Re: [PATCH v2] powerpc/powernv: Cache OPAL check_token() results

[Nikhil Kumar Singh <nikhilks@linux.ibm.com>]

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On 26/06/26 18:05, Shivang Upadhyay wrote:
> diff --git a/arch/powerpc/platforms/powernv/opal.c b/arch/powerpc/platforms/powernv/opal.c
> index 1946dbdc9fa1..1e9cb5271ee7 100644
> --- a/arch/powerpc/platforms/powernv/opal.c
> +++ b/arch/powerpc/platforms/powernv/opal.c
> @@ -1125,6 +1125,36 @@ EXPORT_SYMBOL_GPL(opal_flash_read);
>   EXPORT_SYMBOL_GPL(opal_flash_write);
>   EXPORT_SYMBOL_GPL(opal_flash_erase);
>   EXPORT_SYMBOL_GPL(opal_prd_msg);
> +
> +/**
> + * opal_check_token - Check if an OPAL call token is supported
> + * @token: OPAL token number to check
> + *
> + * Returns 1 if supported, 0 if not.
> + */
> +int64_t opal_check_token(uint64_t token)
> +{
> +	static u8 token_cache[OPAL_LAST];
> +	enum {
> +		SUPP_UNKNOWN = 0,
> +		PRESENT,
> +		ABSENT
> +	};
> +
> +	if (token > OPAL_LAST)
> +		return 0;
> +
> +	if (token_cache[token] == SUPP_UNKNOWN) {

Hi Shivang,
Thanks for the v2 patch. This is a great optimization and definitely 
helps reduce unnecessary overhead. I have one small suggestion.

You can wrap the if (token_cache[token] == SUPP_UNKNOWN) check, as well 
as the if (token > OPAL_LAST) check above it, with unlikely(), since 
this function is in the hot path. Out-of-bounds tokens are very rare, 
and after the cache is populated, the SUPP_UNKNOWN condition is expected 
to be false in almost all cases. Wrapping these checks with unlikely() 
will help the compiler optimize branch prediction.

Thanks,
~ Nikhil

> +		/* Do the actual opal_call here */
> +		if (opal_check_token_call(token)) {
> +			token_cache[token] = PRESENT;
> +		} else {
> +			token_cache[token] = ABSENT;
> +		}
> +	}
> +
> +	return (token_cache[token] == PRESENT);
> +}
>   EXPORT_SYMBOL_GPL(opal_check_token);
>   
>   /* Convert a region of vmalloc memory to an opal sg list */

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