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    Subject[PATCH AUTOSEL 5.0 154/262] ARM: 8833/1: Ensure that NEON code always compiles with Clang
    Date
    From: Nathan Chancellor <natechancellor@gmail.com>

    [ Upstream commit de9c0d49d85dc563549972edc5589d195cd5e859 ]

    While building arm32 allyesconfig, I ran into the following errors:

    arch/arm/lib/xor-neon.c:17:2: error: You should compile this file with
    '-mfloat-abi=softfp -mfpu=neon'

    In file included from lib/raid6/neon1.c:27:
    /home/nathan/cbl/prebuilt/lib/clang/8.0.0/include/arm_neon.h:28:2:
    error: "NEON support not enabled"

    Building V=1 showed NEON_FLAGS getting passed along to Clang but
    __ARM_NEON__ was not getting defined. Ultimately, it boils down to Clang
    only defining __ARM_NEON__ when targeting armv7, rather than armv6k,
    which is the '-march' value for allyesconfig.

    >From lib/Basic/Targets/ARM.cpp in the Clang source:

    // This only gets set when Neon instructions are actually available, unlike
    // the VFP define, hence the soft float and arch check. This is subtly
    // different from gcc, we follow the intent which was that it should be set
    // when Neon instructions are actually available.
    if ((FPU & NeonFPU) && !SoftFloat && ArchVersion >= 7) {
    Builder.defineMacro("__ARM_NEON", "1");
    Builder.defineMacro("__ARM_NEON__");
    // current AArch32 NEON implementations do not support double-precision
    // floating-point even when it is present in VFP.
    Builder.defineMacro("__ARM_NEON_FP",
    "0x" + Twine::utohexstr(HW_FP & ~HW_FP_DP));
    }

    Ard Biesheuvel recommended explicitly adding '-march=armv7-a' at the
    beginning of the NEON_FLAGS definitions so that __ARM_NEON__ always gets
    definined by Clang. This doesn't functionally change anything because
    that code will only run where NEON is supported, which is implicitly
    armv7.

    Link: https://github.com/ClangBuiltLinux/linux/issues/287

    Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
    Acked-by: Nicolas Pitre <nico@linaro.org>
    Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
    Reviewed-by: Stefan Agner <stefan@agner.ch>
    Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    Documentation/arm/kernel_mode_neon.txt | 4 ++--
    arch/arm/lib/Makefile | 2 +-
    arch/arm/lib/xor-neon.c | 2 +-
    lib/raid6/Makefile | 2 +-
    4 files changed, 5 insertions(+), 5 deletions(-)

    diff --git a/Documentation/arm/kernel_mode_neon.txt b/Documentation/arm/kernel_mode_neon.txt
    index 525452726d31..b9e060c5b61e 100644
    --- a/Documentation/arm/kernel_mode_neon.txt
    +++ b/Documentation/arm/kernel_mode_neon.txt
    @@ -6,7 +6,7 @@ TL;DR summary
    * Use only NEON instructions, or VFP instructions that don't rely on support
    code
    * Isolate your NEON code in a separate compilation unit, and compile it with
    - '-mfpu=neon -mfloat-abi=softfp'
    + '-march=armv7-a -mfpu=neon -mfloat-abi=softfp'
    * Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
    NEON code
    * Don't sleep in your NEON code, and be aware that it will be executed with
    @@ -87,7 +87,7 @@ instructions appearing in unexpected places if no special care is taken.
    Therefore, the recommended and only supported way of using NEON/VFP in the
    kernel is by adhering to the following rules:
    * isolate the NEON code in a separate compilation unit and compile it with
    - '-mfpu=neon -mfloat-abi=softfp';
    + '-march=armv7-a -mfpu=neon -mfloat-abi=softfp';
    * issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
    into the unit containing the NEON code from a compilation unit which is *not*
    built with the GCC flag '-mfpu=neon' set.
    diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
    index ad25fd1872c7..0bff0176db2c 100644
    --- a/arch/arm/lib/Makefile
    +++ b/arch/arm/lib/Makefile
    @@ -39,7 +39,7 @@ $(obj)/csumpartialcopy.o: $(obj)/csumpartialcopygeneric.S
    $(obj)/csumpartialcopyuser.o: $(obj)/csumpartialcopygeneric.S

    ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
    - NEON_FLAGS := -mfloat-abi=softfp -mfpu=neon
    + NEON_FLAGS := -march=armv7-a -mfloat-abi=softfp -mfpu=neon
    CFLAGS_xor-neon.o += $(NEON_FLAGS)
    obj-$(CONFIG_XOR_BLOCKS) += xor-neon.o
    endif
    diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
    index 2c40aeab3eaa..c691b901092f 100644
    --- a/arch/arm/lib/xor-neon.c
    +++ b/arch/arm/lib/xor-neon.c
    @@ -14,7 +14,7 @@
    MODULE_LICENSE("GPL");

    #ifndef __ARM_NEON__
    -#error You should compile this file with '-mfloat-abi=softfp -mfpu=neon'
    +#error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
    #endif

    /*
    diff --git a/lib/raid6/Makefile b/lib/raid6/Makefile
    index 4e90d443d1b0..e723eacf7868 100644
    --- a/lib/raid6/Makefile
    +++ b/lib/raid6/Makefile
    @@ -39,7 +39,7 @@ endif
    ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
    NEON_FLAGS := -ffreestanding
    ifeq ($(ARCH),arm)
    -NEON_FLAGS += -mfloat-abi=softfp -mfpu=neon
    +NEON_FLAGS += -march=armv7-a -mfloat-abi=softfp -mfpu=neon
    endif
    CFLAGS_recov_neon_inner.o += $(NEON_FLAGS)
    ifeq ($(ARCH),arm64)
    --
    2.19.1
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