Re: [PATCH] Documentation: kbuild: Add note about using (subst m,y)

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On Tue, Feb 28, 2023 at 12:13 PM David Gow <davidgow@xxxxxxxxxx> wrote:
> THe kbuild documentation already notes that, where there's an obj-y
> target in a subdirectory, it will be orphaned if the subdirectory uses
> obj-m. Suggest a way of forcing the directory to obj-y when it's based
> on a config option which could be 'm'.
> Linus made several suggestions as to how this is usually done here:
> This only documents the first one, which seems most common.
> Signed-off-by: David Gow <davidgow@xxxxxxxxxx>
> ---
>  Documentation/kbuild/makefiles.rst | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
> index e67eb261c9b0..d9eb24799f52 100644
> --- a/Documentation/kbuild/makefiles.rst
> +++ b/Documentation/kbuild/makefiles.rst
> @@ -254,6 +254,17 @@ 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.
> +This means that, if a directory contains both obj-y and obj-m targets,
> +it should be added with obj-y. If this directory nevertheless should
> +only be built when a config option is enabled (typically to support
> +a module which requires one or two files to nevertheless be built-in),
> +'subst m,y' can be used to ensure obj-y is used.
> +
> +Example::
> +
> +  #drivers/Makefile
> +  obj-$(subst m,y,$(CONFIG_HYPERV)) += hv/
> +

I think many subsystems simply do

obj-y  += hv/

I do not think we need to advertise hyperv's way
since it does not look very pretty.

Mostly, it looks like this:

obj-y  += kunit/


obj-$(CONFIG_KUNIT_HOOK)   += hook.o


I think Linus' suggestion is OK, but
the BSD style seems less ugly,
of course, that is just a matter of style.

obj-$(CONFIG_HYPERV:m=y)   += kunit/

>  Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
>  descending into subdirectories. It is a good fit when you know they
>  do not contain kernel-space objects at all. A typical usage is to let
> --

Best Regards
Masahiro Yamada

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