[RFC PATCH 04/12] arm64: Implement ARCH_HAS_KERNEL_FPU_SUPPORT

[Will Deacon]

On Thu, Dec 07, 2023 at 09:54:34PM -0800, Samuel Holland wrote:
> arm64 provides an equivalent to the common kernel-mode FPU API, but in a
> different header and using different function names. Add a wrapper
> header, and export CFLAGS adjustments as found in lib/raid6/Makefile.
> 
> Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
> ---
> 
>  arch/arm64/Kconfig           |  1 +
>  arch/arm64/Makefile          |  9 ++++++++-
>  arch/arm64/include/asm/fpu.h | 17 +++++++++++++++++
>  3 files changed, 26 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/fpu.h
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7b071a00425d..485ac389ac11 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -30,6 +30,7 @@ config ARM64
>  	select ARCH_HAS_GCOV_PROFILE_ALL
>  	select ARCH_HAS_GIGANTIC_PAGE
>  	select ARCH_HAS_KCOV
> +	select ARCH_HAS_KERNEL_FPU_SUPPORT if KERNEL_MODE_NEON
>  	select ARCH_HAS_KEEPINITRD
>  	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>  	select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 9a2d3723cd0f..4a65f24c7998 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -36,7 +36,14 @@ ifeq ($(CONFIG_BROKEN_GAS_INST),y)
>  $(warning Detected assembler with broken .inst; disassembly will be unreliable)
>  endif
>  
> -KBUILD_CFLAGS	+= -mgeneral-regs-only	\
> +# The GCC option -ffreestanding is required in order to compile code containing
> +# ARM/NEON intrinsics in a non C99-compliant environment (such as the kernel)
> +CC_FLAGS_FPU	:= -ffreestanding
> +# Enable <arm_neon.h>
> +CC_FLAGS_FPU	+= -isystem $(shell $(CC) -print-file-name=include)
> +CC_FLAGS_NO_FPU	:= -mgeneral-regs-only
> +
> +KBUILD_CFLAGS	+= $(CC_FLAGS_NO_FPU) \
>  		   $(compat_vdso) $(cc_has_k_constraint)
>  KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
>  KBUILD_AFLAGS	+= $(compat_vdso)

Can you use this to replace the same logic in arch/arm64/lib/Makefile,
like you do for arch/arm/?

Will