[RFC 01/15] riscv: add initial kconfig and build flags for big-endian

[Ben Dooks]

this is the initial kconfig and makefile updates to get a base
big-endian build for arch/riscv. Will require header updates
for IO code and features whcih are known to not work.
---
 arch/riscv/Kconfig  | 22 ++++++++++++++++++++++
 arch/riscv/Makefile | 15 ++++++++++++---
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index fa8f2da87a0a..e352e022ed4a 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -426,6 +426,28 @@ choice
 		bool "medium any code model"
 endchoice
 
+choice
+	prompt "Data endian"
+	default CPU_LITTLE_ENDIAN
+	help
+	  Configure the endiannes of data access performed by the CPU.
+	  This will require system to be booted from M mode in big endian
+	  and the userland to be compiled for the same endian-ness.
+
+config CPU_BIG_ENDIAN
+	bool "Build big-endian kernel"
+	depends on EXPERT
+	help
+	  Say Y if you want to run big-endian kernel and userspace
+	  Set for expert as this experimental
+
+config CPU_LITTLE_ENDIAN
+       bool "Build little-endian kernel (default)"
+       help
+	  Say  Y if you want to run little-endian kernel and userspace
+	  This is the default for most distributions
+endchoice
+
 config MODULE_SECTIONS
 	bool
 	select HAVE_MOD_ARCH_SPECIFIC
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index d469db9f46f4..535cbbb76f5c 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -21,6 +21,11 @@ else
 endif
 endif
 
+ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
+KBUILD_CPPFLAGS	+= -mbig-endian -D__RISCVEB__
+CHECKFLAGS	+= -D__RISCVEB__
+endif
+
 ifeq ($(CONFIG_CMODEL_MEDLOW),y)
 KBUILD_CFLAGS_MODULE += -mcmodel=medany
 endif
@@ -28,13 +33,17 @@ endif
 export BITS
 ifeq ($(CONFIG_ARCH_RV64I),y)
 	BITS := 64
-	UTS_MACHINE := riscv64
+	ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
+		UTS_MACHINE := riscv64_be
+		KBUILD_LDFLAGS += -melf64briscv
+	else	
+		UTS_MACHINE := riscv64
+		KBUILD_LDFLAGS += -melf64lriscv
+	endif
 
 	KBUILD_CFLAGS += -mabi=lp64
 	KBUILD_AFLAGS += -mabi=lp64
 
-	KBUILD_LDFLAGS += -melf64lriscv
-
 	KBUILD_RUSTFLAGS += -Ctarget-cpu=generic-rv64 --target=riscv64imac-unknown-none-elf \
 			    -Cno-redzone
 else
-- 
2.37.2.352.g3c44437643