From owner-freebsd-hackers@freebsd.org Mon Nov 9 23:34:39 2015 Return-Path: Delivered-To: freebsd-hackers@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 5F5D2A2A9CC for ; Mon, 9 Nov 2015 23:34:39 +0000 (UTC) (envelope-from jmg@gold.funkthat.com) Received: from gold.funkthat.com (gate2.funkthat.com [208.87.223.18]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "gold.funkthat.com", Issuer "gold.funkthat.com" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id 4521C11A9 for ; Mon, 9 Nov 2015 23:34:39 +0000 (UTC) (envelope-from jmg@gold.funkthat.com) Received: from gold.funkthat.com (localhost [127.0.0.1]) by gold.funkthat.com (8.14.5/8.14.5) with ESMTP id tA9NYcR3076256 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 9 Nov 2015 15:34:38 -0800 (PST) (envelope-from jmg@gold.funkthat.com) Received: (from jmg@localhost) by gold.funkthat.com (8.14.5/8.14.5/Submit) id tA9NYctW076255; Mon, 9 Nov 2015 15:34:38 -0800 (PST) (envelope-from jmg) Date: Mon, 9 Nov 2015 15:34:38 -0800 From: John-Mark Gurney To: AlexHully Cc: freebsd-hackers@freebsd.org Subject: Re: dma Message-ID: <20151109233438.GM65715@funkthat.com> References: <298361446978177@web22g.yandex.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <298361446978177@web22g.yandex.ru> X-Operating-System: FreeBSD 9.1-PRERELEASE amd64 X-PGP-Fingerprint: 54BA 873B 6515 3F10 9E88 9322 9CB1 8F74 6D3F A396 X-Files: The truth is out there X-URL: http://resnet.uoregon.edu/~gurney_j/ X-Resume: http://resnet.uoregon.edu/~gurney_j/resume.html X-TipJar: bitcoin:13Qmb6AeTgQecazTWph4XasEsP7nGRbAPE X-to-the-FBI-CIA-and-NSA: HI! HOW YA DOIN? can i haz chizburger? User-Agent: Mutt/1.5.21 (2010-09-15) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.7 (gold.funkthat.com [127.0.0.1]); Mon, 09 Nov 2015 15:34:38 -0800 (PST) X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 09 Nov 2015 23:34:39 -0000 AlexHully wrote this message on Sun, Nov 08, 2015 at 11:22 +0100: > I would like a clarification for dma. > > The context: most dma capable devices have 32 bits address range. Depends upon the bus, and now w/ PCIe, 64bit is commonly supported... > Is it correct that, if there was no 3G/1G mapping in the kernel, or that kernel low memory could map 4Gb memory, one could choose any free addresses in that low memory to set up a dma buffer? I assume you're talking about i386... If you are, the kernel/userland mapping split has nothing to do w/ DMA... the 3G/1G split is a virtual address space construct and DMA purely deals w/ the physical memory layout. Or are you talking about the 3G/1G physical memory, PCI device hole? On non-PAE i386 systems (with only 32bit physical addressing), there is no performance impact when dealing w/ a 32bit only capabile DMA device as all of physical memory is addressable. -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."