Re: [PATCH v8 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af()

[Catalin Marinas]

On Sat, Sep 21, 2019 at 09:50:52PM +0800, Jia He wrote:
> We unconditionally set the HW_AFDBM capability and only enable it on
> CPUs which really have the feature. But sometimes we need to know
> whether this cpu has the capability of HW AF. So decouple AF from
> DBM by new helper cpu_has_hw_af().
> 
> Reported-by: kbuild test robot <lkp@xxxxxxxxx>
> Suggested-by: Suzuki Poulose <Suzuki.Poulose@xxxxxxx>
> Signed-off-by: Jia He <justin.he@xxxxxxx>
> ---
>  arch/arm64/include/asm/cpufeature.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index c96ffa4722d3..46caf934ba4e 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -667,6 +667,16 @@ static inline u32 id_aa64mmfr0_parange_to_phys_shift(int parange)
>  	default: return CONFIG_ARM64_PA_BITS;
>  	}
>  }
> +
> +/* Decouple AF from AFDBM. */

We could do with a better comment here or just remove it altogether. The
aim of the patch was to decouple AF check from the AF+DBM but the
comment here should describe what the function does. Maybe something
like: "Check whether hardware update of the Access flag is supported".

> +static inline bool cpu_has_hw_af(void)
> +{
> +	if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
> +		return read_cpuid(ID_AA64MMFR1_EL1) & 0xf;
> +
> +	return false;
> +}

Other than the comment above,

Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>