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

From: Suzuki K Poulose <[email-masked]>
Date: Tue, 8 Oct 2019 16:32:01 +0100

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 <[email-masked]>
>> Sent: 2019年10月1日 21:19
>> To: Will Deacon <[email-masked]>
>> Cc: Justin He (Arm Technology China) <[email-masked]>; Catalin
>> Marinas <[email-masked]>; Mark Rutland
>> <[email-masked]>; James Morse <[email-masked]>;
>> Matthew Wilcox <[email-masked]>; Kirill A. Shutemov
>> <[email-masked]>; [email-masked];
>> [email-masked]; [email-masked]; Punit Agrawal
>> <[email-masked]>; Thomas Gleixner <[email-masked]>;
>> Andrew Morton <[email-masked]>; [email-masked]; Kaly
>> Xin (Arm Technology China) <[email-masked]>
>> 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 <[email-masked]> 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 <[email-masked]>
>>>> Suggested-by: Suzuki Poulose <[email-masked]>
>>>> Reviewed-by: Catalin Marinas <[email-masked]>
>>>> ---
>>>>   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

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