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

[Johan Hovold]

On Wed, Dec 18, 2019 at 09:38:42AM +0100, Uwe Kleine-KÃnig 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 âyâ (built-in) or âmâ
	(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