[PATCH 1/3] RISC-V: Improve use of isa2hwcap[]

[Conor Dooley]

On Fri, Oct 21, 2022 at 12:59:03PM +0200, Andrew Jones wrote:
> Improve isa2hwcap[] by removing it from static storage, as
> riscv_fill_hwcap() is only called once, and by reducing its size
> from 256 bytes to 26. The latter improvement is possible because
> isa2hwcap[] will never be indexed with capital letters and we can
> precompute the offsets from 'a'.

Hey Drew, couple questions for you - mostly due to naivety I think..

How do we know that isa2hwcap will never interact with capital letters?
It pulls the isa string from dt and the no-capitals enforcement comes
from there since one with capitals is invalid? I didn't dig particularly
deeply into the code, but is there a risk that we regress some user that
has a dt with capitals in the isa string? Or is that a "your dt was wrong
and you're out-of-tree so that's your problem" situation?

Secondly, in the UAPI header, the COMPAT_HWCAP_ISA_FOO defines are
computed as I - A rather than i - a. Should those be changed too for the
sake of consistently using the lowercase everywhere, or do you think
that doesn't really matter?

Thanks,
Conor.

> 
> No functional change intended.
> 
> Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> ---
>  arch/riscv/kernel/cpufeature.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 694267d1fe81..4677320d7e31 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -74,15 +74,15 @@ void __init riscv_fill_hwcap(void)
>  	const char *isa;
>  	char print_str[NUM_ALPHA_EXTS + 1];
>  	int i, j, rc;
> -	static unsigned long isa2hwcap[256] = {0};
> +	unsigned long isa2hwcap[26] = {0};
>  	unsigned long hartid;
>  
> -	isa2hwcap['i'] = isa2hwcap['I'] = COMPAT_HWCAP_ISA_I;
> -	isa2hwcap['m'] = isa2hwcap['M'] = COMPAT_HWCAP_ISA_M;
> -	isa2hwcap['a'] = isa2hwcap['A'] = COMPAT_HWCAP_ISA_A;
> -	isa2hwcap['f'] = isa2hwcap['F'] = COMPAT_HWCAP_ISA_F;
> -	isa2hwcap['d'] = isa2hwcap['D'] = COMPAT_HWCAP_ISA_D;
> -	isa2hwcap['c'] = isa2hwcap['C'] = COMPAT_HWCAP_ISA_C;
> +	isa2hwcap['i' - 'a'] = COMPAT_HWCAP_ISA_I;
> +	isa2hwcap['m' - 'a'] = COMPAT_HWCAP_ISA_M;
> +	isa2hwcap['a' - 'a'] = COMPAT_HWCAP_ISA_A;
> +	isa2hwcap['f' - 'a'] = COMPAT_HWCAP_ISA_F;
> +	isa2hwcap['d' - 'a'] = COMPAT_HWCAP_ISA_D;
> +	isa2hwcap['c' - 'a'] = COMPAT_HWCAP_ISA_C;
>  
>  	elf_hwcap = 0;
>  
> @@ -196,8 +196,10 @@ void __init riscv_fill_hwcap(void)
>  			if (unlikely(ext_err))
>  				continue;
>  			if (!ext_long) {
> -				this_hwcap |= isa2hwcap[(unsigned char)(*ext)];
> -				set_bit(*ext - 'a', this_isa);
> +				int nr = *ext - 'a';
> +
> +				this_hwcap |= isa2hwcap[nr];
> +				set_bit(nr, this_isa);
>  			} else {
>  				SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
>  				SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
> -- 
> 2.37.3
> 
> 
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