riscv: add Kconfig entries for the code model

RISC-V has two code models, medium low (medlow) and medium any (medany).
Medlow limits addressable memory to a single 2 GiB range between the
absolute addresses -2 GiB and +2 GiB. Medany limits addressable memory
to any single 2 GiB address range.

By default, medlow is selected for U-Boot on both 32-bit and 64-bit
systems.

The -mcmodel compiler flag is selected according to the Kconfig
configuration.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
[bmeng: adjust to make medlow the default code model for U-Boot]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

arch/riscv/Kconfig

@@ -44,6 +44,24 @@ config ARCH_RV64I
 
 endchoice
 
+choice
+	prompt "Code Model"
+	default CMODEL_MEDLOW
+
+config CMODEL_MEDLOW
+	bool "medium low code model"
+	help
+	  U-Boot and its statically defined symbols must lie within a single 2 GiB
+	  address range and must lie between absolute addresses -2 GiB and +2 GiB.
+
+config CMODEL_MEDANY
+	bool "medium any code model"
+	help
+	  U-Boot and its statically defined symbols must be within any single 2 GiB
+	  address range.
+
+endchoice
+
 config RISCV_ISA_C
 	bool "Emit compressed instructions"
 	default y

arch/riscv/Makefile

@@ -17,8 +17,15 @@ endif
 ifeq ($(CONFIG_RISCV_ISA_C),y)
 	ARCH_C = c
 endif
+ifeq ($(CONFIG_CMODEL_MEDLOW),y)
+	CMODEL = medlow
+endif
+ifeq ($(CONFIG_CMODEL_MEDANY),y)
+	CMODEL = medany
+endif
 
-ARCH_FLAGS = -march=$(ARCH_BASE)$(ARCH_A)$(ARCH_C) -mabi=$(ABI)
+ARCH_FLAGS = -march=$(ARCH_BASE)$(ARCH_A)$(ARCH_C) -mabi=$(ABI) \
+	     -mcmodel=$(CMODEL)
 
 PLATFORM_CPPFLAGS	+= $(ARCH_FLAGS)
 CFLAGS_EFI		+= $(ARCH_FLAGS)