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kconfig.h: explain IS_MODULE(), IS_ENABLED()

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Extend IS_MODULE() and IS_ENABLED comments to explain why one might use
"#if IS_ENABLED(CONFIG_FOO)" instead of "#ifdef CONFIG_FOO".

To wit, "#ifdef CONFIG_FOO" is true only for CONFIG_FOO=y, while
"#if IS_ENABLED(CONFIG_FOO)" is true for both CONFIG_FOO=y and
CONFIG_FOO=m.

This is because "CONFIG_FOO=m" in .config does not result in "CONFIG_FOO"
being defined.  The actual definitions are in autoconf.h, where:

  CONFIG_FOO=y   results in   #define CONFIG_FOO 1
  CONFIG_FOO=m   results in   #define CONFIG_FOO_MODULE 1

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
This commit is contained in:
Bjorn Helgaas 2021-06-01 16:31:43 -05:00 • committed by Masahiro Yamada
commit c7c90e121e
1 changed files with 4 additions and 2 deletions
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