linux-kernel - Re: [PATCH] kbuild: Include external modules compile flags

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Date:   Wed, 5 Feb 2020 14:48:26 -0800
From:   Gregory Rose <gvrose8192@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dev@...nvswitch.org, dsahern@...il.com
Subject: Re: [PATCH] kbuild: Include external modules compile flags

On 2/1/2020 10:07 PM, Masahiro Yamada wrote:
> On Thu, Jan 30, 2020 at 3:09 AM Gregory Rose <gvrose8192@...il.com> wrote:
>>
>> 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@...il.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.
>
> I quickly tested it, and confirmed
> KCFLAGS works for external modules, too.
>
>
> Makefile.modfinal includes scripts/Makefile.lib
>
>
> So,  c_flags contains $(KCFLAGS)
>
>   c_flags -> KBUILD_CFLAGS -> KCFLAGS
>
>
Hi Masahiro,

I found a way to make this work on the openvswitch out of tree kernel 
module.  KCFLAGS
doesn't work because it is added at the end of the gcc command line and 
we need to
add a '-include <file>' directive so that the include file comes 
*before* all other include
files.  I fix this by inserting the '-include <file>' command line 
option to the beginning
of our own Kbuild NOSTDINC variable.

Thanks for your help in debugging this.

- Greg