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Date:   Wed, 18 Sep 2019 07:20:17 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Jia He <justin.he@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        James Morse <james.morse@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Suzuki Poulose <Suzuki.Poulose@arm.com>,
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        Anshuman Khandual <anshuman.khandual@arm.com>,
        Jun Yao <yaojun8558363@gmail.com>,
        Alex Van Brunt <avanbrunt@nvidia.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Ralph Campbell <rcampbell@nvidia.com>, hejianet@gmail.com,
        Kaly Xin <Kaly.Xin@arm.com>
Subject: Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper
 cpu_has_hw_af()
Message-ID: <20190918142017.GC9880@bombadil.infradead.org>
References: <20190918131914.38081-1-justin.he@arm.com>
 <20190918131914.38081-2-justin.he@arm.com>
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On Wed, Sep 18, 2019 at 09:19:12PM +0800, Jia He wrote:
> +/* Decouple AF from AFDBM. */
> +bool cpu_has_hw_af(void)
> +{
> +	return (read_cpuid(ID_AA64MMFR1_EL1) & 0xf);
> +}
> +

Do you really want to call read_cpuid() every time?  I would have thought
you'd want to use the static branch mechanism to do the right thing at
boot time.  See Documentation/static-keys.txt.