[PATCH v3 1/4] powerpc/kbuild: set default generic machine type for 32-bit compile

[Nicholas Piggin <npiggin@gmail.com>]

Some 64-bit toolchains uses the wrong ISA variant for compiling 32-bit
kernels, even with -m32. Debian's powerpc64le is one such case, and
that is because it is built with --with-cpu=power8.

So when cross compiling a 32-bit kernel with a 64-bit toolchain, set
-mcpu=powerpc initially, which is the generic 32-bit powerpc machine
type and scheduling model. CPU and platform code can override this
with subsequent -mcpu flags if necessary.

This is not done for 32-bit toolchains otherwise it would override
their defaults, which are presumably set appropriately for the
environment (moreso than a 64-bit cross compiler).

This fixes a lot of build failures due to incompatible assembly when
compiling 32-bit kernel with th Debian powerpc64le 64-bit toolchain.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

---
Since v2:
- reworded changelog to explain the cause of the problem,
  (thanks Segher).
- moved the flags into the 64-32 cross compile case so as not to
  disturb native 32-bit builds.

 arch/powerpc/Makefile | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 95813df90801..15ca4bafad82 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -24,9 +24,20 @@ ifeq ($(HAS_BIARCH),y)
 ifeq ($(CROSS32_COMPILE),)
 CROSS32CC	:= $(CC) -m32
 KBUILD_ARFLAGS	+= --target=elf32-powerpc
+
+ifdef CONFIG_PPC32
+# These options will be overridden by any -mcpu option that the CPU
+# or platform code sets later on the command line, but they are needed
+# to set a sane 32-bit cpu target for the 64-bit cross compiler which
+# may default to the wrong ISA.
+KBUILD_CFLAGS		+= -mcpu=powerpc
+KBUILD_AFLAGS		+= -mcpu=powerpc
+endif
+
 endif
 endif
 
+
 export CROSS32CC CROSS32AR
 
 ifeq ($(CROSS_COMPILE),)
-- 
2.17.0