External modules with O=... (was: Re: [PATCH] kbuild: Fix include path in scripts/Makefile.modpost)

[Lucas De Marchi]

Hi, I was pointed to this thread since I'm trying something similar
in kmod's testsuite. See below.

On Tue, May 24, 2022 at 02:52:45AM GMT, Masahiro Yamada wrote:
> On Tue, May 17, 2022 at 7:51 PM <3090101217@xxxxxxxxxx> wrote:
> > From: Jing Leng <jleng@xxxxxxxxxxxxx>
> >
> > When building an external module, if users don't need to separate the
> > compilation output and source code, they run the following command:
> > "make -C $(LINUX_SRC_DIR) M=$(PWD)". At this point, "$(KBUILD_EXTMOD)"
> > and "$(src)" are the same.
> >
> > If they need to separate them, they run "make -C $(KERNEL_SRC_DIR)
> > O=$(KERNEL_OUT_DIR) M=$(OUT_DIR) src=$(PWD)". Before running the
> > command, they need to copy "Kbuild" or "Makefile" to "$(OUT_DIR)" to
> > prevent compilation failure.
> >
> > So the kernel should change the included path to avoid the copy operation.
> >
> > Signed-off-by: Jing Leng <jleng@xxxxxxxxxxxxx>
> > ---
> >  scripts/Makefile.modpost | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
> > index 48585c4d04ad..0273bf7375e2 100644
> > --- a/scripts/Makefile.modpost
> > +++ b/scripts/Makefile.modpost
> > @@ -87,8 +87,7 @@ obj := $(KBUILD_EXTMOD)
> >  src := $(obj)
> >
> >  # Include the module's Makefile to find KBUILD_EXTRA_SYMBOLS
> > -include $(if $(wildcard $(KBUILD_EXTMOD)/Kbuild), \
> > -             $(KBUILD_EXTMOD)/Kbuild, $(KBUILD_EXTMOD)/Makefile)
> > +include $(if $(wildcard $(src)/Kbuild), $(src)/Kbuild, $(src)/Makefile)
> >
> >  # modpost option for external modules
> >  MODPOST += -e
> > --
> > 2.17.1
>
>
> I do not think "M=$(OUT_DIR) src=$(PWD)" is the official way,
> but this patch is a clean up.

I tried what is in this patch and also tried to find an official way in
the docs.

In kmod's testsuite we build dummy kernel modules to exercise the API.
https://git.kernel.org/pub/scm/utils/kernel/kmod/kmod.git/tree/testsuite/module-playground

This works:
	make -C /lib/modules/$(uname -r)/build M=$PWD

This doesn't:
	make -C /lib/modules/$(uname -r)/build M=$PWD O=/tmp/kmod_test_modules

I also tried the variants above with setting src, but all of them give
me errors - I used 6.10 and 6.11-rc7 for these tests.

Is there a way to do this?

thanks
Lucas De Marchi

> Applied to linux-kbuild. Thanks.
>
>
> --
> Best Regards
> Masahiro Yamada