From mboxrd@z Thu Jan 1 00:00:00 1970 From: James.Bottomley@HansenPartnership.com (James Bottomley) Date: Sat, 02 Jan 2016 08:17:51 -0800 Subject: [Question about DMA] Consistent memory? In-Reply-To: <20160102103948.GR8644@n2100.arm.linux.org.uk> References: <20160102103948.GR8644@n2100.arm.linux.org.uk> Message-ID: <1451751471.2323.3.camel@HansenPartnership.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Sat, 2016-01-02 at 10:39 +0000, Russell King - ARM Linux wrote: > On Thu, Dec 31, 2015 at 04:50:54PM +0900, Masahiro Yamada wrote: > > Hi. > > > > I am new to the Linux DMA APIs. > > > > First, I started by reading Documentation/DMA-API.txt, > > but I am confused with the term "consistent memory". > > Just read "coherent memory" instead - the documentation confusingly > uses the two terms to refer to the same thing. I think there was a > patch a while back to replace "consistent" with "coherent" in this > document, though I'm not sure what happened to it. It's an standards issue. The Document was originally based on the PCI DMA API. All the PCI standards documentation refers to "consistent memory" instead of "coherent memory". The original DMA API was designed for PA-RISC and its standards documentation refers to "coherent memory" hence the confusion. The two terms are equivalent, but there's no real way of removing either without someone reading the actual specs and wondering what the other term means. James From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751947AbcABQR5 (ORCPT ); Sat, 2 Jan 2016 11:17:57 -0500 Received: from bedivere.hansenpartnership.com ([66.63.167.143]:40476 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751693AbcABQRx (ORCPT ); Sat, 2 Jan 2016 11:17:53 -0500 Message-ID: <1451751471.2323.3.camel@HansenPartnership.com> Subject: Re: [Question about DMA] Consistent memory? From: James Bottomley To: Russell King - ARM Linux , Masahiro Yamada Cc: Linux Kernel Mailing List , dmaengine@vger.kernel.org, Lars-Peter Clausen , Vinod Koul , Nicolas Ferre , Christoph Hellwig , Dan Williams , Sumit Semwal , linux-arm-kernel Date: Sat, 02 Jan 2016 08:17:51 -0800 In-Reply-To: <20160102103948.GR8644@n2100.arm.linux.org.uk> References: <20160102103948.GR8644@n2100.arm.linux.org.uk> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 2016-01-02 at 10:39 +0000, Russell King - ARM Linux wrote: > On Thu, Dec 31, 2015 at 04:50:54PM +0900, Masahiro Yamada wrote: > > Hi. > > > > I am new to the Linux DMA APIs. > > > > First, I started by reading Documentation/DMA-API.txt, > > but I am confused with the term "consistent memory". > > Just read "coherent memory" instead - the documentation confusingly > uses the two terms to refer to the same thing. I think there was a > patch a while back to replace "consistent" with "coherent" in this > document, though I'm not sure what happened to it. It's an standards issue. The Document was originally based on the PCI DMA API. All the PCI standards documentation refers to "consistent memory" instead of "coherent memory". The original DMA API was designed for PA-RISC and its standards documentation refers to "coherent memory" hence the confusion. The two terms are equivalent, but there's no real way of removing either without someone reading the actual specs and wondering what the other term means. James