Difference between revisions of "Device tree"

Latest revision as of 20:20, 19 July 2023

Overview

2021-08-23 Monday Starting notes for Device Tree, an OS independent framework with its own governance and steering committee, community, web site and on-line documentation. As a contextual note, Device Tree has been part of Linux Kernel Project for many more years than part of Zephyr RTOS Project. Some useful starting point references and documents are linked here in table 1:

Device Tree specification at Github:
• https://github.com/devicetree-org/devicetree-specification/releases/tag/v0.3
Zephyr Project Device Tree documentation:
• https://docs.zephyrproject.org/2.6.0/reference/devicetree/api.html
Toradex document on Toradex use of Device Tree:
• https://developer.toradex.com/linux-bsp/how-to/device-tree/device-tree-customization/
A device tree introduction on a git repository of RaspberryPi foundation. This document explains fundamental device tree data types:
• Device_tree_documentation#device-tree.md device-tree.md

^ Some Historical Context

Some device tree historical context given in this Power Point presentation by Thomas Petazzoni of Free Electrons:

• https://elinux.org/images/f/f9/Petazzoni-device-tree-dummies_0.pdf
• https://www.nxp.com/docs/en/application-note/AN5125.pdf

^ DTS Labels

• https://devicetree-specification.readthedocs.io/en/v0.3/source-language.html

^ Device Bindings

Today Monday studying section 4.0, "Device Bindings" and binding guidelines. May have questions for Jared Wolff of CircuitDojo, relating to his Quality Wing Zephyr driver project and specifically the files in Jared's AQW project.

Latest Zephyr documentation as of 2021-08-23 is here:

• https://docs.zephyrproject.org/latest/guides/dts/bindings.html
• https://docs.zephyrproject.org/latest/guides/dts/intro.html
• https://docs.zephyrproject.org/latest/guides/dts/howtos.html#dt-howtos
• https://docs.zephyrproject.org/latest/guides/dts/intro.html#devicetree-in-out-files

Related to or came up in search result in regards to '_P_label' no such symbol defined errors, when board overlay file not specified or not found:

• https://docs.zephyrproject.org/apidoc/latest/group__devicetree-generic-id.html

^ Device Tree Bindings Files

Noting that in AQW bindings file for SHCT3 sensor there is an include line:

Code excerpt, full file from Jared Wolff AQW driver:

#
# Copyright (c) 2021 Circuit Dojo LLC
#

description: Sensirion SHTC3 Temp/Humidity Sensor

compatible: "sensirion,shtc3"

include: i2c-device.yaml

The file i2c-device.yaml is found locally in /usr/local/share/ncs/v1.6.1/zephyr/dts/bindings/i2c/i2c-device.yaml, and this file contains:

Code excerpt, full file from Zephyr project:

# Copyright (c) 2017, Linaro Limited
# SPDX-License-Identifier: Apache-2.0

# Common fields for I2C devices

include: [base.yaml, power.yaml]

on-bus: i2c

properties:
    reg:
      required: true
      description: device address on i2c bus
    label:
      required: true

QUESTION: how does a developer know to include i2c-device.yaml?

A cerca 2022-06-24 post to Jared Wolff community forum:

• https://community.jaredwolff.com/d/330-not-finding-local-device-bindings

^ Element of Device Source

2023-07-19 stub section to encapsulate elements of device tree source language, e.g. nodes, node labels, phandles and similar

^ phandles

In device tree source `phandles` are akin to C pointers, though technically Ted reads that there are no such elements as pointers in DTS. DTS node properties can be of type `phandle`, `phandles` and `phandle-array` respectively (one at a time) and these node property types are discussed in detail in the Zephyr Project documentation at:

• https://docs.zephyrproject.org/latest/build/dts/phandles.html

^ Zephyr Device Tree API - A System Of Macros

• https://docs.zephyrproject.org/2.6.0/reference/devicetree/api.html

^ Device Tree Undocumented Features

0408 - Undocumented Device Tree syntax and features . . . Early Google search result when seeking info on `/delete-property/` token in Device Tree Source file, observed specifically in Jared Wolff (Circuit Dojo) nfed release tag v1.7.1, active_sleep sample app board overlay file.

• https://elinux.org/Device_Tree_Source_Undocumented