Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751883AbWB1RCy (ORCPT ); Tue, 28 Feb 2006 12:02:54 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751880AbWB1RCy (ORCPT ); Tue, 28 Feb 2006 12:02:54 -0500 Received: from sj-iport-4.cisco.com ([171.68.10.86]:50205 "EHLO sj-iport-4.cisco.com") by vger.kernel.org with ESMTP id S1751883AbWB1RCx (ORCPT ); Tue, 28 Feb 2006 12:02:53 -0500 X-IronPort-AV: i="4.02,153,1139212800"; d="scan'208"; a="1780511234:sNHT33151104" To: Jes Sorensen Cc: "Bryan O'Sullivan" , Andrew Morton , Andi Kleen , linux-kernel , Jesse Barnes Subject: Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining X-Message-Flag: Warning: May contain useful information References: <1140841250.2587.33.camel@localhost.localdomain> <44047565.3090202@sgi.com> From: Roland Dreier Date: Tue, 28 Feb 2006 09:02:47 -0800 In-Reply-To: <44047565.3090202@sgi.com> (Jes Sorensen's message of "Tue, 28 Feb 2006 17:08:05 +0100") Message-ID: User-Agent: Gnus/5.1007 (Gnus v5.10.7) XEmacs/21.4.17 (Jumbo Shrimp, linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 X-OriginalArrivalTime: 28 Feb 2006 17:02:48.0509 (UTC) FILETIME=[CD5A16D0:01C63C88] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1075 Lines: 21 Jes> Not quite correct as far as I understand it. mmiowb() is Jes> supposed to guarantee that writes to MMIO space have Jes> completed before continuing. That of course covers the Jes> multi-CPU case, but it should also cover the write-combining Jes> case. I don't believe this is correct. mmiowb() does not guarantee that writes have completed -- they may still be pending in a buffer in a bridge somewhere. The _only_ effect of mmiowb() is to make sure that writes which have been ordered between CPUs using some other mechanism (i.e. a lock) are properly ordered by the rest of the system. This only has an effect systems like very large ia64 systems, where (as I understand it), writes can pass each other on the way to the PCI bus. In fact, mmiowb() is a NOP on essentially every architecture. - R. - 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/