Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751686AbaBCKaj (ORCPT ); Mon, 3 Feb 2014 05:30:39 -0500 Received: from mail-pd0-f174.google.com ([209.85.192.174]:40020 "EHLO mail-pd0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750900AbaBCKah (ORCPT ); Mon, 3 Feb 2014 05:30:37 -0500 Message-ID: <52EF6FCE.8090704@gmail.com> Date: Mon, 03 Feb 2014 18:30:38 +0800 From: Chen Gang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: James Hogan CC: David Laight , "'Dan Carpenter'" , "devel@driverdev.osuosl.org" , "andreas.dilger@intel.com" , Antonio Quartulli , Greg KH , "bergwolf@gmail.com" , "linux-kernel@vger.kernel.org" , David Miller , "oleg.drokin@intel.com" , "jacques-charles.lafoucriere@cea.fr" , "jinshan.xiong@intel.com" , netdev , "linux-metag@vger.kernel.org" Subject: Re: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union References: <20140118142404.GT7444@mwanda> <52DBA3D4.3090308@gmail.com> <52DD0EFF.2010305@imgtec.com> <20140120123045.GV7444@mwanda> <52DD18A5.1090308@imgtec.com> <20140120125603.GD4815@mwanda> <20140120211356.GG4815@mwanda> <52DE4DA3.7090301@imgtec.com> <52E3A642.7010307@gmail.com> <52ECFD53.7010401@gmail.com> <20140203085855.GA26722@mwanda> <063D6719AE5E284EB5DD2968C1650D6D0F6B772B@AcuExch.aculab.com> <52EF6DCC.6040807@imgtec.com> In-Reply-To: <52EF6DCC.6040807@imgtec.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/03/2014 06:22 PM, James Hogan wrote: > On 03/02/14 10:05, David Laight wrote: >> From: Dan Carpenter >>> On Sat, Feb 01, 2014 at 09:57:39PM +0800, Chen Gang wrote: >>>> It seems, our kernel still stick to treate 'pack' region have effect >>>> with both 'align' and 'sizeof'. >>> >>> It's not about packed regions. It's about unions. It's saying the >>> sizeof() a union is a multiple of 4 unless it's packed. >>> >>> union foo { >>> short x; >>> short y; >>> }; >>> >>> The author intended the sizeof(union foo) to be 2 but on metag arch then >>> it is 4. >> >> The same is probably be true of: struct foo { _u16 bar; }; > > Yes indeed. > >> Architectures that define such alignment rules are a right PITA. >> You either need to get the size to 2 without using 'packed', or >> just not define such structures. >> It is worth seeing if adding aligned(2) will change the size - I'm >> not sure. > > __aligned(2) alone doesn't seem to have any effect on sizeof() or > __alignof__() unless it is accompanied by __packed. x86_64 is similar in > that respect (it just packs sanely in the first place). > > Combining __packed with __aligned(2) does the trick though (__packed > alone sets __aligned(1) which is obviously going to be suboptimal). > Oh, thank you for your explanation. And hope this feature issue can be fixed, and satisfy both kernel and ABI. :-) Thanks. -- Chen Gang Open, share and attitude like air, water and life which God blessed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/