From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754643AbXDXUoI (ORCPT ); Tue, 24 Apr 2007 16:44:08 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754645AbXDXUoH (ORCPT ); Tue, 24 Apr 2007 16:44:07 -0400 Received: from mga09.intel.com ([134.134.136.24]:10151 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754642AbXDXUoD (ORCPT ); Tue, 24 Apr 2007 16:44:03 -0400 X-ExtLoop1: 1 X-IronPort-AV: i="4.14,448,1170662400"; d="scan'208"; a="78534032:sNHT20621682" Date: Tue, 24 Apr 2007 13:42:08 -0700 From: Ashok Raj To: Andi Kleen Cc: Ashok Raj , linux-kernel@vger.kernel.org, akpm@osdl.org, gregkh@suse.de, muli@il.ibm.com, asit.k.mallick@intel.com, suresh.b.siddha@intel.com, anil.s.keshavamurthy@intel.com, arjan@linux.intel.com, shaohua.li@intel.com Subject: Re: [Intel IOMMU][patch 4/8] Supporting Zero Length Reads in Intel IOMMU. Message-ID: <20070424204208.GD27911@linux-os.sc.intel.com> References: <20070424060259.426374000@intel.com> <20070424061037.821838000@intel.com> <200704242128.12057.ak@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200704242128.12057.ak@suse.de> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 24, 2007 at 09:28:11PM +0200, Andi Kleen wrote: > On Tuesday 24 April 2007 08:03:03 Ashok Raj wrote: > > PCI specs permit zero length reads (ZLR) even if the mapping for that region > > is write only. Support for this feature is indicated by the presence of a bit > > in the DMAR capability. If a particular DMAR does not support this capability > > we map write-only regions as read-write. > > > > This option can also provides a workaround for some drivers that request > > a write-only mapping when they really should request a read-write. > > (We ran into one such case in eepro100.c in handling rx_ring_dma) > > Better just fix the drivers instead of adding such hacks Some of the early DMAR's dont handle zero-length-reads as required. hardware that supports it correctly will advertise via its capabilities. We could remove the cmdline option since it should not be really required.