[PATCH] kbuild: use ?= to assign CROSS_COMPILE by arch-Makefile

[Masahiro Yamada]

On Mon, Apr 12, 2021 at 4:44 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> Hi Yamada-san,
>
> On Sun, Apr 11, 2021 at 3:56 PM Masahiro Yamada <masahiroy at kernel.org> wrote:
> > Use ?= operator to let arch/*/Makefile to assign CROSS_COMPILE only
> > when CROSS_COMPILE is undefined.
> >
> > This allows arch-Makefiles to drop the ifeq ($(CROSS_COMPILE),)
> > conditional.
> >
> > This slightly changes the behavior; the arch-Makefile previously
> > overrode CROSS_COMPILE when CROSS_COMPILE has already been made empty
> > via an environment variable as in 'export CROSS_COMPILE='.
> >
> > With this commit, arch-Makefle will respect the user's environment
> > set-up, which seems to be a more correct behavior.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy at kernel.org>
>
> Thanks for your patch!
>
> > ---
> >
> >  arch/arc/Makefile    | 4 +---
> >  arch/h8300/Makefile  | 4 +---
> >  arch/m68k/Makefile   | 4 +---
> >  arch/mips/Makefile   | 4 +---
> >  arch/parisc/Makefile | 6 ++----
> >  arch/sh/Makefile     | 4 +---
>
> What about arch/xtensa/Makefile?
>
> > --- a/arch/m68k/Makefile
> > +++ b/arch/m68k/Makefile
> > @@ -17,10 +17,8 @@
> >  KBUILD_DEFCONFIG := multi_defconfig
> >
> >  ifneq ($(SUBARCH),$(ARCH))
> > -       ifeq ($(CROSS_COMPILE),)
> > -               CROSS_COMPILE := $(call cc-cross-prefix, \
> > +       CROSS_COMPILE ?= $(call cc-cross-prefix, \
> >                         m68k-linux-gnu- m68k-linux- m68k-unknown-linux-gnu-)
> > -       endif
> >  endif
>
> This does not seem to work as expected: my standard build scripts
> (using "make ARCH=m68k") no longer pick up the cross-compiler,
> but fall back to the native compiler, thus breaking the build.


Agh, sorry, this patch does not work
because the top Makefile exports CROSS_COMPILE.

export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS
CROSS_COMPILE LD CC



Removing CROSS_COMPILE from that makes ?= work,
but it would break other parts.


Please ignore this patch.




> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-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



-- 
Best Regards
Masahiro Yamada