linux-kernel - Re: [PATCH] kbuild: clarify the difference between obj-y and obj-m w.r.t. descending

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Date:   Thu, 19 Dec 2019 17:49:15 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Johan Hovold <johan@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: clarify the difference between obj-y and obj-m
 w.r.t. descending

On Thu, Dec 19, 2019 at 08:51:00PM +0900, Masahiro Yamada wrote:
> 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@...nel.org>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

Looks good! Thanks for fixing this.

Reviewed-by: Johan Hovold <johan@...nel.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