Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752616Ab1FTTUq (ORCPT ); Mon, 20 Jun 2011 15:20:46 -0400 Received: from relais.videotron.ca ([24.201.245.36]:15086 "EHLO relais.videotron.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751106Ab1FTTUp (ORCPT ); Mon, 20 Jun 2011 15:20:45 -0400 MIME-version: 1.0 Content-transfer-encoding: 7BIT Content-type: TEXT/PLAIN; charset=US-ASCII Date: Mon, 20 Jun 2011 15:20:44 -0400 (EDT) From: Nicolas Pitre X-X-Sender: nico@xanadu.home To: Russell King - ARM Linux Cc: Arnd Bergmann , Alan Stern , gregkh@suse.de, linux-usb@vger.kernel.org, lkml , Rabin Vincent , Alexander Holler , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] USB: ehci: use packed, aligned(4) instead of removing the packed attribute In-reply-to: <20110620190240.GJ26089@n2100.arm.linux.org.uk> Message-id: References: <201106201858.45624.arnd@arndb.de> <20110620190240.GJ26089@n2100.arm.linux.org.uk> 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: 1285 Lines: 27 On Mon, 20 Jun 2011, Russell King - ARM Linux wrote: > On Mon, Jun 20, 2011 at 06:58:45PM +0200, Arnd Bergmann wrote: > > A recent change in gcc changed the default behaviour when compiling the > > ehci driver on ARM, but the behaviour was already nondeterministic > > because the definition of the readl/writel macros on ARM relies on > > unspecified behaviour (cast to pointer with larger aligment). > > It's unspecified behaviour period. If you pass a pointer to readl/writel > which is not word aligned, what you get back is anyones guess. The pointer _is_ aligned. The problem is that it comes from the address of a structure member, which structure is marked __packed. Older GCC would see the cast to unsigned int applied to that pointer within the ARM's readl()/writel() implementation and ignore that the source of the address is marked __packed, which __packed implies an alignment of 1. REcent GCC versions would honnor the alignment of 1 and perform the unsigned int * access using byte sized 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/