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Kernel compilation: make oldconfig

make oldconfig takes the .config and runs it through the rules of the Kconfig files and produces a .config file which is consistent with the Kconfig rules.
If there are config values which are missing, make oldconfig will ask for them.

Remove «inexisting» CONFIG values 

$ grep CONFIG_DOES_NOT_EXIST .config
$ ./scripts/config --enable CONFIG_DOES_NOT_EXIST
$ grep CONFIG_DOES_NOT_EXIST .config
CONFIG_DOES_NOT_EXIST=y
$ make oldconfig
#
# configuration written to .config
#
$ grep CONFIG_DOES_NOT_EXIST .config

See also

Modification of CONFIG symbols
make olddefconfig is similar to make oldconfig, but sets new config symbols to their default values without prompting.

Links

How to understand 'make oldconfig'?

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