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| author | Masahiro Yamada <masahiroy@kernel.org> | 2019-12-19 20:51:00 +0900 |
|---|---|---|
| committer | Masahiro Yamada <masahiroy@kernel.org> | 2019-12-22 00:25:35 +0900 |
| commit | 28f94a44298c99c0db85539874b62f21d94fcaa7 (patch) | |
| tree | cd6fbf7499a88942672ffdcc558138f2da223510 /Documentation/kbuild | |
| parent | 8f268881d7d278047b00eed54bbb9288dbd6ab23 (diff) | |
kbuild: clarify the difference between obj-y and obj-m w.r.t. descending
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, then built-in
objects are linked into vmlinux eventually. This works properly only
when the Makefile specifying 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@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Diffstat (limited to 'Documentation/kbuild')
| -rw-r--r-- | Documentation/kbuild/makefiles.rst | 16 |
1 files 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 |