Re: [PATCH v10 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af()

[Suzuki K Poulose]

On 08/10/2019 02:12, Justin He (Arm Technology China) wrote:
> Hi Will and Marc
> Sorry for the late response, just came back from a vacation.
>
> > -----Original Message-----
> > From: Marc Zyngier <maz@xxxxxxxxxx>
> > Sent: 2019年10月1日 21:19
> > To: Will Deacon <will@xxxxxxxxxx>
> > Cc: Justin He (Arm Technology China) <Justin.He@xxxxxxx>; Catalin
> > Marinas <Catalin.Marinas@xxxxxxx>; Mark Rutland
> > <Mark.Rutland@xxxxxxx>; James Morse <James.Morse@xxxxxxx>;
> > Matthew Wilcox <willy@xxxxxxxxxxxxx>; Kirill A. Shutemov
> > <kirill.shutemov@xxxxxxxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; Punit Agrawal
> > <punitagrawal@xxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>;
> > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; hejianet@xxxxxxxxx; Kaly
> > Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>
> > Subject: Re: [PATCH v10 1/3] arm64: cpufeature: introduce helper
> > cpu_has_hw_af()
> >
> > On Tue, 1 Oct 2019 13:54:47 +0100
> > Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > > On Mon, Sep 30, 2019 at 09:57:38AM +0800, Jia He wrote:
> > > > We unconditionally set the HW_AFDBM capability and only enable it on
> > > > CPUs which really have the feature. But sometimes we need to know
> > > > whether this cpu has the capability of HW AF. So decouple AF from
> > > > DBM by new helper cpu_has_hw_af().
> > > >
> > > > Signed-off-by: Jia He <justin.he@xxxxxxx>
> > > > Suggested-by: Suzuki Poulose <Suzuki.Poulose@xxxxxxx>
> > > > Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > > ---
> > > >   arch/arm64/include/asm/cpufeature.h | 10 ++++++++++
> > > >   1 file changed, 10 insertions(+)
> > > >
> > > > diff --git a/arch/arm64/include/asm/cpufeature.h
> > b/arch/arm64/include/asm/cpufeature.h
> > > > index 9cde5d2e768f..949bc7c85030 100644
> > > > --- a/arch/arm64/include/asm/cpufeature.h
> > > > +++ b/arch/arm64/include/asm/cpufeature.h
> > > > @@ -659,6 +659,16 @@ static inline u32
> > id_aa64mmfr0_parange_to_phys_shift(int parange)
> > > >    default: return CONFIG_ARM64_PA_BITS;
> > > >    }
> > > >   }
> > > > +
> > > > +/* Check whether hardware update of the Access flag is supported */
> > > > +static inline bool cpu_has_hw_af(void)
> > > > +{
> > > > + if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
> > > > +         return read_cpuid(ID_AA64MMFR1_EL1) & 0xf;
> > >
> > > 0xf? I think we should have a mask in sysreg.h for this constant.
> >
> > We don't have the mask, but we certainly have the shift.
> >
> > GENMASK(ID_AA64MMFR1_HADBS_SHIFT + 3,
> > ID_AA64MMFR1_HADBS_SHIFT) is a bit
> > of a mouthful though. Ideally, we'd have a helper for that.
> Ok, I will implement the helper if there isn't so far.
> And then replace the 0xf with it.

Or could we simpl reuse existing cpuid_feature_extract_unsigned_field() ?

u64 mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);

return cpuid_feature_extract_unsigned_field(mmfr1, ID_AA64MMFR1_HADBS_SHIFT) ?

Cheers
Suzuki