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

[Johan Hovold <johan@kernel.org>]

On Thu, Dec 19, 2019 at 05:55:54PM +0900, Masahiro Yamada wrote:
> On Thu, Dec 19, 2019 at 5:18 PM Johan Hovold <johan@kernel.org> wrote:
> >
> > On Thu, Dec 19, 2019 at 11:00:15AM +0900, Masahiro Yamada wrote:
> > > Hi.
> > >
> > > On Thu, Dec 19, 2019 at 2:23 AM Johan Hovold <johan@kernel.org> wrote:
> >
> > > > 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?
> >
> > > 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.
> >
> > Well, any objects in serdev/ specified in obj-y would currently be built
> > but never included in either built-in.a or any module.
> 
> Right.
> 
> > > I think it is better to make the document
> > > clarify this.
> >
> > Yeah, I don't have a use case for this, but the documentation would need
> > to be updated to not have more people look into this.
> >
> > And at least this behaviour allowed us to catch this bug, but only
> > because we had dependent modules that failed to build. I see now that
> > you posted a patch adding a general warning about obj-y under obj-m last
> > fall, but that was apparently never merged.
> 
> This one?
> 
> https://lore.kernel.org/patchwork/patch/1126959/

Yep, that's the one I meant.
 
> This patch detected several Makefile/Kconfig bugs,
> and it is good.
> 
> But, it also turned out to cause false positive warnings
> on some architectures.
> I thought it was not so easy to fix it.
> So, I decided to postpone it.

Ok.

> > But when was this behaviour changed? And was that done on purpose?
> 
> No. Not changed at all.
> 
> In my understanding, the current behavior has been kept
> from the beginning.

Ok, thanks for confirming. The contradicting documentation I refer to
above has been there since before git at least however.

Here's a random example of someone being bit by this five years ago:

	https://www.spinics.net/lists/linux-kbuild/msg10887.html

Would you mind taking a stab at updating the docs?

Johan