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

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Message-ID: <aowpzz4rbedjkwkl7b4a2f5pf2p5cgsu6knsgxnayckhbumxf3@aznrm7oliydb>
Date: Tue, 10 Sep 2024 09:00:29 -0500
From: Lucas De Marchi <lucas.demarchi@...el.com>
To: Masahiro Yamada <masahiroy@...nel.org>
CC: <3090101217@....edu.cn>, Michal Marek <michal.lkml@...kovi.net>, "Nick
 Desaulniers" <ndesaulniers@...gle.com>, Linux Kbuild mailing list
	<linux-kbuild@...r.kernel.org>, Linux Kernel Mailing List
	<linux-kernel@...r.kernel.org>, Jing Leng <jleng@...arella.com>
Subject: External modules with O=... (was: Re: [PATCH] kbuild: Fix include
 path in scripts/Makefile.modpost)

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@....edu.cn> wrote:
>>
>> From: Jing Leng <jleng@...arella.com>
>>
>> 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@...arella.com>
>> ---
>>  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