[PATCH v2 1/3] riscv: Add Kconfig options to distinguish Zaamo and Zalrsc

[Yao Zi]

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 # [1]
Link: https://lore.kernel.org/u-boot/20250729162035.209849-9-uros.stajic@htecgroup.com/ # [2]
Signed-off-by: Yao Zi <ziyao at disroot.org>
---
 arch/riscv/Kconfig  | 17 +++++++++++++++++
 arch/riscv/Makefile |  7 ++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 8c6feae5735..8b5a4074724 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -339,10 +339,27 @@ 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 6f80f4a7108..fdda6da1df3 100644
--- 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)
-- 
2.50.1