riscv: Add Kconfig options to distinguish Zaamo and Zalrsc

Ratified on Apr. 2024, the original RISC-V "A" extension is now split
into two separate extensions, "Zaamo" for atomic operations and "Zalrsc"
for load-reserved/store-conditional instructions.

For now, we've already seen real-world designs implement the Zalrsc
extension only[2]. As U-Boot mainly runs with only one HART, we could
easily support these designs by not using AMO instructions in the
hard-written assembly if necessary, for which this patch introduces two
new Kconfig options to indicate the availability of "Zaamo" and "Zalrsc".

Note that even with this patch, "A" extension is specified in the ISA
string passed to the compiler as long as one of "Zaamo" or "Zalrsc" is
available, since they're only recognized with a quite recent version of
GCC/Clang. The compiler usually doesn't automatically generate atomic
instructions unless the source explicitly instructs it to do so, thus
this should be safe.

Link: https://github.com/riscv/riscv-zaamo-zalrsc/commit/d94c64c63e9120d56bdeb540caf2e5dae60a8126
Link: https://lore.kernel.org/u-boot/20250729162035.209849-9-uros.stajic@htecgroup.com/
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 04eb0e6f23cfb669bd1b6ced123242f49a9a62b2..f0d2c90434955c87326e76c30239c0d67ff32d8e 100644 (file)
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -343,10 +343,27 @@ endmenu
 
 config RISCV_ISA_A
        bool "Standard extension for Atomic Instructions"
+       depends on RISCV_ISA_ZAAMO && RISCV_ISA_ZALRSC
        default y
        help
          Adds "A" to the ISA string passed to the compiler.
 
+config RISCV_ISA_ZAAMO
+       bool "Standard extension for Atomic Memory Operations"
+       default y
+       help
+         Indicates the platform supports Zaamo extension for atomic memory
+         operations. Hand-written Assembly routines won't use AMO
+         instructions if set to n.
+
+config RISCV_ISA_ZALRSC
+       bool "Standard extension for LR/SC instructions"
+       default y
+       help
+         Indicates the platform supports Zalrsc extension for load-reserved
+         and store-conditional isntructions. Hand-written assembly routines
+         won't use LR/SC instructions if set to n.
+
 config RISCV_ISA_ZICBOM
        bool "Zicbom support"
        depends on !SYS_DISABLE_DCACHE_OPS

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 6f80f4a7108c110f8f125e1cff2ecbdcd9bbba87..fdda6da1df3264dee87eab450e64e5533cd38748 100644 (file)
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -11,7 +11,12 @@ ifeq ($(CONFIG_ARCH_RV32I),y)
        ARCH_BASE = rv32im
        ABI_BASE = ilp32
 endif
-ifeq ($(CONFIG_RISCV_ISA_A),y)
+# GCC starts to recognize "Zaamo" and "Zalrsc" from version 15, which is quite
+# recent. We don't bother checking the exact compiler version, but pass "A"
+# extension for -march as long as one of "Zaamo" or "Zalrsc" is available.
+ifeq ($(findstring y,$(CONFIG_RISCV_ISA_A)     \
+                    $(CONFIG_RISCV_ISA_ZAAMO)  \
+                    $(CONFIG_RISCV_ISA_ZALRSC)),y)
        ARCH_A = a
 endif
 ifeq ($(CONFIG_RISCV_ISA_F),y)