Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af()

[Catalin Marinas <catalin.marinas@arm.com>]

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