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SubjectRe: kbuild obj-m directory descend (was: Re: [PATCH] serdev: fix builds with CONFIG_SERIAL_DEV_BUS=m)

On Thu, Dec 19, 2019 at 2:23 AM Johan Hovold <> 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

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