Re: [PATCH 05/23] kbuild: doc: describe the -C option precisely for external module builds

[Nicolas Schier]

On Tue, Sep 17, 2024 at 11:16:33PM +0900, Masahiro Yamada wrote:
> Building external modules is typically done using this command:
> 
>   $ make -C <KERNEL_DIR> M=<EXTMOD_DIR>
> 
> Here, <KERNEL_DIR> refers to the output directory where the kernel was
> built, not the kernel source directory.
> 
> When the kernel is built in-tree, there is no ambiguity, as the output
> directory and the source directory are the same.
> 
> If the kernel was built in a separate build directory, <KERNEL_DIR>
> should be the kernel output directory. Otherwise, Kbuild cannot locate
> necessary build artifacts such as the .config file, etc. This has been
> the method for building external modules based on the kernel compiled in
> a separate directory for over 20 years. [1]
> 
> If you pass the kernel source directory to the -C option, you must also
> specify the kernel build directory using the O= option. This approach
> works as well, though it results in a slightly longer command:
> 
>   $ make -C <KERNEL_SOURCE_DIR> O=<KERNEL_BUILD_DIR> M=<EXTMOD_DIR>
> 
> Some people mistakenly believe that O= should point to a separate output
> directory for external modules when used together with M=. This commit
> adds more clarification to Documentation/kbuild/kbuild.rst.
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=e321b2ec2eb2993b3d0116e5163c78ad923e3c54
> 
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> ---
> 
>  Documentation/kbuild/kbuild.rst  | 5 +++++
>  Documentation/kbuild/modules.rst | 9 ++++++---
>  2 files changed, 11 insertions(+), 3 deletions(-)

Reviewed-by: Nicolas Schier <n.schier@xxxxxx>