arch: add support for RISC-V 64-bit (riscv64) architecture

This enables a riscv64 system to be built with a Buildroot generated
toolchain (gcc >= 7.x, binutils >= 2.30, glibc only).

This configuration has been used to successfully build a qemu-bootable
riscv-linux-4.15 kernel (https://github.com/riscv/riscv-linux.git).

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
[Thomas:
 - simplify arch.mk.riscv by directly setting GCC_TARGET_ARCH
 - simplify glibc.mk changes by using GLIBC_CONF_ENV.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

3 files changed, 145 insertions, 0 deletions

diff --git a/arch/Config.in b/arch/Config.in
index 7d1aeb2174..8bd57fe1cc 100644
--- a/arch/Config.in
+++ b/arch/Config.in
@@ -198,6 +198,17 @@ config BR2_powerpc64le
 	  http://www.power.org/
 	  http://en.wikipedia.org/wiki/Powerpc
 
+config BR2_riscv
+	bool "RISCV"
+	select BR2_ARCH_HAS_MMU_MANDATORY
+	select BR2_ARCH_NEEDS_GCC_AT_LEAST_7
+	help
+	  RISC-V is an open, free Instruction Set Architecture created
+	  by the UC Berkeley Architecture Research group and supported
+	  and promoted by RISC-V Foundation.
+	  https://riscv.org/
+	  https://en.wikipedia.org/wiki/RISC-V
+
 config BR2_sh
 	bool "SuperH"
 	select BR2_ARCH_HAS_MMU_OPTIONAL
@@ -423,6 +434,10 @@ if BR2_powerpc || BR2_powerpc64 || BR2_powerpc64le
 source "arch/Config.in.powerpc"
 endif
 
+if BR2_riscv
+source "arch/Config.in.riscv"
+endif
+
 if BR2_sh
 source "arch/Config.in.sh"
 endif
diff --git a/arch/Config.in.riscv b/arch/Config.in.riscv
new file mode 100644
index 0000000000..4361890bf4
--- /dev/null
+++ b/arch/Config.in.riscv
@@ -0,0 +1,102 @@
+# RISC-V CPU ISA extensions.
+
+config BR2_RISCV_ISA_RVI
+	bool
+
+config BR2_RISCV_ISA_RVM
+	bool
+
+config BR2_RISCV_ISA_RVA
+	bool
+
+config BR2_RISCV_ISA_RVF
+	bool
+
+config BR2_RISCV_ISA_RVD
+	bool
+
+config BR2_RISCV_ISA_RVC
+	bool
+
+choice
+	prompt "Target Architecture Variant"
+	default BR2_riscv_g
+
+config BR2_riscv_g
+	bool "General purpose (G)"
+	select BR2_RISCV_ISA_RVI
+	select BR2_RISCV_ISA_RVM
+	select BR2_RISCV_ISA_RVA
+	select BR2_RISCV_ISA_RVF
+	select BR2_RISCV_ISA_RVD
+	help
+	  General purpose (G) is equivalent to IMAFD.
+
+config BR2_riscv_custom
+	bool "Custom architecture"
+	select BR2_RISCV_ISA_RVI
+	select BR2_RISCV_ISA_CUSTOM_RVA
+
+endchoice
+
+if BR2_riscv_custom
+
+comment "Instruction Set Extensions"
+
+config BR2_RISCV_ISA_CUSTOM_RVM
+	bool "Integer Multiplication and Division (M)"
+	select BR2_RISCV_ISA_RVM
+
+config BR2_RISCV_ISA_CUSTOM_RVA
+	bool "Atomic Instructions (A)"
+	select BR2_RISCV_ISA_RVA
+
+config BR2_RISCV_ISA_CUSTOM_RVF
+	bool "Single-precision Floating-point (F)"
+	select BR2_RISCV_ISA_RVF
+
+config BR2_RISCV_ISA_CUSTOM_RVD
+	bool "Double-precision Floating-point (D)"
+	depends on BR2_RISCV_ISA_RVF
+	select BR2_RISCV_ISA_RVD
+
+config BR2_RISCV_ISA_CUSTOM_RVC
+	bool "Compressed Instructions (C)"
+	select BR2_RISCV_ISA_RVC
+endif
+
+config BR2_RISCV_64
+	bool
+	default y
+	select BR2_ARCH_IS_64
+
+choice
+	prompt "Target ABI"
+	default BR2_RISCV_ABI_LP64
+
+config BR2_RISCV_ABI_LP64
+	bool "lp64"
+	depends on BR2_ARCH_IS_64
+
+config BR2_RISCV_ABI_LP64F
+	bool "lp64f"
+	depends on BR2_ARCH_IS_64 && BR2_RISCV_ISA_RVF
+
+config BR2_RISCV_ABI_LP64D
+	bool "lp64d"
+	depends on BR2_ARCH_IS_64 && BR2_RISCV_ISA_RVD
+endchoice
+
+config BR2_ARCH
+	default "riscv64" if BR2_ARCH_IS_64
+
+config BR2_ENDIAN
+	default "LITTLE"
+
+config BR2_GCC_TARGET_ABI
+	default "lp64" if BR2_RISCV_ABI_LP64
+	default "lp64f" if BR2_RISCV_ABI_LP64F
+	default "lp64d" if BR2_RISCV_ABI_LP64D
+
+config BR2_READELF_ARCH_NAME
+	default "RISC-V"
diff --git a/arch/arch.mk.riscv b/arch/arch.mk.riscv
new file mode 100644
index 0000000000..022d1a6809
--- /dev/null
+++ b/arch/arch.mk.riscv
@@ -0,0 +1,28 @@
+#
+# Configure the GCC_TARGET_ARCH variable and append the
+# appropriate RISC-V ISA extensions.
+#
+
+ifeq ($(BR2_riscv),y)
+
+ifeq ($(BR2_ARCH_IS_64),y)
+GCC_TARGET_ARCH := rv64i
+endif
+
+ifeq ($(BR2_RISCV_ISA_RVM),y)
+GCC_TARGET_ARCH := $(GCC_TARGET_ARCH)m
+endif
+ifeq ($(BR2_RISCV_ISA_RVA),y)
+GCC_TARGET_ARCH := $(GCC_TARGET_ARCH)a
+endif
+ifeq ($(BR2_RISCV_ISA_RVF),y)
+GCC_TARGET_ARCH := $(GCC_TARGET_ARCH)f
+endif
+ifeq ($(BR2_RISCV_ISA_RVD),y)
+GCC_TARGET_ARCH := $(GCC_TARGET_ARCH)d
+endif
+ifeq ($(BR2_RISCV_ISA_RVC),y)
+GCC_TARGET_ARCH := $(GCC_TARGET_ARCH)c
+endif
+
+endif