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SubjectRe: [PATCH v4 22/22] arm64: Add work around for Arm Cortex-A55 Erratum 1024718
On Tue, Mar 13, 2018 at 11:51:20AM +0000, Suzuki K Poulose wrote:
> Some variants of the Arm Cortex-55 cores (r0p0, r0p1, r1p0) suffer
> from an erratum 1024718, which causes incorrect updates when DBM/AP
> bits in a page table entry is modified without a break-before-make
> sequence. The work around is to skip enabling the hardware DBM feature
> on the affected cores. The hardware Access Flag management features
> is not affected. There are some other cores suffering from this
> errata, which could be added to the midr_list to trigger the work
> around.
>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: ckadabi@codeaurora.org
> Cc: Dave Martin <dave.martin@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>

[...]

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 4e73c610a409..0fc097e2b72e 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -963,9 +963,23 @@ static inline void __cpu_enable_hw_dbm(void)
> isb();
> }
>
> +static bool cpu_has_broken_dbm(void)
> +{
> + /* List of CPUs which have broken DBM support. */
> + static const struct midr_range cpus[] = {
> +#ifdef CONFIG_ARM64_ERRATUM_1024718
> + MIDR_RANGE(MIDR_CORTEX_A55, 0, 0, 1, 0), // A55 r0p0 -r1p0
> +#endif
> + {},
> + };
> +
> + return is_midr_in_range_list(read_cpuid_id(), cpus);
> +}
> +
> static bool cpu_can_use_dbm(const struct arm64_cpu_capabilities *cap)
> {
> - return has_cpuid_feature(cap, SCOPE_LOCAL_CPU);
> + return has_cpuid_feature(cap, SCOPE_LOCAL_CPU) &&
> + !cpu_has_broken_dbm();

This refactoring seems sensbile to me. The mapping between erratum IDs
and features that don't work is never guaranteed to be 1:1, so it
makes sense for the check function to have a generic name and to paste
in the relevant erratum ID ranges depending on the kernel config.

[...]

Reviewed-by: Dave Martin <Dave.Martin@arm.com>

Cheers
---Dave

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