linux-kernel - Re: [PATCH v10 1/3] arm64: cpufeature: introduce helper cpu_has_hw

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Message-ID: <20191001141848.762296bd@why>
Date:   Tue, 1 Oct 2019 14:18:48 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Jia He <justin.he@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Punit Agrawal <punitagrawal@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, hejianet@...il.com,
        Kaly Xin <Kaly.Xin@....com>
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@...nel.org> 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@....com>
> > Suggested-by: Suzuki Poulose <Suzuki.Poulose@....com>
> > Reviewed-by: Catalin Marinas <catalin.marinas@....com>
> > ---
> >  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.

	M.
-- 
Without deviation from the norm, progress is not possible.