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kbuild: doc: document subdir-y syntax 

There is no explanation about subdir-y.

Let's document it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
This commit is contained in:
Masahiro Yamada 2020-11-28 20:51:08 +09:00
parent d0e628cd81
commit c0ea806f87
1 changed files with 15 additions and 0 deletions
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15
Documentation/kbuild/makefiles.rst
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@ -319,6 +319,21 @@ more details, with real examples.
that directory specifies obj-y, those objects will be left orphan.
It is very likely a bug of the Makefile or of dependencies in Kconfig.
Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
descending into subdirectories. It is a good fit when you know they
do not contain kernel-space objects at all. A typical usage is to let
Kbuild descend into subdirectories to build tools.
Examples::
# scripts/Makefile
subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
subdir-$(CONFIG_MODVERSIONS) += genksyms
subdir-$(CONFIG_SECURITY_SELINUX) += selinux
Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
syntax is always used for directories.
It is good practice to use a `CONFIG_` variable when assigning directory
names. This allows kbuild to totally skip the directory if the
corresponding `CONFIG_` option is neither 'y' nor 'm'.