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

[Geert Uytterhoeven]

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.

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