linux-kernel - Re: [question] extract the feature bits width to 4

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Message-ID: <e110a4fa-2537-9ac0-0dc1-dded5d88f34f@huawei.com>
Date:   Tue, 28 Nov 2017 20:38:39 +0800
From:   gengdongjiu <gengdongjiu@...wei.com>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>
CC:     "mark.rutland@....com" <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [question] extract the feature bits width to 4


On 2017/11/28 19:40, Suzuki K Poulose wrote:
> Cc: linux-arm-kernel
> 
> On 28/11/17 11:17, gengdongjiu wrote:
>> Hi,suzuki/mark,
> 
> Hello!
> 
> Please Cc linux-arm-kernel mailing list in the future for any arm/arm64 kernel
> related queries.
Thanks a lot for the reply, Ok

> 
>>
>>    very sorry to disturb you, I have a question that want to consult with you. For the CPU feature detection,
>> why we use extract 4 bits width for the feature match instead of the actual bits number[1]?  may be the actual hardware feature bit more than 4 bits.
> 
> 
> There are features which could be more or less than 4bit wide. However,
> all the arm64_cpu_capabilities features are 4bits and hence the hard coded
> bits there. If we ever come across handling capabilities which are not
> standard size, we could add the necessary code to support it.
yes, normally the arm64_cpu_capabilities features are 4bits. However, in my
platform, there is a private CPU feature which is more than 4bits.

> 
> Are you planning to add something which does require the change ? Please
> feel free to send patches.

If I support this feature, I have to change it and not use 4bits. This CPU
feature is private, I does not want to upstream the whole feature.

If I only send the change about using actual width instead of 4 bits when detection feature ,
I am not sure you are agree.


> 
> Cheers
> 
> Suzuki
> 
> 
>> thanks!
>>
>> static inline int __attribute_const__
>> cpuid_feature_extract_field(u64 features, int field, bool sign)
>> {
>>          return cpuid_feature_extract_field_width(features, field, 4, sign);
>> }
>>
>> static bool
>> feature_matches(u64 reg, const struct arm64_cpu_capabilities *entry)
>> {
>>          int val = cpuid_feature_extract_field(reg, entry->field_pos, entry->sign);
>>
>>          return val >= entry->min_field_value;
>> }
>>
>>
> 
> 
> .
>