From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.kundenserver.de ([212.227.126.187]:54643 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753743AbaEHKot (ORCPT ); Thu, 8 May 2014 06:44:49 -0400 From: Arnd Bergmann To: Bjorn Helgaas Cc: linux-doc@vger.kernel.org, Greg Kroah-Hartman , Joerg Roedel , Randy Dunlap , Liviu Dudau , linux-kernel@vger.kernel.org, James Bottomley , linux-pci@vger.kernel.org, David Woodhouse Subject: Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments Date: Thu, 08 May 2014 12:44:12 +0200 Message-ID: <15249748.Q2CaU9aBE9@wuerfel> In-Reply-To: <20140508001814.GB4951@google.com> References: <20140506223250.17968.27054.stgit@bhelgaas-glaptop.roam.corp.google.com> <45161364.3Gyo1xJJLV@wuerfel> <20140508001814.GB4951@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Sender: linux-pci-owner@vger.kernel.org List-ID: On Wednesday 07 May 2014 18:18:14 Bjorn Helgaas wrote: > On Wed, May 07, 2014 at 09:58:58AM +0200, Arnd Bergmann wrote: > > On Tuesday 06 May 2014 16:48:40 Bjorn Helgaas wrote: > > > Convert the "iova" arguments of iommu_map(), iommu_unmap(), etc., from > > > "unsigned long" to dma_addr_t. > > > > > > bb5547acfcd8 ("iommu/fsl: Make iova dma_addr_t in the iommu_iova_to_phys > > > API") did this for iommu_iova_to_phys(), but didn't fix the rest of the > > > IOMMU API. > > > > This patch looks 100% correct, but I'm not convinced it's a good idea: > > On 32-bit platforms (i.e. most of the ones you change), doing 64-bit > > arithmetic has a noticeable overhead. I am not aware of an IOMMU that > > actually uses 64-bit DMA addresses on its slave side, usually they > > are used to translate addresses from 32-bit masters into 64-bit > > memory addresses, so using 'unsigned long' seems better from a practical > > point of view as opposed to the strict type correctness. > > The current x86 IOMMUs support DMA addresses larger than 32 bits, but > obviously those platforms usually run 64-bit kernels so "unsigned > long" is already 64 bits. > > I guess you're thinking about cases where "unsigned long" is 32 bits, > the IOMMU only supports 32 bit DMA addresses, and dma_addr_t is 64 > bits. If the IOMMU only supports 32-bit DMA addresses, is there any > value in having a 64-bit dma_addr_t? Two cases: a) You can have a system with some DMA masters that are 64-bit capable, and other masters that can only do 32-bit DMA and for this reason use an IOMMU. I expect to see more of these in the future. b) We build kernels that run on a multitude of ARM32 platforms these days. We have some that require a 32-bit dma_addr_t and some that don't (because they always use swiotlb or an IOMMU). Arnd