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 D8279C433DF for ; Thu, 13 Aug 2020 09:20:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BD54F2078B for ; Thu, 13 Aug 2020 09:20:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726604AbgHMJUr convert rfc822-to-8bit (ORCPT ); Thu, 13 Aug 2020 05:20:47 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:56591 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726334AbgHMJUr (ORCPT ); Thu, 13 Aug 2020 05:20:47 -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-156-hgKTbYm5MtyJduu9P-gWoA-1; Thu, 13 Aug 2020 10:20:44 +0100 X-MC-Unique: hgKTbYm5MtyJduu9P-gWoA-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 10:20:43 +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 10:20:43 +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: AQHWcFyE8iGJWCR9sU2N3RtiiWBxsak0KslwgAFy2oCAABdfEP//+uqAgAAS+MA= Date: Thu, 13 Aug 2020 09:20:43 +0000 Message-ID: <20ab5d871b154423a4dcefef1ddd64e2@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> <54216e492cec4f84bc43dee176130e89@AcuMS.aculab.com> <20200813090409.GA1080@yilunxu-OptiPlex-7050> In-Reply-To: <20200813090409.GA1080@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 10:04 > > On Thu, Aug 13, 2020 at 08:28:05AM +0000, David Laight wrote: > > 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.) > > I got it. Thanks for your detailed explanation. I think we may change them to > u32. Is it the same case for u8? Think we may also change the dfl_device_id.type. Loosely 'yes' but it isn't worth the churn of 'random' changes. And they aren't often passed to/from functions - which I'm 98% sure requires masking. I commented because the compiler was going to add pad fields after your u16 values - so you'd get do space saving and probably more code. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)