From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753934Ab0JMO4p (ORCPT ); Wed, 13 Oct 2010 10:56:45 -0400 Received: from moutng.kundenserver.de ([212.227.17.10]:52716 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753843Ab0JMO4o (ORCPT ); Wed, 13 Oct 2010 10:56:44 -0400 From: Arnd Bergmann To: Casey Dahlin Subject: Re: A question about ZONE_DMA Date: Wed, 13 Oct 2010 16:57:17 +0200 User-Agent: KMail/1.12.2 (Linux/2.6.35-16-generic; KDE/4.3.2; x86_64; ; ) Cc: linux-kernel@vger.kernel.org References: <20101013145240.GE12467@foucault.redhat.com> In-Reply-To: <20101013145240.GE12467@foucault.redhat.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201010131657.18469.arnd@arndb.de> X-Provags-ID: V02:K0:j0TQv4BarrNgeAPBFl3GLMakEMPXHM/ASIDqqPwenLo d+DXWaw5SVNlVYHBhOoi3my0UCYJTP/yx/7pGiH2XY6uKq34K1 zi6u1XU+PTg8Y8b0wdGgw43I03TebesWACMCURiM95vNeKUmbx 6paPehDKUsYdt9/F/B08h76SA7kmx3dWeyuTzchAQ91QfEILhM tE9lTWbpEcHEThB8E6NMA== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday 13 October 2010, Casey Dahlin wrote: > So if I understand correctly, ZONE_DMA exists to separate and preserve the > chunk of memory to which older ISA cards are capable of doing DMA. The question > that arises for me is: > > 1) Why do all 32-bit boxes have a ZONE_DMA when surely only a tiny and > shrinking number of them have a need for it? > > 2) Why do any 64-bit boxes have a ZONE_DMA? Is there really some godless > monster out there who put an early ISA bridge and a 64-bit CPU on the same > motherboard?! > > Can someone shed light on what I'm missing? There are some ISA devices that have become standard components, like floppy drives. A lot of PCs still have connectors for floppies even though they are hardly used these days. Some architectures define ZONE_DMA in a different way, e.g. as the lower 2 GB because of device restrictions on 64 bit systems. Arnd