Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754795Ab1FSVjh (ORCPT ); Sun, 19 Jun 2011 17:39:37 -0400 Received: from relais.videotron.ca ([24.201.245.36]:25548 "EHLO relais.videotron.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752929Ab1FSVje (ORCPT ); Sun, 19 Jun 2011 17:39:34 -0400 MIME-version: 1.0 Content-transfer-encoding: 7BIT Content-type: TEXT/PLAIN; charset=US-ASCII Date: Sun, 19 Jun 2011 17:39:33 -0400 (EDT) From: Nicolas Pitre X-X-Sender: nico@xanadu.home To: Arnd Bergmann Cc: linux-arm-kernel@lists.infradead.org, Alan Stern , gregkh@suse.de, linux-usb@vger.kernel.org, lkml , Rabin Vincent , Alexander Holler Subject: Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute In-reply-to: <201106192202.23989.arnd@arndb.de> Message-id: References: <201106192202.23989.arnd@arndb.de> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3359 Lines: 92 On Sun, 19 Jun 2011, Arnd Bergmann wrote: > On Sunday 19 June 2011 21:00:01 Alan Stern wrote: > > On Sun, 19 Jun 2011, Nicolas Pitre wrote: > > > On Thu, 16 Jun 2011, Arnd Bergmann wrote: > > > > On Thursday 16 June 2011 22:10:53 Alexander Holler wrote: > > > > At least I would be happier without the patch. I'm trying to convince > > > > people to not use these attributes unless required because too much > > > > harm is done when they are used without understanding the full > > > > consequences. I also recommend using __packed as localized as possible, > > > > i.e. set it for the members that need it, not the entire struct. > > > > > > > > I agree that your patch is harmless, it's just the opposite of > > > > a cleanup in my opinion. > > > > > > The question is: does the structure really has to be packed? > > > > What do you mean? The structure really does need to be allocated > > without padding between the fields; is that the same thing? So do a > > bunch of other structures that currently have no annotations at all. > > I guess the issue is that some ABIs actually require a minimum alignment, > like the old ARM ABI that you can still use to build the kernel. > > If a structure is not a multiple of four bytes in size, that ABI > will add padding at the end, e.g. in > > struct s { > char c[2]; > }; > > struct t { > struct s t1; > unsigned short t2[3]; > }; > > On most architectures, struct s will be two bytes in size and one byte > aligned, while struct t is eight bytes and two byte aligned. > > On ARM oABI, struct s ends up with four byte size and alignment while > struct t is twelve bytes long. All this is ok for regular structures, > but not when they are used to describe memory layout of hardware > registers on on-wire packets. Agreed. Is that the case with EHCI though? In your example, you'd have to mark that structure as packed,aligned(2). > > > If it does, then the follow-up question is: is a packing on word > > > boundaries sufficient? > > > > > If the answer is yes in both cases, then having packed,aligned(4) is not > > > a frivolity but rather a correctness issue. > > > > Why so? Current systems work just fine without it. > > I think Nicolas got it backwards here, adding both packed and > aligned(4) would make a structure like the one above consistently > incorrect when used to describe a tightly packed hardware structure. I didn't look at the details, but your example above requires aligned(2). That tells the compiler that it may use instructions to access the data (or hardware) that are up to 2-byte wide (on ARM that means STRh/LDRH) for the data of that width instead of the byte per byte loads. > > In this case, we would have to do > > struct s { > char c[2]; > } __packed; > > struct t { > struct s t1; > unsigned short t2[3] __aligned(2); > } __packed; > > To tell the compiler that t2 is indeed aligned, while struct t > is packed to include no padding around t. ... and give the aligned(2) attribute to struct t so those shorts are accessed with a 16-bit wide load/store not byte loads/stores. Nicolas -- 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/