From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932412AbdK1LkK (ORCPT ); Tue, 28 Nov 2017 06:40:10 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:51848 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932291AbdK1LkI (ORCPT ); Tue, 28 Nov 2017 06:40:08 -0500 Subject: Re: [question] extract the feature bits width to 4 To: gengdongjiu References: <17ab0a11-7ca2-ff22-35d6-7446dbccc3ea@huawei.com> Cc: "mark.rutland@arm.com" , Linux Kernel Mailing List , "linux-arm-kernel@lists.infradead.org" From: Suzuki K Poulose Message-ID: Date: Tue, 28 Nov 2017 11:40:05 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <17ab0a11-7ca2-ff22-35d6-7446dbccc3ea@huawei.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > 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. Are you planning to add something which does require the change ? Please feel free to send patches. 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; > } > >