Re: [PATCH v4 1/2] RISC-V: clarify what some RISCV_ISA* config options do

[Charlie Jenkins <charlie@rivosinc.com>]

On Thu, Oct 24, 2024 at 11:19:40AM +0100, Conor Dooley wrote:
> From: Conor Dooley <conor.dooley@microchip.com>
> 
> During some discussion on IRC yesterday and on Pu's bpf patch [1]
> I noticed that these RISCV_ISA* Kconfig options are not really clear
> about their implications. Many of these options have no impact on what
> userspace is allowed to do, for example an application can use Zbb
> regardless of whether or not the kernel does. Change the help text to
> try and clarify whether or not an option affects just the kernel, or
> also userspace. None of these options actually control whether or not an
> extension is detected dynamically as that's done regardless of Kconfig
> options, so drop any text that implies the option is required for
> dynamic detection, rewording them as "do x when y is detected".
> 
> Link: https://lore.kernel.org/linux-riscv/20240328-ferocity-repose-c554f75a676c@spud/ [1]
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

This has been sitting around for a while but is a good change.

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>

> Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
> ---
>  arch/riscv/Kconfig | 36 +++++++++++++++++++-----------------
>  1 file changed, 19 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 62545946ecf43..278a38c94c5a6 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -527,7 +527,8 @@ config RISCV_ISA_C
>  	help
>  	  Adds "C" to the ISA subsets that the toolchain is allowed to emit
>  	  when building Linux, which results in compressed instructions in the
> -	  Linux binary.
> +	  Linux binary. This option produces a kernel that will not run on
> +	  systems that do not support compressed instructions.
>  
>  	  If you don't know what to do here, say Y.
>  
> @@ -537,8 +538,8 @@ config RISCV_ISA_SVNAPOT
>  	depends on RISCV_ALTERNATIVE
>  	default y
>  	help
> -	  Allow kernel to detect the Svnapot ISA-extension dynamically at boot
> -	  time and enable its usage.
> +	  Enable support for the Svnapot ISA-extension when it is detected
> +	  at boot.
>  
>  	  The Svnapot extension is used to mark contiguous PTEs as a range
>  	  of contiguous virtual-to-physical translations for a naturally
> @@ -556,9 +557,8 @@ config RISCV_ISA_SVPBMT
>  	depends on RISCV_ALTERNATIVE
>  	default y
>  	help
> -	   Adds support to dynamically detect the presence of the Svpbmt
> -	   ISA-extension (Supervisor-mode: page-based memory types) and
> -	   enable its usage.
> +	   Add support for the Svpbmt ISA-extension (Supervisor-mode:
> +	   page-based memory types) in the kernel when it is detected at boot.
>  
>  	   The memory type for a page contains a combination of attributes
>  	   that indicate the cacheability, idempotency, and ordering
> @@ -577,14 +577,15 @@ config TOOLCHAIN_HAS_V
>  	depends on AS_HAS_OPTION_ARCH
>  
>  config RISCV_ISA_V
> -	bool "VECTOR extension support"
> +	bool "Vector extension support"
>  	depends on TOOLCHAIN_HAS_V
>  	depends on FPU
>  	select DYNAMIC_SIGFRAME
>  	default y
>  	help
> -	  Say N here if you want to disable all vector related procedure
> -	  in the kernel.
> +	  Add support for the Vector extension when it is detected at boot.
> +	  When this option is disabled, neither the kernel nor userspace may
> +	  use vector procedures.
>  
>  	  If you don't know what to do here, say Y.
>  
> @@ -667,8 +668,8 @@ config RISCV_ISA_ZBB
>  	depends on RISCV_ALTERNATIVE
>  	default y
>  	help
> -	   Adds support to dynamically detect the presence of the ZBB
> -	   extension (basic bit manipulation) and enable its usage.
> +	   Add support for enabling optimisations in the kernel when the
> +	   Zbb extension is detected at boot.
>  
>  	   The Zbb extension provides instructions to accelerate a number
>  	   of bit-specific operations (count bit population, sign extending,
> @@ -707,9 +708,9 @@ config RISCV_ISA_ZICBOM
>  	select RISCV_DMA_NONCOHERENT
>  	select DMA_DIRECT_REMAP
>  	help
> -	   Adds support to dynamically detect the presence of the ZICBOM
> -	   extension (Cache Block Management Operations) and enable its
> -	   usage.
> +	   Add support for the Zicbom extension (Cache Block Management
> +	   Operations) and enable its use in the kernel when it is detected
> +	   at boot.
>  
>  	   The Zicbom extension can be used to handle for example
>  	   non-coherent DMA support on devices that need it.
> @@ -722,7 +723,7 @@ config RISCV_ISA_ZICBOZ
>  	default y
>  	help
>  	   Enable the use of the Zicboz extension (cbo.zero instruction)
> -	   when available.
> +	   in the kernel when it is detected at boot.
>  
>  	   The Zicboz extension is used for faster zeroing of memory.
>  
> @@ -760,8 +761,9 @@ config FPU
>  	bool "FPU support"
>  	default y
>  	help
> -	  Say N here if you want to disable all floating-point related procedure
> -	  in the kernel.
> +	  Add support for floating point operations when an FPU is detected at
> +	  boot. When this option is disabled, neither the kernel nor userspace
> +	  may use the floating point unit.
>  
>  	  If you don't know what to do here, say Y.
>  
> -- 
> 2.45.2
> 

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