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

[Masahiro Yamada]

On Mon, Apr 12, 2021 at 5:15 PM Masahiro Yamada <masahiroy at kernel.org> wrote:
>
> 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
>


The following will make this patch work, but probably it is better to
not do this...




diff --git a/Makefile b/Makefile
index cc77fd45ca64..26bf482f0d88 100644
--- a/Makefile
+++ b/Makefile
@@ -506,7 +506,7 @@ KBUILD_LDFLAGS_MODULE :=
 KBUILD_LDFLAGS :=
 CLANG_FLAGS :=

-export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS
CROSS_COMPILE LD CC
+export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS LD CC
 export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS
LEX YACC AWK INSTALLKERNEL
 export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
 export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
@@ -681,6 +681,8 @@ export RETPOLINE_VDSO_CFLAGS

 include arch/$(SRCARCH)/Makefile

+export CROSS_COMPILE
+
 ifdef need-config
 ifdef may-sync-config
 # Read in dependencies to all Kconfig* files, make sure to run syncconfig if






-- 
Best Regards
Masahiro Yamada