Re: [PATCH 1/2] riscv: Add Kconfig options to distinguish Zaamo and Zalrsc

[Yixun Lan <dlan@gentoo.org>]

Hi Yao,

On 09:21 Sat 02 Aug     , Yao Zi wrote:
> 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@disroot.org>
> ---
>  arch/riscv/Kconfig             | 17 +++++++++++++++++
>  arch/riscv/Makefile            |  7 ++++++-
>  configs/ibex-ast2700_defconfig |  3 ++-
>  3 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 8c6feae5735..b1c2d657e99 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. 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 instructions. Assembly rutines 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)
> diff --git a/configs/ibex-ast2700_defconfig b/configs/ibex-ast2700_defconfig
> index f088aec8716..eb5cab43645 100644
> --- a/configs/ibex-ast2700_defconfig
> +++ b/configs/ibex-ast2700_defconfig
> @@ -23,7 +23,8 @@ CONFIG_SYS_MEM_TOP_HIDE=0x10000000
>  CONFIG_BUILD_TARGET=""
>  CONFIG_TARGET_ASPEED_AST2700_IBEX=y
>  # CONFIG_RISCV_ISA_F is not set
> -# CONFIG_RISCV_ISA_A is not set
> +# CONFIG_RISCV_ISA_ZAAMO is not set
> +# CONFIG_RISCV_ISA_ZALRSC is not set

this is confusing, while in this patchset it actually equal to :
# CONFIG_RISCV_ISA_ZAAMO is not set
CONFIG_RISCV_ISA_ZALRSC=y

also I believe changes for configs/ibex-ast2700_defconfig should go as
an independent patch, please separate it

>  # CONFIG_SPL_SMP is not set
>  CONFIG_XIP=y
>  CONFIG_SPL_XIP=y
> -- 
> 2.50.1
> 

-- 
Yixun Lan (dlan)