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Subject: Re: [PATCH] kbuild: Include external modules compile flags
From:   Gregory Rose <gvrose8192@gmail.com>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dev@openvswitch.org, dsahern@gmail.com
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Date:   Wed, 29 Jan 2020 10:09:05 -0800
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On 1/28/2020 7:37 AM, Gregory Rose wrote:
> On 1/27/2020 7:35 PM, Masahiro Yamada wrote:
>> On Tue, Jan 28, 2020 at 6:50 AM Greg Rose <gvrose8192@gmail.com> wrote:
>>> Since this commit:
>>> 'commit 9b9a3f20cbe0 ("kbuild: split final module linking out into 
>>> Makefile.modfinal")'
>>> at least one out-of-tree external kernel module build fails
>>> during the modfinal make phase because Makefile.modfinal does
>>> not include the ccflags-y variable from the exernal module's Kbuild.
>> ccflags-y is passed only for compiling C files in that directory.
>>
>> It is not used for compiling *.mod.c
>> This is true for both in-kernel and external modules.
>>
>> So, ccflags-y is not a good choice
>> for passing such flags that should be globally effective.
>>
>>
>> Maybe, KCFLAGS should work.
>>
>>
>> module:
>>         $(MAKE) KCFLAGS=...  M=$(PWD) -C /lib/modules/$(uname 
>> -r)/build modules
>>

Hi Masahiro,

I'm unable to get that to work.  KCFLAGS does not seem to be used in 
Makefile.modfinal.

[snip]
>>> --- a/scripts/Makefile.modfinal
>>> +++ b/scripts/Makefile.modfinal
>>> @@ -21,6 +21,10 @@ __modfinal: $(modules)
>>>   modname = $(notdir $(@:.mod.o=))
>>>   part-of-module = y
>>>
>>> +# Include the module's Makefile to find KBUILD_EXTRA_SYMBOLS
>>> +include $(if $(wildcard $(KBUILD_EXTMOD)/Kbuild), \
>>> +             $(KBUILD_EXTMOD)/Kbuild)
>>> +

I continue to wonder why this it is so bad to include the external 
module's Kbuild.
It used to be included in Makefile.modpost and did no harm, and in fact 
was what
made our external build work at all in the past.  Without the ability to 
define our
local kernel module build environment during the modfinal make I see no 
way forward.

That said, I'm no expert on the Linux kernel Makefile 
interdependencies.  If you
have some other idea we could try I'm game.

Thanks,

- Greg