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>

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index 3f1697c..d36768c 100644 (file)
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -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'.