Introduction

Whenever we go to connect, we bring a bunch of goodies, boards, T-Shirts and of course some interesting works also. So, this time I got a Zephyr issue assigned to me. This is about adapting 96Boards board support to Zephyr’s default board configuration guidelines. This blog will provide information about Zephyr’s configuration mechanism, default board configuration guidelines and how I adapted it for 96Boards.

Zephyr’s Configuration Mechanism

Like Linux kernel, Zephyr also adopted the Kconfig for configuring the whole RTOS in order to make it highly customizable. Kconfig system uses the Kconfig files for providing the configuration symbols, menu structure, default values and so on. In Zephyr, Kconfig is used in conjunction with Devicetree files. If you are coming from Linux background (ARM, PowerPC…), it may sound little strange. In Linux, devicetree provides the runtime configuration to the kernel. But Zephyr restricts its usage to build time only.

In a nutshell, the configuration of Zephyr happens in below order:

1. Kconfig
2. Board devicetree
3. Common devicetree overlays
4. Device specific devicetree overlays

More information about Kconfig’s structure can be found in Zephyr docs.

Default Board Configuration Guidelines

Kconfig definitions of a particular board is provided in 3 different ways:

1. Kconfig.defconfig
2. Kconfig.board
3. board_defconfig

Kconfig.defconfig

This Kconfig file provides configuration values for invisible Kconfig symbols. Invisible option means, it is not configurable by the user and it has no prompt. Visible in the sense that this option can be configured from the menuconfig interface and has a prompt.

Kconfig.board

This Kconfig option provides board name and the SoC it depends on. For instance, below is the Kconfig.board content of 96Boards Carbon board.

# Kconfig - 96Boards Carbon STMF401RE board configuration
#
# Copyright (c) 2016 Linaro Limited.
#
# SPDX-License-Identifier: Apache-2.0
#

config BOARD_96B_CARBON
        bool "96Boards Carbon (STM32F401)"
        depends on SOC_STM32F401XE

board_defconfig

This file implements the .config syntax and provides values for visible Kconfig symbols default for a board.

In general, a SoC will expose all available interfaces and it will be limited by the interfaces provided by the board. So, care should be taken to initialize and expose only available interfaces on the board. This will be a huge performance factor for an RTOS like Zephyr since exposing all unavailable interfaces will waste memory and time.

That’s why Zephyr has provided the default board configuration guidelines for all boards and it makes much sense to adapt to it.

Adopting 96Boards to Guidelines

First an issue was created for tracking the whole progress of the guideline adoption work. Then sub-issues were created for tracking the individual board family work. For 96Boards, issue #9233 was created and assigned to me at Linaro Connect YVR18 by Erwan. This issue requires us to adopt the following supported 96Boards:

1. 96Boards Argonkey
2. 96Boards Carbon
3. 96Boards Carbon nRF51
4. 96Boards Neonkey
5. 96Boards Nitrogen

So I submitted a Pull Request to Zephyr GitHub repository and it got accepted after the review. There isn’t much work involved in this PR since all of our boards were in good shape. I just enabled the peripherals which were missing, disabled the ones which were not exposed and updated the board documentation to reflect these changes.

Conclusion

Finally, all of our 96Boards were adopted to the Zephyr’s default board configuration guidelines and the board support looks much cleaner now. This will only make the user experience better with 96Boards and we are hoping that the community will benefit from this.