[RFC PATCH 02/12] ARM: Implement ARCH_HAS_KERNEL_FPU_SUPPORT

[Samuel Holland]

ARM 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/arm/Kconfig           |  1 +
 arch/arm/Makefile          |  7 +++++++
 arch/arm/include/asm/fpu.h | 17 +++++++++++++++++
 3 files changed, 25 insertions(+)
 create mode 100644 arch/arm/include/asm/fpu.h

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index f8567e95f98b..92e21a4a2903 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -14,6 +14,7 @@ config ARM
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KEEPINITRD
 	select ARCH_HAS_KCOV
+	select ARCH_HAS_KERNEL_FPU_SUPPORT if KERNEL_MODE_NEON
 	select ARCH_HAS_MEMBARRIER_SYNC_CORE
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
 	select ARCH_HAS_PTE_SPECIAL if ARM_LPAE
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 5ba42f69f8ce..1dd860dba5f5 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -130,6 +130,13 @@ endif
 # Accept old syntax despite ".syntax unified"
 AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
 
+# 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 $(CC) -print-file-name=include)
+CC_FLAGS_FPU	+= -march=armv7-a -mfloat-abi=softfp -mfpu=neon
+
 ifeq ($(CONFIG_THUMB2_KERNEL),y)
 CFLAGS_ISA	:=-Wa,-mimplicit-it=always $(AFLAGS_NOWARN)
 AFLAGS_ISA	:=$(CFLAGS_ISA) -Wa$(comma)-mthumb
diff --git a/arch/arm/include/asm/fpu.h b/arch/arm/include/asm/fpu.h
new file mode 100644
index 000000000000..d01ca06e700a
--- /dev/null
+++ b/arch/arm/include/asm/fpu.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * linux/arch/arm/include/asm/fpu.h
+ *
+ * Copyright (C) 2023 SiFive
+ */
+
+#ifndef __ASM_FPU_H
+#define __ASM_FPU_H
+
+#include <asm/neon.h>
+
+#define kernel_fpu_available()	cpu_has_neon()
+#define kernel_fpu_begin()	kernel_neon_begin()
+#define kernel_fpu_end()	kernel_neon_end()
+
+#endif /* ! __ASM_FPU_H */
-- 
2.42.0