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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>
alistair/sunxi64-5.5-dsi
Masahiro Yamada 2019-12-19 20:51:00 +09:00
parent 8f268881d7
commit 28f94a4429
1 changed files with 13 additions and 3 deletions
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16
Documentation/kbuild/makefiles.rst
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@ -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