S_node_refs
 *
 * Binding (compatible = custom-props-phandles):
 *   /path/to/dts/bindings/custom-props-phandles.yaml
 */
// --snip--
/* Generic property macros: */
// --snip--

The properties path-by-path and path-by-label both have the type path. We might therefore expect a similar output that we got when specifying paths in the /chosen node, namely node identifiers. However, the only output we get for those nodes is the following:

#define DT_N_S_node_refs_P_path_by_path_EXISTS 1
#define DT_N_S_node_refs_P_path_by_label_EXISTS 1

So basically, the type: path exists in Zephyr, but at the time of writing does not produce any output – except for the _EXISTS macros, which are useless without any additional macros. This may seem surprising but makes a lot of sense recalling that Zephyr’s Devicetree is not compiled into a Devicetree blob dtb, but is instead used or resolved at compile time. Paths are of no use since nodes exist only as node identifiers and are therefore nothing but tokens that are used to access a node’s properties. At the time of writing, there is in fact not a single binding in Zephyr that uses a path.

Let’s move on to the generated output for the properties phandle-by-path and phandle-by-label:

#define DT_N_S_node_refs_P_phandle_by_path DT_N_S_node_a
#define DT_N_S_node_refs_P_phandle_by_path_IDX_0 DT_N_S_node_a
#define DT_N_S_node_refs_P_phandle_by_path_IDX_0_PH DT_N_S_node_a
#define DT_N_S_node_refs_P_phandle_by_path_IDX_0_EXISTS 1
#define DT_N_S_node_refs_P_phandle_by_path_LEN 1
#define DT_N_S_node_refs_P_phandle_by_path_EXISTS 1

#define DT_N_S_node_refs_P_phandle_by_label DT_N_S_node_a
#define DT_N_S_node_refs_P_phandle_by_label_IDX_0 DT_N_S_node_a
#define DT_N_S_node_refs_P_phandle_by_label_IDX_0_PH DT_N_S_node_a
#define DT_N_S_node_refs_P_phandle_by_label_IDX_0_EXISTS 1
#define DT_N_S_node_refs_P_phandle_by_label_LEN 1
#define DT_N_S_node_refs_P_phandle_by_label_EXISTS 1

Zephyr’s generator translated both property values into the node’s identifier, just like we’ve seen for the /aliases and /chosen nodes. The format in which the property’s value is provided in the Devicetree does not matter for the generated output – which is of the same type phandle.

It is a bit surprising, though, that Zephyr also generated macros containing _IDX_0 and the macro with the suffix _LEN: After all, phandle-typed properties only support a single value (try adding more!). The reason for this becomes clear when looking at the output for our property phandles of the same-named type:

#define DT_N_S_node_refs_P_phandles_IDX_0 DT_N_S_node_a
#define DT_N_S_node_refs_P_phandles_IDX_0_PH DT_N_S_node_a
#define DT_N_S_node_refs_P_phandles_IDX_0_EXISTS 1
#define DT_N_S_node_refs_P_phandles_IDX_1 DT_N_S_node_b
#define DT_N_S_node_refs_P_phandles_IDX_1_PH DT_N_S_node_b
#define DT_N_S_node_refs_P_phandles_IDX_1_EXISTS 1
/* --snip-- phandles_FOREACH */
#define DT_N_S_node_refs_P_phandles_LEN 2
#define DT_N_S_node_refs_P_phandles_EXISTS 1

Thus, the generated output for phandle and phandles is essentially handled in the same way. The only difference is, that for phandles only the _IDX_n macros are generated, and thus the node identifiers can only be accessed by index, whereas for the phandle type it is possible to retrieve the node’s identifier using the node identifier and property name. We’ll see this when we’ll access phandles via the Devicetree API in a later section.

In contrast to array, uint8-array, and string-array, no initializer expression is generated. Makes sense: phandles are not values but only tokens and thus cannot be used as such by an application.

In summary: paths practically don’t exist in Zephyr, phandle and phandles essentially produce the same output, and all Devicetree references and paths translate to node identifiers – which are just tokens.

phandle-array

Let’s briefly recall why we need a phandle-array: In the previous article, we’ve seen how gpios are represented by the nRF52840’s Devicetree (for a detailed explanation, please read the corresponding section in the previous article):

phandle-array in Zephyr

zephyr/dts/arm/nordic/nrf52840.dtsi

/ {
  soc {
    gpio0: gpio@50000000 {
      compatible = "nordic,nrf-gpio";
      #gpio-cells = ;
      port = ;
    };

    gpio1: gpio@50000300 {
      compatible = "nordic,nrf-gpio";
      #gpio-cells = ;
      port = ;
    };
  };
};

Each instance handles all pins in the corresponding port. However, some other Devicetree node, e.g., an LED, needs a way to control and configure a single pin of a given port. We can’t do this using a normal phandle, e.g., , we need a mechanisim to pass some parameters or metadata along with the phandle. phandle-arrays implement this exact use case, allowing to pass a predefined number of cells with the phandle, e.g., as we can see in the nRF52840’s development kit DTS file for /leds/led_0:

zephyr/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts

/ {
  leds {
    led0: led_0 {
      gpios = ;
    };
  };
};

What we haven’t seen yet, is how we can know that the first cell 13 is the pin within gpio0, and the second cell GPIO_ACTIVE_LOW is a configuration flag. This is part of the semantics defined by the binding of the gpio nodes that are compatible with nordic,nrf-gpio:

zephyr/dts/bindings/gpio/nordic,nrf-gpio.yaml

description: NRF5 GPIO node
compatible: "nordic,nrf-gpio"
# --snip--

gpio-cells:
  - pin
  - flags

Here we see that gpio-cells consists of two items pin and flags, which are matched exactly against the provided metadata in the given order. Thus, 13 refers to the pin and GPIO_ACTIVE_LOW to its flags.

You may have noticed that the gpio nodes do not conform to the naming convention that we’ve seen in the last article. This makes it a bit awkward to explain what needs to be provided for a phandle-array and the referenced nodes. Therefore, we’ll build our own from scratch.

Note: Have a look at Zephyr’s documentation about specifier cells in case you want to know how gpios opts out of the mentioned naming convention.

A phandle-array from scratch

Let’s briefly review the rules for using phandle-arrays that we’ve seen in the previous article. Notice that there’s no need to understand them out of context, we’ll see all of them in action in our example:

  • By convention, a phandle-array property is plural and its name must thus end in s.
  • The value of a phandle-array property is an array of phandles, but each phandle is followed by the pre-defined number of cells for each referenced node.
  • The number of cells that can follow a node’s reference is specified by the node’s specifier cells property
  • The specifier cells property has a defined naming convention: The name is formed by removing the plural ‘s’ and attaching ‘-cells’ to the name of the phandle-array property: #-cells

Let’s extend our props-phandles.overlay with a new property phandle-array-of-refs. Within this property, we’ll reference /node_a and pass along two cells, and /node_b with just one cell as metadata:

dts/playground/props-phandles.overlay

/ {
  label_a: node_a { /* Empty. */ };
  label_b: node_b { /* Empty. */ };
  node_refs {
    compatible = "custom-props-phandles";
    /* --snip-- */
    phandle-array-of-refs = ;
  };
};

Within our binding for custom-props-phandles, we can now add the property phandle-array-of-refs of type phandle-array:

dts/bindings/custom-props-phandles.yaml

description: Custom properties
compatible: "custom-props-phandles"

properties:
  # --snip--
  phandle-array-of-refs:
    type: phandle-array

With that, we fulfill our first rule, namely that our property name ends with an ‘s’. If we violate this rule, e.g., by using phandle-array-of-ref as the property name, the Devicetree compiler would reject the binding with the following error:

-- Found devicetree overlay: dts/playground/props-phandles.overlay
devicetree error: 'phandle-array-of-ref' in 'properties:'
in /path/to/dts/bindings/custom-props-phandles.yaml has type 'phandle-array'
and its name does not end in 's', but no 'specifier-space' was provided.

The mentioned specifier-space is a way to work around the plural ‘s’ rule and is used for properties that would otherwise result in weird names. We won’t explain the workings of specifier-space in this article, so I’ll leave you with a reference to Zephyr’s documentation on specifier-space in case you want to know how this works in detail.

For now, let’s revert the name change and use phandle-array-of-refs as the property name and try to compile the project (the command didn’t change). Now the Devicetree compiler complains that our specifier-cells are missing:

-- Found devicetree overlay: dts/playground/props-phandles.overlay
devicetree error: 
lacks #phandle-array-of-ref-cells

Looking at our value assignment phandle-array-of-refs = ; this error makes perfect sense since the compiler has no way of knowing how many cells are supposed to follow each node’s reference. We need to update our overlay with the required specifier cells property for each referenced node:

dts/playground/props-phandles.overlay

/ {
  label_a: node_a { #phandle-array-of-ref-cells = ; };
  label_b: node_b { #phandle-array-of-ref-cells = ; };
  node_refs {
    /* --snip-- */
    phandle-array-of-refs = ;
  };
};

Now the Devicetree compiler knows, that any reference to /node_a in a property of type phandle-array must be followed by exactly 2 cells, whereas a reference to /node_b only expects one cell. Does this do the trick? Let’s try to recompile the example:

-- Found devicetree overlay: dts/playground/props-phandles.overlay
devicetree error: phandle-array-of-ref controller

for 
lacks binding

For any node referenced in a phandle-array, Zephyr requires a binding since it otherwise isn’t able to generate the required macros. Without the bindings for our referenced nodes, Zephyr could have been able to generate a partial set of macros, but thankfully it was decided that such incomplete bindings are rejected entirely.

Since the number of required cells in our nodes is different, we need two separate bindings. We’ll again use the binding name as a filename and create two files dts/bindings/custom-cells-a.yaml and dts/bindings/custom-cells-b.yaml:

$ tree --charset=utf-8 --dirsfirst.
├── dts
│   ├── bindings
│   │   ├── custom-cells-a.yaml
│   │   ├── custom-cells-b.yaml
│   │   ├── custom-props-basics.yaml
│   │   └── custom-props-phandles.yaml
│   └── playground
│       ├── props-basics.overlay
│       └── props-phandles.overlay
├── src
│   └── main.c
├── CMakeLists.txt
└── prj.conf

Within the binding files, as usual, we declare compatibility using compatible key. We might expect that #phandle-array-of-ref-cells are placed under the properties key in the binding. They are, however, top-level keys and give a name to each cell that follows the reference:

dts/bindings/custom-cells-a.yaml

description: Dummy for matching "cells"
compatible: "custom-cells-a"

phandle-array-of-ref-cells:
  - name-of-cell-one
  - name-of-cell-two

dts/bindings/custom-cells-a.yaml

description: Dummy for matching "cells"
compatible: "custom-cells-b"

phandle-array-of-ref-cells:
  - name-of-cell-one

The above binding thus defines a name and semantics for each value that follows the corresponding phandle when used in a phandle-array, just like we’ve seen for the gpios example and the cells pin and flags. E.g., for phandle-array-of-refs = ;, we now know that:

  • For /node_a, the value 1 is assigned to a cell named name-of-cell-one and the value 2 to a cell named name-of-cell-two;
  • for /node_a, the cell name-of-cell-one is assigned the value 1.

We’re therefore providing the means for the Devicetree API to access the metadata passed with the phandle using a descriptive name.

Note: One thing that is not immediately clear is, that number of elements in the -cells key of the binding must match the number of -cells = specified in the Devicetree. It is, e.g., not possible to provide more elements in a binding’s list and restrict the exact number of cells using the matching Devicetree property. Thus, for any node that has a binding, the property -cells in the Devicetree is actually redundant.

We can now go ahead and assign the bindings to our nodes with the compatible property, and recompile the project.

dts/playground/props-phandles.overlay

/ {
  label_a: node_a {
    compatible = "custom-cells-a";
    #phandle-array-of-ref-cells = ;
  };
  label_b: node_b {
    compatible = "custom-cells-b";
    #phandle-array-of-ref-cells = ;
  };
  node_refs {
    compatible = "custom-props-phandles";
    /* --snip-- */
    phandle-array-of-refs = ;
  };
};

For our node /node_refs, Zephyr’s generator now produces the following macros:

#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_0_EXISTS 1
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_0_PH DT_N_S_node_a
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_0_VAL_name_of_cell_one 1
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_0_VAL_name_of_cell_one_EXISTS 1
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_0_VAL_name_of_cell_two 2
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_0_VAL_name_of_cell_two_EXISTS 1
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_1_EXISTS 1
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_1_PH DT_N_S_node_b
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_1_VAL_name_of_cell_one 1
#define DT_N_S_node_refs_P_phandle_array_of_refs_IDX_1_VAL_name_of_cell_one_EXISTS 1
/* --snip-- phandle_array_of_refs_FOREACH */
#define DT_N_S_node_refs_P_phandle_array_of_refs_LEN 2
#define DT_N_S_node_refs_P_phandle_array_of_refs_EXISTS 1

Unsurprisingly, the output looks a lot like what we get for phandles: We get a set of macros for each index n _IDX_n, but now the length n _LEN is not the total number of cells within the array but matches the number of “tuples” of phandles and metadata.

Along the macro with the suffix _PH containing the phandle and thus node’s identifier, _VAL_x macros are generated for each cell that follows the phandle; the specifier name x matches the name provided for each specifier in the binding.

Due to the fact that the index within a phandle-array does not refer to a cell’s value, values for phandle-array properties are sometimes assigned using the alternative syntax that we’ve already seen in the previous article:

  • phandle-array-of-refs = , ; is used instead of
  • phandle-array-of-refs = ;.

The generated output is identical.

Admittedly, the chosen names name_of_cell_x are not very descriptive, and the fact that one is assigned the value 1 and two the value 2 doesn’t help, so let’s have a quick lock at what’s generated for the gpios = ; property of the /leds/led0:

#define DT_N_S_leds_S_led_0_P_gpios_IDX_0_EXISTS 1
#define DT_N_S_leds_S_led_0_P_gpios_IDX_0_PH DT_N_S_soc_S_gpio_50000000
#define DT_N_S_leds_S_led_0_P_gpios_IDX_0_VAL_pin 13
#define DT_N_S_leds_S_led_0_P_gpios_IDX_0_VAL_pin_EXISTS 1
#define DT_N_S_leds_S_led_0_P_gpios_IDX_0_VAL_flags 1
#define DT_N_S_leds_S_led_0_P_gpios_IDX_0_VAL_flags_EXISTS 1
/* --snip-- gpios_FOREACH */
#define DT_N_S_leds_S_led_0_P_gpios_LEN 1
#define DT_N_S_leds_S_led_0_P_gpios_EXISTS 1

Here, we clearly see that we’ll be using the pin with the number 13 from the selected node /soc/gpio@50000000, and we’ll use the value 1 for its flags – which is what GPIO_ACTIVE_LOW expands to.

Deleting properties

In our props-basics.overlay example we’ve seen the boolean property existent-boolean. We’ve learned that a boolean is set to true if it exists in the Devicetree, otherwise, it is false. Let’s assume that a boolean property exists in some include file, how could you set this property to false in your overlay? You’d somehow need to be able to tell the compiler to remove the property since that is the only way to set it to false.

We can try this in our existing dts/playground/props-basics.overlay, where node_with_props has existent-boolean in its properties and thus sets its value to true. Without modifying node_with_props directly, we can delete the property after the node’s declaration using /delete-property/. We’ll do this for both, existent-boolean and string, as follows:

dts/playground/props-basics.overlay

/ {
  label_with_props: node_with_props {
    compatible = "custom-props-basics";
    existent-boolean;
    /* --snip-- */
  };
};

&label_with_props {
  /delete-property/ existent-boolean;
  /delete-property/ string;
}

This leads to the same generated output as if the properties were not defined in the node at all. As we’ve seen, for booleans the property’s value is simply set to 0. For the property string, no more macros are generated.

#define DT_N_S_node_with_props_P_existent_boolean 0
#define DT_N_S_node_with_props_P_existent_boolean_EXISTS 1
/* No macros DT_N_S_node_with_props_P_string ... */

Full example

Instead of providing the full example as text, I’ll leave you with a link to the example application 03_devicetree_semantics on GitHub. The overlays and bindings have the same name and content as used throughout the previous sections.

Zephyr’s Devicetree API

Now that we’ve seen both, DTS files and their bindings, we’ll wrap up this article by looking at how the generated macros are accessed in an application. We’ll cover the most common macros from zephyr/include/zephyr/devicetree.h. Those macros are the basis for most Zephyr modules, e.g., gpios, that in turn provide their own Devicetree API built on top of the mentioned macros.

Except for the macros used to access a node’s identifier, in practice you’ll use macros provided by the corresponding subystem. E.g., the gpio subsystem has its own Devicetree macros. It is nevertheless important to understand the basics, especially if you’re planning to write your own drivers. In the next article, we’ll see some examples of practical, subsystem-specific Devicetree APIs.

Note: For details, you’ll always need to refer to the official Devicetree API documentation and to the corresponding subsystems. The documentation is really great and has lots of examples. We’ll cover the basics here so that you can easily navigate the documentation. There are simply too many macros to cover them all!

In the following sections, we’ll be accessing the properties of the nodes that we’ve created throughout this article. In case you didn’t follow along but still would like to try it out yourself, use the freestanding application in the reference repository. It also contains most code snippets in its main.c file.

Macrobatics

Having seen the macros provided by Zephyr’s generator script, it is now pretty obvious how nodes and property values are accessed by Zephyr’s Devicetree API: The big “mystery” behind the Devicetree API is token pasting. In its simplest form, tokens are pasted or concatenated using the token-pasting operator ##:

// Let's assume we use macros with the format _,
// e.g., a "node" with property "string_value" set to "123-test"
#define node_string_value "123-test"

// Tokens in C macros are pasted using `##`.
#define TOKEN_CAT_3(a1, a2, a3)      a1##a2##a3
#define NODE_PROP(node_id, property) TOKEN_CAT_3(node, _, property)

// The following resolves to `node_string_value` and thus to "123-test"
static const char *str = NODE_PROP(node, string_value);

Zephyr takes token pasting to another level in its Devicetree API, using variadic macro arguments, macro repetitions, and other neat tricks. In his presentation “Zephyr Devicetree Mysteries, Solved” from the Zephyr Development Summit 2022, Martí Bolívar fittingly describes those mechanisms as “macrobatics”.

Note: I highly recommend watching his presentation (maybe after reading through this article), and if you’re curious, digging into util_macro.h and util_loops.h in zephyr/include/zephyr/sys. The files are very well documented, and the macros MACRO_MAP_CAT (used by the DT_PATH) and IS_ENABLED (resolving the _EXISTS macros) are especially interesting.

Node identifiers

Let’s review how we can reference a node in the Devicetree using our props-basics.overlay:

dts/playground/props-basics.overlay

/ {
  aliases {
    alias-by-label = &label_with_props;
    alias-by-path = &{/node_with_props};
    alias-as-string = "/node_with_props";
  };

  chosen {
    chosen-by-label = &label_with_props;
    chosen-by-path = &{/node_with_props};
    chosen-as-string = "/node_with_props";
  };

  label_with_props: node_with_props {
    /* ... */
  };
};

Note: We’re deliberately skipping references to a node using phandle properties since those are properties, and properties are accessed differently, as we’ll see in the next section (yes, technically node references in /chosen and /aliases are properties too, but those nodes are predefined by the Devicetree specification).

Having thoroughly explored Devicetree, we know that we have the following possibilities to refer to a node:

  • Using the /aliases or /chosen nodes,
  • using a globally unique node label,
  • or via the node’s full path.

In the previous article, we’ve learned that it is up to you to decide which way to reference nodes in your application. It typically makes sense to use /aliases or /chosen nodes, since paths and even node labels can be different for, e.g., different boards.

How can we reference nodes? We know that nodes as such do not exist in Zephyr’s generated Devicetree: Nodes are translated into tokens and thus do not have any value on their own. In Zephyr, these tokens are referred to as node identifiers and they are used as parameters by other macros, e.g., to retrieve a property value. Such tokens are practically just prefixes for property macros.

Enough talk. Let’s start with /aliases: Zephyr provides the DT_ALIAS macro to get a node’s identifier via its alias:

// Node identifier by /aliases node.
// DT_ALIAS(alias) = DT_N_ALIAS_ ## alias
#define NODE_PROPS_ALIAS_BY_LABEL   DT_ALIAS(alias_by_label)
#define NODE_PROPS_ALIAS_BY_PATH    DT_ALIAS(alias_by_path)
#define NODE_PROPS_ALIAS_BY_STRING  DT_ALIAS(alias_as_string)

The DT_ALIAS macro simply pastes DT_N_ALIAS_ and the given node identifier and therefore resolves to the macros that we’ve already seen in the section about /aliases and /chosen. In turn, all alias macros resolve to the node identifier DT_N_S_node_with_props.

API Pasted macro Node identifier
DT_ALIAS(alias_by_label) DT_N_ALIAS_alias_by_label DT_N_S_node_with_props
DT_ALIAS(alias_by_path) DT_N_ALIAS_alias_by_path DT_N_S_node_with_props
DT_ALIAS(alias_as_string) DT_N_ALIAS_alias_as_string DT_N_S_node_with_props

You might have noticed that the properties in the DTS file use dashes, e.g., “alias-by-label”, whereas in the code we need to replace the dashes with underscores for the alias name, resulting in alias_by_label. We’ve learned this transformation in the section about understanding Devicetree macro names. The macros in the API, however, cannot change the format of their parameters and thus identifiers. Therefore, you have to perform the “lowercase-and-underscores” transformation for all identifiers passed to Devicetree API macros:

  • All letters are converted to lowercase,
  • and non-alphanumeric characters are converted to underscores “_”.

No other transformations are needed, e.g., you’ll never need to provide a path with / replaced by _S_ or prefix Devicetree macros yourself. The Devicetree API does that for you and provides macros for all such use cases.

As mentioned before, the resulting node identifier DT_N_S_node_with_props is only a token. No macro with the same name exists, the following search does not yield any results:

$ grep -E 'DT_N_S_node_with_props[ t]' 
  ../build/zephyr/include/generated/devicetree_generated.h

Therefore, we cannot use a node identifier on its own anywhere in the code. Well, since it is a token, technically we could use it, e.g., as variable name, but that’s not the point (and please don’t do that). The point is, that the C preprocessor cannot further resolve the token, and wherever you’d use the node identifier DT_N_S_node_with_props, it would be placed as text (not as string literal) in code.

Moving on! Just like for /aliases, Zephyr provides the DT_CHOSEN macro to get a node’s identifier via the /chosen node:

// Node identifier by /chosen node.
// DT_CHOSEN(chosen) = DT_N_CHOSEN_ ## chosen
#define NODE_PROPS_CHOSEN_BY_LABEL  DT_CHOSEN(chosen_by_label)
#define NODE_PROPS_CHOSEN_BY_PATH   DT_CHOSEN(chosen_by_path)
#define NODE_PROPS_CHOSEN_AS_STRING DT_CHOSEN(chosen_as_string)

The DT_CHOSEN macro simply pastes DT_N_CHOSEN_ and the given node identifier and again resolves to the macros that we’ve seen in the section about aliases and chosen. Just like the aliases, all chosen nodes resolve to the node identifier DT_N_S_node_with_props:

API Pasted macro Node identifier
DT_CHOSEN(chosen_by_label) DT_N_CHOSEN_chosen_by_label DT_N_S_node_with_props
DT_CHOSEN(chosen_by_path) DT_N_CHOSEN_chosen_by_path DT_N_S_node_with_props
DT_CHOSEN(chosen_as_string) DT_N_CHOSEN_chosen_as_string DT_N_S_node_with_props

This leaves us with node labels and retrieving a node’s identifier by its path. Zephyr provides the two macros DT_NODELABEL and DT_PATH for this:

// Node identifier by label.
// DT_NODELABEL(label) = DT_N_NODELABEL_ ## label
#define NODE_PROPS_BY_LABEL DT_NODELABEL(label_with_props)

// Node identifier by path.
// DT_PATH(...) = DT_N_ pasted with S_ for all nodes in the path.
#define NODE_PROPS_BY_PATH  DT_PATH(node_with_props)

The DT_NODELABEL is again a simple token pasting of DT_N_NODELABEL_ and the provided label, resulting in the macro DT_N_NODELABEL_label_with_props that we’ve seen in the section about labels and paths, which again resolves to the node identifier DT_N_S_node_with_props.

The macro DT_PATH is a variadic macro that allows retrieving a node’s identifier using its full path. The path to a node is specified by the sequence of nodes, starting at the root node /. Each argument is thus a node identifier; the root node / is omitted. The Devicetree API simply pastes DT_N and _S_ for each node (in the “lowercase-and-underscores” form) in the path, resulting in the nodde identifier DT_N_S_node_with_props.

Note: The path /soc/uart@40002000 is an example of a node that is two levels deep, obtained using DT_PATH(soc, uart_40002000). Notice that it is not possible to use node labels in paths.

Property values

Using the node identifiers and the property’s name in the “lowercase-and-underscores” form, we can access the property’s value using the macro DT_PROP. The macro is very straight-forward since all it needs to do is paste the node ID, separated by _P_ with the provided property name.

Note: Have a look at the documentation of DT_PROP. You’ll find that its documentation matches with what we’ve seen for each type in the section about basic types (only enumerations are not explicitly mentioned in DT_PROP’s documentation).

In props-basics.overlay, we defined several properties that we’ll now access using the Devicetree API:

dts/playground/props-phandles.overlay

/ {
  label_with_props: node_with_props {
    compatible = "custom-props-basics";
    existent-boolean;
    int = ;
    array = ;
    uint8-array = [ 12 34  ];
    string = string_value: "foo bar baz";
    string-array = "foo", "bar", "baz";
    enum-int = ;
    enum-string = "whatever";
  };
};

Simple values

An unsurprising but common pattern that you’ll see in Zephr modules that use the Devicetree API are structures that map to a node’s properties. This is especially prominent in device drivers that use instance-based APIs, e.g., DT_INST_PROP.

Note: Instance-based APIs are in general recommended for device drivers. For the sake of simplicity, we’ve only specified individual nodes and don’t use multiple instances. Refer to the official documentation for details.

The following is an overly simplified example that retrieves all one-dimensional values from /node_with_props. For demonstration purposes, we’re passing different macros for the node’s identifier:

typedef struct
{
    bool boolean_exists;
    int32_t int_value;
    char *string_value;
    int32_t enum_int_value;
    char *enum_string_value;
} values_t;

const values_t values = {
    .boolean_exists    = DT_PROP(NODE_PROPS_ALIAS_BY_LABEL, existent_boolean),
    .int_value         = DT_PROP(NODE_PROPS_ALIAS_BY_PATH, int),
    .string_value      = DT_PROP(NODE_PROPS_ALIAS_BY_STRING, string),
    .enum_int_value    = DT_PROP(NODE_PROPS_CHOSEN_BY_PATH, enum_int),
    .enum_string_value = DT_PROP(NODE_PROPS_CHOSEN_BY_LABEL, enum_string),
};

printk("values = {n");
printk("  .boolean_exists     = %dn", values.boolean_exists);    // = 1
printk("  .int_value          = %dn", values.int_value);         // = 1
printk("  .string_value       = %sn", values.string_value);      // = "foo bar baz"
printk("  .enum_int_value     = %dn", values.enum_int_value);    // = 200
printk("  .enum_string_value  = %sn", values.enum_string_value); // = "whatever"
printk("}n");

Knowing that all of the given node identifiers resolve to DT_N_S_node_with_props, we can easily determine the macros and thus values pasted by DT_PROP that we’ve seen in the section about basic types:

  • DT_N_S_node_with_props_P_existent_boolean
  • DT_N_S_node_with_props_P_int
  • DT_N_S_node_with_props_P_string
  • DT_N_S_node_with_props_P_enum_int
  • DT_N_S_node_with_props_P_enum_string

For our enumerations enum-int and enum-string, additional macros such as DT_ENUM_IDX exist to access the index of the selected value within its list of possible values. This can be useful, e.g., to avoid string comparisons in enumerations that use the type string.

// DT_ENUM_IDX resolves to DT_N_S_node_with_props_P_enum_int_ENUM_IDX
// "The index within 'enum_int' of the selected value '200' is 1."
printk(
    "The index within 'enum_int' of the selected value '%d' is %d.n",
    values.enum_int_value,
    DT_ENUM_IDX(NODE_PROPS_CHOSEN_AS_STRING, enum_int));

// DT_ENUM_IDX resolves to DT_N_S_node_with_props_P_enum_string_ENUM_IDX
// "The index within 'enum_string' of the selected value 'whatever' is 0."
printk(
    "The index within 'enum_string' of the selected value '%s' is %d.n",
    values.enum_string_value,
    DT_ENUM_IDX(NODE_PROPS_CHOSEN_AS_STRING, enum_string));

Enumerations and strings also allow to retrieve a token matching its value. Such tokens can be used, e.g., to create unique variable names during compile time. Tokens can also be used to declare structure fields (though this is mostly done using specifier cells). For the sake of completeness, we’ll declare a variable using the string property’s value as a token:

// Resolves to DT_N_S_node_with_props_P_string_STRING_TOKEN
// and thus the variable name foo_bar_baz.
#define STRING_TOKEN DT_STRING_TOKEN(NODE_PROPS_BY_PATH, string)

uint8_t STRING_TOKEN = 0U;
STRING_TOKEN += 1U;
printk("STRING_TOKEN = %dn", STRING_TOKEN);

Scalar values

As we’ve seen in the basic types section, the macros for scalar types provide initializer lists and can therefore also be assigned to variable-length arrays. In addition, a _LEN macro is generated, accessible via the DT_PROP_LEN:

// cell_array = {10 /* 0xa */, 11 /* 0xb */, 12 /* 0xc */};
const uint32_t cell_array[] = DT_PROP(NODE_PROPS_BY_LABEL, array);
// bytestring = {18 /* 0x12 */, 52 /* 0x34 */};
const uint8_t bytestring[]  = DT_PROP(NODE_PROPS_BY_PATH, uint8_array);

const size_t cell_array_exp_length = DT_PROP_LEN(NODE_PROPS_BY_LABEL, array);
const size_t bytestring_exp_length = DT_PROP_LEN(NODE_PROPS_BY_LABEL, uint8_array);

if ((cell_array_exp_length != (sizeof(cell_array) / sizeof(uint32_t))) ||
    (bytestring_exp_length != (sizeof(bytestring) / sizeof(uint8_t))))
{
    // This is unreachable code, since the `_LEN` macro matches the
    // number of elements in the generated initializer list.
    printk("Something's wrong!n");
}

Scalar types also provide _FOREACH macros. These macros are expanded for each element in the scalar type during compile time and thus apply a given macro for each element. The following shows how to use the DT_FOREACH_PROP_ELEM macro, which in turn uses the generated macro of the corresponding property:

#define PRINTK_STRING(node_id, prop, idx)                                
    do                                                                   
    {                                                                    
        printk("[%d] -- %sn", idx, DT_PROP_BY_IDX(node_id, prop, idx)); 
    } while (0);

// This expands to one printk statement for each element in /node_with_props's
// property string-array _at compile-time_.
DT_FOREACH_PROP_ELEM(NODE_PROPS_BY_LABEL, string_array, PRINTK_STRING);
// [0] -- foo
// [1] -- bar
// [2] -- baz

phandles

We’ve seen that properties of the types phandle and phandles produce the same output, with the only difference being that the output of phandle only generates a single set of macros. Since the values of phandle macros are still just node identifiers, we need to make a tiny adaption for our referenced /node_a to show how they are used; we’re adding the property dummy-value of type int in the overlay:

dts/playground/props-phandles.overlay

/ {
  label_a: node_a {
    compatible = "custom-cells-a";
    #phandle-array-of-ref-cells = ;
    dummy-value = ;
  };
  label_b: node_b {
    compatible = "custom-cells-b";
    #phandle-array-of-ref-cells = ;
  };

  node_refs {
    compatible = "custom-props-phandles";
    // --snip--
    phandle-by-path = ;
    phandle-by-label = ;
    phandles = ;
    // --snip--
  };
};

An update of the binding is also necessary since otherwise Zephyr’s generator won’t produce any output:

dts/bindings/custom-cells-a.yaml

description: Dummy for matching "cells"
compatible: "custom-cells-a"

properties:
  dummy-value:
    type: int
# --snip--

For phandles from properties of type phandle or phandles it is possible to retrieve the node identifier using the DT_PHANDLE and DT_PHANDLE_BY_IDX Devicetree macros. The former can only be used for phandle types, whereas the latter is usable by both since both types provide the required macros including indices _IDX:

// Identifier of /node_refs.
#define NODE_REFS DT_PATH(node_refs)

// Identifiers of /node_a and /node_b via the phandle properties in /node_refs.
#define NODE_A_PHANDLE_BY_LABEL DT_PHANDLE(NODE_REFS, phandle_by_label)
#define NODE_A_PHANDLE_BY_PATH  DT_PHANDLE(NODE_REFS, phandle_by_path)
#define NODE_A_PHANDLES         DT_PHANDLE_BY_IDX(NODE_REFS, phandle_array, 0)
#define NODE_B_PHANDLES         DT_PHANDLE_BY_IDX(NODE_REFS, phandle_array, 1)

The macros with the prefix NODE_A_ resolve to DT_N_S_node_a, and NODE_B_PHANDLES to DT_N_S_node_b. Once you have the node’s identifier, you can access its properties as usual, e.g., our newly added dummy-value, using the DT_PROP macro:

uint32_t val_from_prop = DT_PROP(NODE_A_PHANDLE_BY_LABEL, dummy_value);

Since this is a very common use case, Zephyr’s Devicetree API also contains the macros DT_PROP_BY_PHANDLE and DT_PROP_BY_PHANDLE_IDX, which allow reading a property’s value without having to retrieve the node identifier using the corresponding DT_PHANDLE macros first – which is the recommended approach:

uint32_t val_from_phandle_by_label = DT_PROP_BY_PHANDLE(NODE_REFS, phandle_by_label, dummy_value);
uint32_t val_from_phandle_by_path  = DT_PROP_BY_PHANDLE(NODE_REFS, phandle_by_path, dummy_value);
uint32_t val_from_phandles         = DT_PROP_BY_PHANDLE_IDX(NODE_REFS, phandles, 0, dummy_value);

Specifier cells in phandle-arrays

We’ve seen how phandle-arrays are specified in DTS files, and how to create the corresponding bindings. Now, we finally see how to access the entries in phandle-array-of-ref-cells in our props-phandles.overlay:

dts/playground/props-phandles.overlay

/ {
  label_a: node_a {
    compatible = "custom-cells-a";
    #phandle-array-of-ref-cells = ;
    // - name for cell 0: "name-of-cell-one"
    // - name for cell 1: "name-of-cell-two"
    dummy-value = ;
  };
  label_b: node_b {
    compatible = "custom-cells-b";
    #phandle-array-of-ref-cells = ;
    // - name for cell 0: "name-of-cell-one"
  };

  node_refs {
    compatible = "custom-props-phandles";
    // --snip--
    phandle-array-of-refs = ;
  };
};

In the above Devicetree source we see that phandle-array-of-refs has two entries:

  • The phandle for /node_a is followed by two specifier cells 1 and 2,
  • the phandle for /node_b is followed by the specifier cell 1.

Note: Remember that in order to know where one entry starts and the next entry ends, it is necessary to check the #-cells property of the referenced node. Typically, a phandle marks the start of a new entry. The alternative syntax , can be used to make the separation more visible.

Looking into the binding, we can also retrieve the names of the corresponding cells:

  • In the metadata for /node_a:
    • 1 is the value for the cell with the name name-of-cell-one
    • 2 is the value for the cell with the name name-of-cell-two
  • In the metadata for /node_b:
    • 1 is the value for the cell with the name name-of-cell-one

The above example is a bit unfortunate since it is very uncommon for a phandle-array to contain specifiers for different node types: /node_a and /node_b in the above example have nothing in common, and the specifier cell with index 0 wouldn’t even need to have the same name.

Nevertheless, we’ll now squash them into the same self-defined structure using the Devicetree macros DT_PHA_BY_IDX and DT_PHA_BY_IDX_OR:

typedef struct
{
    uint32_t cell_one;
    uint32_t cell_two;
} node_spec_t;

#define NODE_DT_SPEC_GET_BY_IDX(node_id, prop, idx)                            
    {                                                                          
        .cell_one = DT_PHA_BY_IDX(node_id, prop, idx, name_of_cell_one),       
        .cell_two = DT_PHA_BY_IDX_OR(node_id, prop, idx, name_of_cell_two, 0), 
    }

// node_a = {.cell_one = 1, .cell_two = 2};
node_spec_t node_a = NODE_DT_SPEC_GET_BY_IDX(NODE_REFS, phandle_array_of_refs, 0);
// node_b = {.cell_one = 1, .cell_two = 0};
node_spec_t node_b = NODE_DT_SPEC_GET_BY_IDX(NODE_REFS, phandle_array_of_refs, 1);

// configure_node(node_a);
// configure_node(node_b);

As we can see, the names of the specifier cells must be known by the application; it is not possible to access the specifier cells by their index. The above example is again a simplified version of how Devicetree APIs are used by device drivers: The node’s specification is retrieved using a Devicetree macro, and the specification is then passed along to the corresponding API function.

We’ll see how this works in detail in the next article, where we’ll retrieve the associated gpio using GPIO_DT_SPEC_GET, and pass the specification to the matching gpio_pin_configure_dt function. We could, e.g., imagine writing a driver for a board’s LEDs:

zephyr/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts

/ {
  leds {
    led0: led_0 {
      // gpio0: compatible = "nordic,nrf-gpio";
      gpios = ;
      // gpio-cells:
      //   - pin
      //   - flags
    };
  };
};

The following is some pseudo code, showing how a driver could provide a macro to assemble the GPIO configuration for the LED pin:

#define LED_GPIO_SPEC_GET_BY_IDX(node_id, idx)                 
    {                                                          
        .pin   = DT_PHA_BY_IDX(node_id, gpios, idx, pin),      
        .flags = DT_PHA_BY_IDX_OR(node_id, gpios, idx, flags), 
    }

// The application can then assemble its GPIO device specification ...
const device_spec_t led_gpio = LED_GPIO_SPEC_GET_BY_IDX(DT_NODELABEL(led0), 0);
// ... and use it in the corresponding API function.
(void) led_gpio_configure_dt(&led_gpio);

What’s also shown in our own example is an _OR type macro: Such macros exist for practically all “normal” Devicetree macros and allow providing a default value in case the property does not exist – meaning its _EXISTS macro is not defined. Since /node_b does not have a specifier cell named name-of-cell-two, the field cell_two in its node_spec_t is always set to zero.

Without using conditional compilation or the _OR macro, any access would not compile, e.g.:

(void) DT_PHA_BY_IDX(NODE_REFS, phandle_array_of_refs, 1, name_of_cell_two);

With that said and done, let’s wrap up this article.

Conclusion

In this article, we’ve learned about Devicetree bindings and how they are used by Zephyr’s generator script to produce code that can, in turn, be used by the application using Zephyr’s Devicetree API. We’ve seen:

  • How Devicetree nodes are associated with bindings,
  • what macros are generated for all property types,
  • and how to use the generated macros with Zephyr’s Devicetree API.

We haven’t used specific subsystems, such as gpio, which add yet another layer of complexity on top of the basic Devicetree API. There are also other, more advanced concepts in Devicetree still that we haven’t seen, e.g., nexus nodes.

However, with what you’ve learned from this and the previous article about Devicetree and bindings, you should have no problems looking things up in Zephyr’s official documentation, without having to follow too many links due to the missing basics.

Note: The full example application including all the Devicetree files that we’ve seen throughout this article is available in the 03_devicetree_semantics folder of the accompanying GitHub repository.

Further reading

The following are great resources when it comes to Zephyr and are worth a read or watch:

Finally, have a look at the files in the accompanying GitHub repository and I hope I’ll see you again in the next article, where we’ll finally see a practical application of everything we’ve learned in this article!

Benjamin Cabé