From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757272Ab3BKNOQ (ORCPT ); Mon, 11 Feb 2013 08:14:16 -0500 Received: from multi.imgtec.com ([194.200.65.239]:44832 "EHLO multi.imgtec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757086Ab3BKNOP (ORCPT ); Mon, 11 Feb 2013 08:14:15 -0500 Message-ID: <5118EA3B.9010904@imgtec.com> Date: Mon, 11 Feb 2013 12:55:23 +0000 From: James Hogan User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 MIME-Version: 1.0 To: James Bottomley CC: , Subject: Re: [RESEND PATCH] scsi: make struct scsi_varlen_cdb_hdr packed References: <1349946933-30314-1-git-send-email-james.hogan@imgtec.com> <1349951069.2425.44.camel@dabdike.int.hansenpartnership.com> <5076AE3B.5040509@imgtec.com> <1349960296.2425.53.camel@dabdike.int.hansenpartnership.com> <5076D360.2010804@imgtec.com> In-Reply-To: <5076D360.2010804@imgtec.com> X-Enigmail-Version: 1.4.6 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [192.168.154.65] X-SEF-Processed: 7_3_0_01181__2013_02_11_12_55_24 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi James, On 11/10/12 15:10, James Hogan wrote: > On 11/10/12 13:58, James Bottomley wrote: >> On Thu, 2012-10-11 at 12:32 +0100, James Hogan wrote: >>> On 11/10/12 11:24, James Bottomley wrote: >>>> On Thu, 2012-10-11 at 10:15 +0100, James Hogan wrote: >>>>> The struct scsi_varlen_cdb_hdr is expected to be exactly 10 bytes when >>>>> used in struct osd_cdb_head, but it isn't marked as packed. Some >>>>> architectures will round the struct size up which triggers BUILD_BUG_ON >>>>> compile errors in osd_initiator.c when the outer structs are unexpected >>>>> sizes. This is fixed by marking struct scsi_varlen_cdb_hdr as __packed. >>>> >>>> What actual problem have you encountered? The structure is {u8[8], u16} >>>> which is naturally packed on every architecture I know about. I've even >>>> built osd_initiator without problem on parisc, which has some of the >>>> most rigid alignment rules I've seen. >>> >>> Hi James, >>> >>> The alignment is fine (the offset of the u16 is 8 bytes), but >>> unfortunately with the metag port of gcc, sizeof(struct >>> scsi_varlen_cdb_hdr) is rounded up to a 4 byte boundary (even though the >>> largest data member alignment is only 2 bytes), which is 12 bytes >>> instead of 10. >> >> That sounds to be a bug in your compiler ... it shouldn't be rounding up >> structure sizes if the structure can fit in 10 bytes. This isn't >> happening in any other architecture that I know of (otherwise we'd have >> had a reported build break). > > This was my initial thought, and I share your feeling that this isn't > ideal compiler behaviour, however it's pretty much set in stone as part > of our ABI now. Having talked to one of our compiler folk about it > here's what he had to say: >> In GCC 4.2 the following backends have STRUCTURE_SIZE_BOUNDARY set to >=32 which will trigger this behaviour: >> >> Alpha+unicos >> Arm prior to AAPCS >> Mt >> >> This is within standards to my knowledge. What are the chances of getting this patch accepted? I believe it is correct and the compiler behaviour is within standards, and hopefully arch/metag will be merged for v3.9 so without this patch the build of osd_initiator.c will be broken on an in-tree architecture. Thanks James