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Date:   Fri, 10 Jun 2022 17:47:12 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Ionela Voinescu <ionela.voinescu@arm.com>
Cc:     Will Deacon <will@kernel.org>, James Morse <james.morse@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: errata: add detection for AMEVCNTR01 incrementing
 incorrectly
Message-ID: <YqN1kJlIkhNAEl/K@arm.com>
References: <20220607125340.13635-1-ionela.voinescu@arm.com>
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On Tue, Jun 07, 2022 at 01:53:40PM +0100, Ionela Voinescu wrote:
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 14a8f3d93add..80e0c700cecf 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -881,11 +881,16 @@ static inline bool cpu_has_pan(void)
>  #ifdef CONFIG_ARM64_AMU_EXTN
>  /* Check whether the cpu supports the Activity Monitors Unit (AMU) */
>  extern bool cpu_has_amu_feat(int cpu);
> +extern bool cpu_has_broken_amu_constcnt(void);
>  #else
>  static inline bool cpu_has_amu_feat(int cpu)
>  {
>  	return false;
>  }
> +static inline bool cpu_has_broken_amu_constcnt(void)
> +{
> +	return false;
> +}
>  #endif
>  
>  /* Get a cpu that supports the Activity Monitors Unit (AMU) */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 42ea2bd856c6..b9e4b2bd2c63 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1791,6 +1791,19 @@ int get_cpu_with_amu_feat(void)
>  	return cpumask_any(&amu_cpus);
>  }
>  
> +bool cpu_has_broken_amu_constcnt(void)
> +{
> +	/* List of CPUs which have broken AMEVCNTR01 (constant counter) */
> +	static const struct midr_range cpus[] = {
> +#ifdef CONFIG_ARM64_ERRATUM_2457168
> +		MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1),
> +#endif
> +		{},
> +	};
> +
> +	return is_midr_in_range(read_cpuid_id(), cpus);
> +}

I'd rather not have this in cpufeature.c as it's not really a feature.
We have some precedent with checking errata in cpufeature.c but IIRC we
did that only to check whether to enable a feature or not in that file
(DBM).

> +
>  static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
>  {
>  	if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 9ab78ad826e2..d4b0b0a40515 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -127,7 +127,8 @@ int __init parse_acpi_topology(void)
>  
>  #ifdef CONFIG_ARM64_AMU_EXTN
>  #define read_corecnt()	read_sysreg_s(SYS_AMEVCNTR0_CORE_EL0)
> -#define read_constcnt()	read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0)
> +#define read_constcnt()	(cpu_has_broken_amu_constcnt() ? 0UL : \
> +			read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0))

How often is this called? You end up reading the cpuid, comparing the
range on each call. I guess you can't use a cpucap in the arm64_errata[]
array as you want a check per-CPU? Does it matter if we return 0UL on
for all CPUs if one is affected?

-- 
Catalin