From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161276AbWGJAEj (ORCPT ); Sun, 9 Jul 2006 20:04:39 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161273AbWGJAEj (ORCPT ); Sun, 9 Jul 2006 20:04:39 -0400 Received: from mx2.suse.de ([195.135.220.15]:49838 "EHLO mx2.suse.de") by vger.kernel.org with ESMTP id S1161272AbWGJAEh (ORCPT ); Sun, 9 Jul 2006 20:04:37 -0400 From: Andi Kleen To: Robert Hancock Subject: Re: [RFC 0/8] Optional ZONE_DMA Date: Mon, 10 Jul 2006 02:02:09 +0200 User-Agent: KMail/1.9.3 Cc: Christoph Lameter , linux-kernel@vger.kernel.org, Nick Piggin , Christoph Hellwig , Marcelo Tosatti , Arjan van de Ven , Martin Bligh , KAMEZAWA Hiroyuki References: <44AFF286.6020601@shaw.ca> In-Reply-To: <44AFF286.6020601@shaw.ca> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200607100202.09787.ak@suse.de> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org > > On x86_64 systems also usually we do not need ZONE_DMA since there > > are barely any ISA DMA devices around (or are you still using a floppy?). > > So for most cases the zone can be dropped. Also if the x86_64 systems > > has less than 4G RAM or DMA controllers that actually can do 64 bit > > then we also do not need ZONE_DMA32. My x86_64 system has 1G of > > memory therefore I can run with a single zone. > > Keep in mind that: Yes we can't really make it optional. There are reasons to use GFP_DMA even without ISA. Also on x86-64 CONFIG_ISA is never set so it would completely eliminate GFP_DMA, which we can't do. That said however nearly users of GFP_DMA outside arch/* are wrong. They should be all audited and convered to use the PCI DMA API instead. > -LPC devices like the floppy controller, maybe enhanced parallel, etc. > may have 24-bit DMA restrictions even if there is no physical ISA bus. > > -Even in totally ISA and LPC-free systems, some PCI devices (like those > that were a quick hack of an ISA device onto PCI) still have 24-bit > address restrictions. There are other devices that have sub-32-bit DMA > capabilities, like Broadcom wireless chips that only address 31 bits > (although I think they are fixing this in the driver). Without the DMA > zone there is no way to ensure that these requests can be satisfied. There are also devices with 31 or 30 bits that also fall back to GFP_DMA and a couple of other cases. -Andi