Contribution description

Besides the autoconf.h header file, which contains the configurations
synbols in the form of C macros, Kconfig generates a .config file. This file has Makefile syntax and contains the same configuration symbols in the form of variables.

This is useful to check if a certain option is being set. E.g. to decide if a CFLAG should be used to configure something:

ifndef CONFIG_FOO
  CFLAGS += -DCONFIG_FOO=BAR
endif

Note on how this is working

When make all is called, make starts reading all Makefiles. At some point it finds the -include directive for the merged.config file. If the file is there (i.e. it's not a clean build) it will jump to the file and read, if not, it will continue reading kconfig.mk. Once make has read all the files it will try to remake all the included files. For that, it will look for implicit and explicit rules to remake the files. In the particular case of merged.config it will find this:

If the file is updated (either because it was not there or because of a change on a source file) make will start from scratch reading all the (now updated) makefiles. This way, the user can check for Kconfig symbols in the application's Makefile (after including Makefile.include).

The problem with clean

In the case make clean all is called merged.config is not included.

Why? The reason is that it depends on the generated folder being created, which in turn depends on $(CLEAN) (which is actually clean if it was called). So if we always included the file, make would try to remake it, it would run clean, and create it again. The it would start over and try to remake it, it would run clean again, updating the file and re-triggering this mechanism forever.

Not including the file on clean has the side effect of not being able to include Kconfig symbols on build, when running make clean all. So far, if users want to use Kconfig they will have to call make clean && make all. Any proposal to solve this is appreciated :-)

Testing procedure

You can add these messages to the tests/kconfig application:

diff --git a/tests/kconfig/Makefile b/tests/kconfig/Makefile
index e386d876b..eb8fcdab8 100644
--- a/tests/kconfig/Makefile
+++ b/tests/kconfig/Makefile
@@ -1,2 +1,8 @@
 include ../Makefile.tests_common
 include $(RIOTBASE)/Makefile.include
+
+ifdef CONFIG_APP_MSG_2
+  $(info "MSG 2 is active")
+else
+  $(info "MSG 2 is not active")
+endif

app.config sets the MSG 2 as active by default.

• Run make clean && make all, you should see that first MSG 2 is not active and then make starts over with MSG 2 is active.
• Run make all again, you should only see MSG 2 is active once.
• Run make menuconfig and disable the message. You should see MSG 2 is not active (once).
• Run make clean all, as the file is not included you will see MSG 2 is not active.

Issues/PRs references

None