From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752530AbbDCPpf (ORCPT ); Fri, 3 Apr 2015 11:45:35 -0400 Received: from mail-ig0-f180.google.com ([209.85.213.180]:38661 "EHLO mail-ig0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752091AbbDCPpa (ORCPT ); Fri, 3 Apr 2015 11:45:30 -0400 Date: Fri, 3 Apr 2015 10:45:26 -0500 From: Bjorn Helgaas To: David Miller Cc: bjorn@helgaas.com, bjorn.helgaas@gmail.com, sparclinux@vger.kernel.org, linux-pci@vger.kernel.org, yinghai@kernel.org, david.ahern@oracle.com, linux-kernel@vger.kernel.org Subject: Re: d63e2e1f3df breaks sparc/T5-8 Message-ID: <20150403154526.GB10892@google.com> References: <20150327.145016.86183910134380870.davem@davemloft.net> <20150329.113250.2136683943336875789.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150329.113250.2136683943336875789.davem@davemloft.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Mar 29, 2015 at 11:32:50AM -0700, David Miller wrote: > From: Bjorn Helgaas > Date: Sun, 29 Mar 2015 08:30:40 -0500 > > > Help me understand the sparc64 situation: are you saying that BAR > > addresses, i.e., MMIO transactions from a CPU or a peer-to-peer DMA can be > > 64 bits, but a DMA to main memory can only be 32 bits? > > > > I assume this would work if we made dma_addr_t 64 bits on sparc64. What > > would be the cost of doing that? > > The cost is 4 extra bytes in every datastructure, kernel wide, that > stores DMA addresses. That much is fairly obvious. What I don't know is how much difference this makes in the end. > Don't use DMA addresses for PCI addresses. They are absolutely not > the same, especially when an IOMMU is always present because in that > case all DMA addresses are virtual and exist in a different realm > and set of constraints/restrictions. I'm still trying to figure out a clear description of how a DMA address is different from a PCI address. If you capture a transaction with a PCI analyzer, I don't think you can tell a DMA address from a PCI address unless you know how bridge windows are programmed. Even then, I'm not sure you can tell a CPU-generated PCI address from a DMA address in a device-generated peer-to-peer transaction. Bjorn