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

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Message-ID: <CAK7LNASBznyHmAwSRApOHw_6dyAXFuskmtKav65xFwKZdvNWJw@mail.gmail.com>
Date:   Tue, 28 Feb 2023 17:30:09 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     David Gow <davidgow@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        Jonathan Corbet <corbet@....net>,
        Sadiya Kazi <sadiyakazi@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation: kbuild: Add note about using (subst m,y)

On Tue, Feb 28, 2023 at 12:13 PM David Gow <davidgow@...gle.com> 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:
> https://lore.kernel.org/linux-kselftest/CAHk-=wgK07PQ_DBBbAbSJ41t__6de1xZ6q7RRu-JS=2SU0wqLQ@mail.gmail.com/
>
> This only documents the first one, which seems most common.
>
> Signed-off-by: David Gow <davidgow@...gle.com>
> ---
>  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/

and

obj-$(CONFIG_KUNIT_HOOK)   += hook.o







Bikeshed:

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
> --
> 2.39.2.722.g9855ee24e9-goog
>


--
Best Regards
Masahiro Yamada