linux-kernel - Re: [PATCH v3 2/2] arm64: Support systems without FP/ASIMD

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Message-ID: <20161114114858.GB3096@e104818-lin.cambridge.arm.com>
Date:   Mon, 14 Nov 2016 11:48:58 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ard.biesheuvel@...aro.org, marc.zyngier@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu,
        Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v3 2/2] arm64: Support systems without FP/ASIMD

Hi Suzuki,

On Tue, Nov 08, 2016 at 01:56:21PM +0000, Suzuki K. Poulose wrote:
> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> index 87b4465..4174f09 100644
> --- a/arch/arm64/include/asm/cpucaps.h
> +++ b/arch/arm64/include/asm/cpucaps.h
> @@ -34,7 +34,8 @@
>  #define ARM64_HAS_32BIT_EL0			13
>  #define ARM64_HYP_OFFSET_LOW			14
>  #define ARM64_MISMATCHED_CACHE_LINE_SIZE	15
> +#define ARM64_HAS_NO_FPSIMD			16
>  
> -#define ARM64_NCAPS				16
> +#define ARM64_NCAPS				17
>  
>  #endif /* __ASM_CPUCAPS_H */
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 9890d20..ce45770 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -213,6 +213,11 @@ static inline bool system_supports_mixed_endian_el0(void)
>  	return id_aa64mmfr0_mixed_endian_el0(read_system_reg(SYS_ID_AA64MMFR0_EL1));
>  }
>  
> +static inline bool system_supports_fpsimd(void)
> +{
> +	return !cpus_have_const_cap(ARM64_HAS_NO_FPSIMD);
> +}

Any particular reason why using negation instead of a ARM64_HAS_FPSIMD?
A potential problem would be the default cpus_have_const_cap()
implementation and the default static key having a slight performance
impact.

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index fc2bd19..f89385d 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -746,6 +746,14 @@ static bool hyp_offset_low(const struct arm64_cpu_capabilities *entry,
>  	return idmap_addr > GENMASK(VA_BITS - 2, 0) && !is_kernel_in_hyp_mode();
>  }
>  
> +static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unused)
> +{
> +	u64 pfr0 = read_system_reg(SYS_ID_AA64PFR0_EL1);
> +
> +	return cpuid_feature_extract_signed_field(pfr0,
> +					ID_AA64PFR0_FP_SHIFT) < 0;
> +}
> +
>  static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "GIC system register CPU interface",
> @@ -829,6 +837,13 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.def_scope = SCOPE_SYSTEM,
>  		.matches = hyp_offset_low,
>  	},
> +	{
> +		/* FP/SIMD is not implemented */
> +		.capability = ARM64_HAS_NO_FPSIMD,
> +		.def_scope = SCOPE_SYSTEM,
> +		.min_field_value = 0,
> +		.matches = has_no_fpsimd,
> +	},

If we go for negation, I don't think we need a min_field_value at all,
the matching is done by the has_no_fpsimd() function.

-- 
Catalin