linux-kernel - [PATCH] ARM: Fix instruction set selection for GCC 11

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Date:   Tue, 10 Aug 2021 08:13:50 +0200
From:   Juerg Haefliger <juerg.haefliger@...onical.com>
To:     linux-arm-kernel@...ts.infradead.org, linux@...linux.org.uk
Cc:     linux-kernel@...r.kernel.org,
        Juerg Haefliger <juergh@...onical.com>
Subject: [PATCH] ARM: Fix instruction set selection for GCC 11

GCC 11 on ARM now complains like the following when trying to determine if
an arch is supported. Presumably because it enforces the default option
'--with-float=hard' which GCC 10 didn't do?

  $ arm-linux-gnueabihf-gcc-11 -march=armv7-a -c -x c /dev/null
  cc1: error: ‘-mfloat-abi=hard’: selected architecture lacks an FPU

Due to that, the kernel build system selects the wrong compiler options
which throws errros like this during the build:

  /tmp/ccrHfZPj.s: Assembler messages:
  /tmp/ccrHfZPj.s:116: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:150: Error: selected processor does not support `isb ' in ARM mode
  /tmp/ccrHfZPj.s:160: Error: selected processor does not support `mrrc p15,1,r4,r5,c14' in ARM mode
  /tmp/ccrHfZPj.s:245: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:503: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:527: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:698: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:731: Error: selected processor does not support `isb ' in ARM mode

Fix that by adding '-msoft-float' to KBUILD_CFLAGS before the definition of
the 'arch-$(CONFIG_CPU_<foo>)' instruction selection macros.

Signed-off-by: Juerg Haefliger <juergh@...onical.com>
---
 arch/arm/Makefile | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 173da685a52e..d21198833997 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -56,6 +56,9 @@ endif
 #
 KBUILD_CFLAGS	+= $(call cc-option,-fno-ipa-sra)
 
+# Need -msoft-float for gcc 11 for the below instruction set selection
+KBUILD_CFLAGS	+= -msoft-float
+
 # This selects which instruction set is used.
 # Note that GCC does not numerically define an architecture version
 # macro, but instead defines a whole series of macros which makes
@@ -125,7 +128,7 @@ AFLAGS_ISA	:=$(CFLAGS_ISA)
 endif
 
 # Need -Uarm for gcc < 3.x
-KBUILD_CFLAGS	+=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
+KBUILD_CFLAGS	+=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -Uarm
 KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(AFLAGS_ISA) $(arch-y) $(tune-y) -include asm/unified.h -msoft-float
 
 CHECKFLAGS	+= -D__arm__
-- 
2.30.2