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

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Message-ID: <17ab0a11-7ca2-ff22-35d6-7446dbccc3ea@huawei.com>
Date:   Tue, 28 Nov 2017 19:17:40 +0800
From:   gengdongjiu <gengdongjiu@...wei.com>
To:     <suzuki.poulose@....com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [question] extract the feature bits width to 4

Hi,suzuki/mark,

  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.
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;
}