Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751999AbaBCKWS (ORCPT ); Mon, 3 Feb 2014 05:22:18 -0500 Received: from multi.imgtec.com ([194.200.65.239]:11215 "EHLO multi.imgtec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751258AbaBCKWQ (ORCPT ); Mon, 3 Feb 2014 05:22:16 -0500 Message-ID: <52EF6DCC.6040807@imgtec.com> Date: Mon, 3 Feb 2014 10:22:04 +0000 From: James Hogan User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: David Laight CC: "'Dan Carpenter'" , Chen Gang , "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> In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6D0F6B772B@AcuExch.aculab.com> X-Enigmail-Version: 1.5.2 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-Originating-IP: [192.168.154.65] X-SEF-Processed: 7_3_0_01192__2014_02_03_10_22_05 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). Cheers James -- 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/