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doc/kconfig.md : Added Appendix D

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Added a new section, Appendix D, to describe a few key aspects
while exposing a macro to Kconfig.

Co-authored-by: Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>
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Akshai M 2020-05-27 22:49:49 +05:30
parent a770bf0f89
commit 879263c1d2
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doc/doxygen/src/kconfig/kconfig.md
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@ -375,6 +375,23 @@ menuconfig, the file will not be re-built anymore, and any changes by manually
editing the source files will have no effect. To go back to manual edition
a `make clean` has to be issued in the application directory.
## Appendix D: A few key aspects while exposing a macro to Kconfig {#kconfig-appendix-d}
A macro that holds a 0 or 1 is modelled in Kconfig as a `bool` symbol. References to this macro
can then make use of IS_ACTIVE macro from kernel_defines.h with C conditionals
for conditional compilation.
[FXOS8700 driver exposure to Kconfig](https://github.com/RIOT-OS/RIOT/pull/13914)
can be considered as an example. If the macro is defined as `TRUE` by default,
a new symbol gets introduced to invert the semantics. The recommended
practice is to add a new symbol and expose it to Kconfig while the old one is
tagged to be deprecated. The process is documented in this
[commit](https://github.com/RIOT-OS/RIOT/pull/13129/commits/c7b6dc587cf20f3177abe0417a408b6ab90d0ff8)
There may be cases where a macro is expected to hold only specific values, e.g.
'GNRC_IPV6_MSG_QUEUE_SIZE' expressed as the power of two. These may be modelled
in such a way that a new macro is introduced to hold the restricted figures
while operators are added to arrive at the desired value. The process is
documented in this [pull request.](https://github.com/RIOT-OS/RIOT/pull/14086)
# Useful references {#kconfig-useful-references}
- [Kconfig language specification](https://www.kernel.org/doc/html/latest/kbuild/kconfig-language.html)
- [Kconfig macro language specification](https://www.kernel.org/doc/html/latest/kbuild/kconfig-macro-language.html)