linux-kernel - Re: kbuild obj-m directory descend (was: Re: [PATCH] serdev: fix builds with CONFIG_SERIAL_DEV

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Message-ID: <CAK7LNATztV-a3maL+vqQhbVsLBD_dsy+wbOZQ5ofQfbZQGGuLA@mail.gmail.com>
Date:   Thu, 19 Dec 2019 11:00:15 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Rob Herring <robh@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sascha Hauer <kernel@...gutronix.de>,
        kbuild test robot <lkp@...el.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: kbuild obj-m directory descend (was: Re: [PATCH] serdev: fix
 builds with CONFIG_SERIAL_DEV_BUS=m)

Hi.

On Thu, Dec 19, 2019 at 2:23 AM Johan Hovold <johan@...nel.org> wrote:
>
> On Wed, Dec 18, 2019 at 09:38:42AM +0100, Uwe Kleine-König wrote:
> > Commit 54edb425346a ("serdev: simplify Makefile") broke builds with
> > serdev configured as module. I don't understand it completely yet, but
> > it seems that
> >
> >       obj-$(CONFIG_SERIAL_DEV_BUS) += serdev/
> >
> > in drivers/tty/Makefile with CONFIG_SERIAL_DEV_BUS=m doesn't result in
> > code that is added using obj-y in drivers/tty/serdev/Makefile being
> > compiled. So instead of dropping $(CONFIG_SERIAL_DEV_BUS) in serdev's
> > Makefile, drop it in drivers/tty/Makefile.
>
> I was just looking into this when you commented on this off-list.
>
> The offending patch is broken since it effectively makes
> CONFIG_SERIAL_DEV_BUS bool (built-in or disabled), but for some reason
> those symbols do not end up in vmlinux (despite being compiled) when you
> add a built-in object goal under a directory that was entered using
> obj-m.
>
> That seems like a bug to me and contradicts the kbuild documentation
> (3.6):
>
>         Example:
>
>         #fs/Makefile
>         obj-$(CONFIG_EXT2_FS) += ext2/
>
>         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.
>
> I tried adding other targets to obj-y directly and they are also are not
> included, seemingly since the directory was entered using obj-m.
>
> Masahiro or Michal, can you shed some light?
>
> Johan


I saw similar questions in ML in the past.
Yes, this is how Kbuild works;
Kbuild only links objects in Makefiles visited by obj-y.

If you use
    obj-m += serdev/
all objects in serdev/ are considered as modular.

I think it is better to make the document
clarify this.


-- 
Best Regards
Masahiro Yamada