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Subject[PATCH] kbuild: clarify the difference between obj-y and obj-m w.r.t. descending
Date
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@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

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
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