Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Wed, 3 Oct 2001 20:24:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 3 Oct 2001 20:24:30 -0400 Received: from pizda.ninka.net ([216.101.162.242]:62869 "EHLO pizda.ninka.net") by vger.kernel.org with ESMTP id ; Wed, 3 Oct 2001 20:24:18 -0400 Date: Wed, 03 Oct 2001 17:24:39 -0700 (PDT) Message-Id: <20011003.172439.66056954.davem@redhat.com> To: James.Bottomley@HansenPartnership.com Cc: jes@sunsite.dk, linuxopinion@yahoo.com, linux-kernel@vger.kernel.org Subject: Re: how to get virtual address from dma address From: "David S. Miller" In-Reply-To: <200110032244.f93MiI103485@localhost.localdomain> In-Reply-To: <200110032244.f93MiI103485@localhost.localdomain> X-Mailer: Mew version 2.0 on Emacs 21.0 / Mule 5.0 (SAKAKI) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org From: James Bottomley Date: Wed, 03 Oct 2001 17:44:18 -0500 (although I can see it may be expensive to walk iommu page tables) I know of hardware where doing the reverse mapping would not even be possible, the page tables are in hardware registers and are "write only". This means you can't even read the PTEs back, you'd have to keep track of them in software and that is totally unacceptable overhead when it won't even be used %99 of the time. The DMA API allows us to support such hardware cleanly and efficiently, but once we add this feature which "everyone absolutely needs" we have a problem with the above mentioned piece of hardware. Franks a lot, David S. Miller davem@redhat.com - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/