From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E0952C433E3 for ; Thu, 13 Aug 2020 08:28:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CB0752053B for ; Thu, 13 Aug 2020 08:28:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726678AbgHMI2L convert rfc822-to-8bit (ORCPT ); Thu, 13 Aug 2020 04:28:11 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:38728 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726044AbgHMI2L (ORCPT ); Thu, 13 Aug 2020 04:28:11 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-207--2uOLIbwMSWiy-x8Xnehmg-1; Thu, 13 Aug 2020 09:28:06 +0100 X-MC-Unique: -2uOLIbwMSWiy-x8Xnehmg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Thu, 13 Aug 2020 09:28:05 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Thu, 13 Aug 2020 09:28:05 +0100 From: David Laight To: 'Xu Yilun' CC: 'Moritz Fischer' , "linux-fpga@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "trix@redhat.com" , "lgoncalv@redhat.com" Subject: RE: [PATCH v4 1/4] fpga: dfl: change data type of feature id to u16 Thread-Topic: [PATCH v4 1/4] fpga: dfl: change data type of feature id to u16 Thread-Index: AQHWcFyE8iGJWCR9sU2N3RtiiWBxsak0KslwgAFy2oCAABdfEA== Date: Thu, 13 Aug 2020 08:28:05 +0000 Message-ID: <54216e492cec4f84bc43dee176130e89@AcuMS.aculab.com> References: <1597027273-25288-1-git-send-email-yilun.xu@intel.com> <1597027273-25288-2-git-send-email-yilun.xu@intel.com> <20200812035604.GA2544@epycbox.lan> <3810fb75b42e45928a39a97449a01520@AcuMS.aculab.com> <20200813075843.GB7383@yilunxu-OptiPlex-7050> In-Reply-To: <20200813075843.GB7383@yilunxu-OptiPlex-7050> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Xu Yilun > Sent: 13 August 2020 08:59 > On Wed, Aug 12, 2020 at 08:52:39AM +0000, David Laight wrote: > > From: Moritz Fischer > > > Sent: 12 August 2020 04:56 > > > > > > On Mon, Aug 10, 2020 at 10:41:10AM +0800, Xu Yilun wrote: > > > > The feature id is stored in a 12 bit field in DFH. So a u16 variable is > > > > enough for feature id. > > > > > > > > This patch changes all feature id related places to fit u16. > > > > How much bigger does it make the kernel? > > The patch changes the definition of feature id from u64 to u16, and will > make the kernel slightly smaller. Unlikely. Most of the structures will gain a 'pad' field. Using u16 for function parameters and results almost certainly requires instructions to mask the value. Any arithmetic on u16 will require masking instructions on (probably) all architectures except x86. Using 'unsigned int' is probably best. u16 is never a good idea unless you are defining enough of them in a structure (eg as an array) to reduce the structure size below some threshold. (Or are matching some hardware layout.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)