Add a base.conf to application configuration directory

[nrbrook]

Currently, application configuration directory supports prj.conf and prj_<FILE_SUFFIX>.conf. However, both may share some or most of the configuration. It's therefore desirable to have a shared configuration file for all FILE_SUFFIXs. I suggest this patch to cmake/modules/configuration_files.cmake:

 zephyr_get(CONF_FILE SYSBUILD LOCAL)
 if(NOT DEFINED CONF_FILE)
+  # Load base.conf first if it exists, for shared configuration
+  zephyr_file(CONF_FILES ${APPLICATION_CONFIG_DIR} KCONF CONF_FILE NAMES "base.conf" SUFFIX ${FILE_SUFFIX})
   zephyr_file(CONF_FILES ${APPLICATION_CONFIG_DIR} KCONF CONF_FILE NAMES "prj.conf" SUFFIX ${FILE_SUFFIX} REQUIRED)
   zephyr_file(CONF_FILES ${APPLICATION_CONFIG_DIR}/socs KCONF CONF_FILE QUALIFIERS SUFFIX ${FILE_SUFFIX})

In this way, different build configurations can share most of the configuration in base.conf and only customise as needed in the prj.conf files. Additionally, it still supports FILE_SUFFIX, so if there was a FILE_SUFFIX that didn't want to use the shared config, it could define its own empty base_<FILE_SUFFIX>.conf file

[henrikbrixandersen]

Not sure if this is acceptable or not, but I would consider naming it app.conf to match app.overlay if implemented.

[nrbrook]

My concern with that naming would be it isn't clear what the difference between app.conf and prj.conf is. Base.conf or perhaps shared.conf or common.conf confer the purpose of the file. I think prj.conf would be better named app.conf, but it's not...

[nordicjm]

Not acceptable, use a base prj.conf file with defaults for all then use EXTRA_CONF_FILE to specify a different file to include on top

[nordicjm]

However, I still believe it doesn't completely remove the need for a shared conf file, because Kconfig defaults is only for global application configuration, not per-board.

???

config SOME_CONFIG
        default y if BOARD_NRF52840DK_NRF52840

if BOARD_NRF52840DK

config OTHER_CONFIG
        default 183

source "some/other/Kconfig.nrf52840dk.file"

endif # BOARD_NRF52840DK

Lots of ways to do it

[nrbrook]

So now you have a Kconfig in the root of the project with board specific ifs, and board specific config directories. It's not a very clean solution is it?
A single line change in zephyr to support a shared base config seems better...

[henrikbrixandersen]

It doesn't have to be board-specific Kconfig that you if on. I would typically use feature-based Kconfig options defined in the app Kconfig file, and select those in the boards/*.conf files for the relevant boards. This provides a nice decoupling, I think.

[henrikbrixandersen]

By the way, using the app Kconfig file for setting defaults shared across multiple prj*.conf selected via FILE_SUFFIX is how the CANnectivity firmware does things - you may find some inspiration there: https://github.com/CANnectivity/cannectivity/blob/main/app/Kconfig

[nrbrook]

Thanks, and yes I already do this, although not as much as I could because some defaults weren't set due to setting in Kconfig after sourcing.
I still don't think it's a complete solution to the problem though.

[nrbrook]

@nordicjm pointed to the smp_svr sample, which evidences the issue – there is over 90% duplication between its prj.conf and prj_ram_load.conf files. The primary issue with that is the opportunity for shared configuration that should the same across configs to deviate when things change. It is difficult to ensure that the conf files remain consistent, especially when considering multiple conf files (using FILE_SUFFIX) and multiple boards.

Considering alternative solutions that already exist, suggested by @nordicjm:

EXTRA_CONF_FILE

One solution is to use prj.conf as the "base" config, and specify an additional "overlay" config with EXTRA_CONF_FILE.

My issues with this approach:

1. You cannot name the additional file prj_<FILE_SUFFIX>.conf if you are using FILE_SUFFIX as it would then be used instead of prj.conf. This means that you need to create your own naming convention, either with a different prefix, or with none. Either way might mean the additional conf files are not sorted next to prj.conf in file explorer.
2. It requires specifying the additional conf file explicitly, increasing build command complexity. If you supply FILE_SUFFIX but not EXTRA_CONF_FILE you may have an invalid build.
3. If you are also using FILE_SUFFIX for other configuration files then it defeats the purpose of the automatic file discovery that provides
4. One solution to work around issue 2 would be for the user to automate the discovery of an optional additional conf file in CMakeLists.txt. However, this is non-trivial, as APPLICATION_CONFIG_DIR is typically set with, for example, set(APPLICATION_CONFIG_DIR "${CMAKE_CURRENT_LIST_DIR}/configuration/\${NORMALIZED_BOARD_TARGET}") before find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE}), where NORMALIZED_BOARD_TARGET is substituted by Zephyr following board discovery. However, Zephyr also discovers the conf files during package load, giving no opportunity for the application CMake script to use the resolved path to discover an optional FILE_SUFFIX specific conf file to use as EXTRA_CONF_FILE. The solution to that is to only load the zephyr module that resolves the app config dir, then checking for a conf file, setting EXTRA_CONF_FILE, then loading zephyr normally. The same applies for FILE_SUFFIX when using sysbuild - it can't be used before a module is loaded. This makes it quite complicated to be a real solution. It is also a non-standard approach which requires modifying CMakeLists.txt

Kconfig defaults

Another solution is to specify board-specific configuration as defaults on the Kconfig options, and then override in prj_<FILE_SUFFIX>.conf as required. For example:

config SOME_CONFIG
        default y if BOARD_NRF52840DK_NRF52840

if BOARD_NRF52840DK

config OTHER_CONFIG
        default 183

source "some/other/Kconfig.nrf52840dk.file"

endif # BOARD_NRF52840DK

My issues with this approach:

1. You need to understand:
   1. Default sort ordering in Kconfig
   2. That you must source Kconfig.zephyr AFTER your defaults
   3. How to do board-specific conditionals
2. You need to know the board's Kconfig config name
3. It requires board specific config to be located in a file at the project root instead of in the board config directory, or it requires sourcing a Kconfig in the board config directory(s)
4. It is quite complex, requiring the if statement and the additional Kconfig syntax, compared to just setting variables in a conf file
5. It requires significantly more cognitive overhead compared to just putting settings in a shared conf file, especially for newcomers.

Alternative suggestions

After evaluating these proposed alternatives, I still believe my proposal here, or a similar one, would be beneficial.

Perhaps an improved approach would be to keep prj.conf as the "base config", and add an optional prj_ex.conf overlay which is automatically discovered and can use FILE_SUFFIX. For example,

• prj.conf - base config
• prj_ex.conf - overlay on prj.conf, used when no FILE_SUFFIX is provided (maybe also when prj_ex_<FILE_SUFFIX>.conf does not exist?)
• prj_ex_release.conf - when FILE_SUFFIX is "release", this file is overlayed on prj.conf.
• prj_other.conf - when FILE_SUFFIX is "other", this file will be used and prj.conf is not used.

The same could then be supported with sysbuild_ex.conf.

Alternative names could be prj_extra.conf, prj_overlay.conf, or perhaps prj.ex.conf/prj.overlay.conf etc to avoid any conflict with FILE_SUFFIX. The reason for keeping prj in the name is to ensure they are sorted next to prj.conf.

This

• Doesn't break any existing behaviour
• Is easy to understand and adopt
• Is additive to existing functionality
• Maintains the automatic discovery that FILE_SUFFIX provides