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

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Message-ID: <e4fc5be5-8594-121a-198b-17ca7486dd3a@arm.com>
Date:   Tue, 8 Oct 2019 16:32:01 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     "Justin He (Arm Technology China)" <Justin.He@....com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        "Kaly Xin (Arm Technology China)" <Kaly.Xin@....com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        James Morse <James.Morse@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        "hejianet@...il.com" <hejianet@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v10 1/3] arm64: cpufeature: introduce helper
 cpu_has_hw_af()



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@...nel.org>
>> Sent: 2019年10月1日 21:19
>> To: Will Deacon <will@...nel.org>
>> Cc: Justin He (Arm Technology China) <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 (Arm Technology China) <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.
>>
> 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