Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752750AbaBCKfs (ORCPT ); Mon, 3 Feb 2014 05:35:48 -0500 Received: from mx0.aculab.com ([213.249.233.131]:44832 "HELO mx0.aculab.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1750965AbaBCKfp convert rfc822-to-8bit (ORCPT ); Mon, 3 Feb 2014 05:35:45 -0500 From: David Laight To: "'James Hogan'" 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 Thread-Topic: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union Thread-Index: AQHPH1WqrPdkB9hsu0CwpyENM0ZOOpqjPaqAgAARGPCAAAYjAIAAApBA Date: Mon, 3 Feb 2014 10:35:02 +0000 Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6B777A@AcuExch.aculab.com> 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> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.202.99.200] Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: James Hogan > 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). Compile some code for a cpu that doesn't support misaligned transfers (probably one of sparc, arm, ppc) and see if the compiler generates a single 16bit request or two 8 bits ones. You don't want the compiler generating multiple byte-sized memory transfers. David -- 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/