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    SubjectRe: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af()
    On Wed, Sep 18, 2019 at 03:20:41PM +0100, Suzuki K Poulose wrote:
    > On 18/09/2019 14:19, Jia He wrote:
    > > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
    > > index c96ffa4722d3..206b6e3954cf 100644
    > > --- a/arch/arm64/include/asm/cpufeature.h
    > > +++ b/arch/arm64/include/asm/cpufeature.h
    > > @@ -390,6 +390,7 @@ extern DECLARE_BITMAP(boot_capabilities, ARM64_NPATCHABLE);
    > > for_each_set_bit(cap, cpu_hwcaps, ARM64_NCAPS)
    > > bool this_cpu_has_cap(unsigned int cap);
    > > +bool cpu_has_hw_af(void);
    > > void cpu_set_feature(unsigned int num);
    > > bool cpu_have_feature(unsigned int num);
    > > unsigned long cpu_get_elf_hwcap(void);
    > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
    > > index b1fdc486aed8..c5097f58649d 100644
    > > --- a/arch/arm64/kernel/cpufeature.c
    > > +++ b/arch/arm64/kernel/cpufeature.c
    > > @@ -1141,6 +1141,12 @@ static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap,
    > > return true;
    > > }
    > > +/* Decouple AF from AFDBM. */
    > > +bool cpu_has_hw_af(void)
    > > +{
    > Sorry for not having asked this earlier. Are we interested in,
    >
    > "whether *this* CPU has AF support ?" or "whether *at least one*
    > CPU has the AF support" ? The following code does the former.
    >
    > > + return (read_cpuid(ID_AA64MMFR1_EL1) & 0xf);

    In a non-preemptible context, the former is ok (per-CPU).

    --
    Catalin

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