Kbuild descends into a directory by either 'y' or 'm', but there is an important difference. Kbuild combines the built-in objects into built-in.a in each directory. The built-in.a in the directory visited by obj-y is merged into the built-in.a in the parent directory. This merge happens recursively when Kbuild is ascending back towards the top directory, so built-in objects are linked into vmlinux eventually. This works properly only when the Makefile that specifies obj-y is reachable by the chain of obj-y. On the other hand, Kbuild does not take built-in.a from the directory visited by obj-m. This it, all the objects in that directory are supposed to be modular. If Kbuild descends into a directory by obj-m, but the Makefile in the sub-directory specifies obj-y, those objects are just left orphan. The current statement "Kbuild only uses this information to decide that it needs to visit the directory" is misleading. Clarify the difference. Reported-by: Johan Hovold <johan@xxxxxxxxxx> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx> --- Documentation/kbuild/makefiles.rst | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst index b9b50553bfc5..d7e6534a8505 100644 --- a/Documentation/kbuild/makefiles.rst +++ b/Documentation/kbuild/makefiles.rst @@ -297,9 +297,19 @@ more details, with real examples. If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) the corresponding obj- variable will be set, and kbuild will descend down in the ext2 directory. - Kbuild only uses this information to decide that it needs to visit - the directory, it is the Makefile in the subdirectory that - specifies what is modular and what is built-in. + + Kbuild uses this information not only to decide that it needs to visit + the directory, but also to decide whether or not to link objects from + the directory into vmlinux. + + When Kbuild descends into the directory with 'y', all built-in objects + from that directory are combined into the built-in.a, which will be + eventually linked into vmlinux. + + When Kbuild descends into the directory with 'm', in contrast, nothing + from that directory will be linked into vmlinux. If the Makefile in + 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. It is good practice to use a `CONFIG_` variable when assigning directory names. This allows kbuild to totally skip the directory if the -- 2.17.1
