linux-kernel - Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM

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Message-ID: <CAMuHMdVzNFqAdxK+QTp7ub7LyhDL_3GbVMoAah_s3nGuJ5JN_Q@mail.gmail.com>
Date:   Tue, 27 Jul 2021 09:07:35 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Christoph Hellwig <hch@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for
 LLVM=1 LLVM_IAS=1

Hi Eric,

On Mon, Jul 26, 2021 at 10:27 PM Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Masahiro Yamada <masahiroy@...nel.org> writes:
> > On Wed, Jul 21, 2021 at 4:58 AM Arnd Bergmann <arnd@...nel.org> wrote:
> >> On Tue, Jul 20, 2021 at 7:43 PM Linus Torvalds
> >> <torvalds@...ux-foundation.org> wrote:
> >> > On Tue, Jul 20, 2021 at 1:05 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >> >
> >> > We do most of the other heavy lifting in this area in Kconfig anyway,
> >> > why not add that compiler choice?
> >> >
> >> > Obviously it would be gated by the tests to see which compilers are
> >> > _installed_ (and that they are valid versions), so that it doesn't ask
> >> > stupid things ("do you want gcc or clang" when only one of them is
> >> > installed and/or viable).
> >>
> >> I don't see a good way of making Kconfig options both select the
> >> compiler and defining variables based on the compiler, since that
> >> would mean teaching Kconfig about re-evaluating all compiler
> >> dependent settings whenever the first option changes.
> >>
> >> I do have another idea that I think would work though.
> >>
> >> > Hmm? So then any "LLVM=1" thing would be about the "make config"
> >> > stage, not the actual build stage.
> >> >
> >> > (It has annoyed me for years that if you want to cross-compile, you
> >> > first have to do "make ARCH=xyz config" and then remember to do "make
> >> > ARCH=xyz" for the build too, but I cross-compile so seldom that I've
> >> > never really cared).
> >>
> >> The best thing that I have come up with is a pre-configure step, where
> >> an object tree gets seeded with a makefile fragment that gets included
> >> for any 'make' invocation. This would set 'ARCH=', 'CROSS_COMPILE',
> >> 'CC=' and possibly any other option that gets passed to 'make' as
> >> a variable and has to exist before calling 'make *config'.
> >
> >
> > There is no need to add a hook to include such makefile fragment(s).
> >
> > Quite opposite, you can put your Makefile (in a different filename)
> > that includes the top Makefile.
> >
> >
> > I think this is what people are already doing:
> >
> >
> > GNU Make looks for 'GNUmakefile', 'makefile', and 'Makefile'
> > in this order.
> >
> >
> > So, you can put 'GNUmakefile' with your favorite setups.
> >
> >
> > $ cat GNUmakefile
> > ARCH=arm64
> > CROSS_COMPILE=aarch64-linux-gnu-
> > CC=clang
> > include Makefile
>
> Very weird.
>
> I just tested this and it does not work.
> I did this:
>
> $ cat GNUmakefile
> ARCH = alpha
> CROSS_COMPILE = $(arch-prefix alpha)
> include Makefile
>
> In one of my build directories and the main makefile simply does not see
> the value of ARCH or CROSS_COMPILE I set.  I have confirmed that my
> GNUmakefile is being read, because everything breaks if I remove the
> include line.
>
> Does anyone have any ideas?
>
> Something so we don't have to specify all of these variables on the make
> command line would be nice.

Just including the main Makefile does not work.
That's why I went with the more convoluted solution in
https://lore.kernel.org/linux-kbuild/CAMuHMdXJBqrpzaSNDJgic14ESiHV6cCcb_5E-st6iniXdmm9_g@mail.gmail.com/

Please try the attached, which combines everything above in a single
file, and which works for me. Note that "$(arch-prefix alpha)" didn't
work for me (it resolved to "gcc"?), so I used "alpha-linux-gnu-"
instead.

Good luck!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Download attachment "GNUmakefile" of type "application/octet-stream" (257 bytes)