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doc/kconfig: add information on default configurations

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Leandro Lanzieri 2021-12-10 16:09:25 +01:00 committed by MrKevinWeiss
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doc/doxygen/src/kconfig/kconfig.md
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@ -63,7 +63,10 @@ files. Two files will be sources of configuration during the generation of the
final header file: `app.config` and `user.config`, which should be placed
inside the application's folder. `app.config` sets default configuration
values for the particular application, the user can override them by setting
them in `user.config`.
them in `user.config`. Additionally, further `.config` files can be added to
the variable `KCONFIG_ADD_CONFIG`, which will be applied _after_ default CPU and
board configurations, `app.config` and `user.config`. This means that they will
have priority.
Let's say that the `SOCK_UTIL_SCHEME_MAXLEN` symbol in `sock_util` module needs
to be configured. The `user.config` file could look like:
@ -458,6 +461,34 @@ config BOARD_SAMR21_XPRO
select HAS_RIOTBOOT
```
### Default configurations
Boards, common board directories, CPUs and common CPU directories may need to
override default configuration values. Visible configuration symbols are
configurable by the user and show on the menuconfig interface. `.config` files
are used to set their values. To allow multiple sources of `.config` files,
there are two Makefile variables developers should use: `KCONFIG_CPU_CONFIG` for
sources added by the CPU or common CPU directories, and `KCONFIG_BOARD_CONFIG`
for sources added by the board or common board directories. This ensures the
correct priority of the configurations.
Currently the `Makefile.features` infrastructure is used to populate the
configuration sources. As the order in which `.config` files are merged matters,
configuration sources should be ordered from more generic to more specific.
Because board's `Makefile.features` is included before CPU's `Makefile.features`
it is important to utilize two different lists of configuration sources. For
instance, if `cpu/cortexm_common` adds its configuration, `cpu/stm32` should add
its configuration after it, and `boards/stm32f769i-disco` after it.
```Makefile
include $(RIOTCPU)/cortexm_common/Makefile.features
# Add stm32 configs after including cortexm_common so stm32 takes precedence
ifeq (1, $(TEST_KCONFIG))
KCONFIG_CPU_CONFIG += $(RIOTCPU)/stm32/stm32.config
endif
```
## Summary of reserved Kconfig prefixes
The following symbol prefixes have been assigned particular semantics and are
reserved for the cases described below: