From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ie0-f169.google.com ([209.85.223.169]:33663 "EHLO mail-ie0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751874AbaEHASS (ORCPT ); Wed, 7 May 2014 20:18:18 -0400 Received: by mail-ie0-f169.google.com with SMTP id rl12so1874455iec.0 for ; Wed, 07 May 2014 17:18:18 -0700 (PDT) Date: Wed, 7 May 2014 18:18:14 -0600 From: Bjorn Helgaas To: Arnd Bergmann 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 Message-ID: <20140508001814.GB4951@google.com> References: <20140506223250.17968.27054.stgit@bhelgaas-glaptop.roam.corp.google.com> <20140506224840.17968.29010.stgit@bhelgaas-glaptop.roam.corp.google.com> <45161364.3Gyo1xJJLV@wuerfel> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <45161364.3Gyo1xJJLV@wuerfel> Sender: linux-pci-owner@vger.kernel.org List-ID: 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? Bjorn