From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932330AbZHYXOW (ORCPT ); Tue, 25 Aug 2009 19:14:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932239AbZHYXOW (ORCPT ); Tue, 25 Aug 2009 19:14:22 -0400 Received: from perceval.irobotique.be ([92.243.18.41]:47106 "EHLO perceval.irobotique.be" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932209AbZHYXOV (ORCPT ); Tue, 25 Aug 2009 19:14:21 -0400 From: Laurent Pinchart To: "David Xiao" Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?") Date: Wed, 26 Aug 2009 01:17:27 +0200 User-Agent: KMail/1.12.0 (Linux/2.6.30.3; KDE/4.3.0; x86_64; ; ) Cc: "Steven Walter" , "Russell King - ARM Linux" , "Ben Dooks" , "Hugh Dickins" , "Robin Holt" , "linux-kernel@vger.kernel.org" , "v4l2_linux" , "linux-arm-kernel@lists.arm.linux.org.uk" References: <200908061208.22131.laurent.pinchart@ideasonboard.com> <1251237768.8877.26.camel@david-laptop> In-Reply-To: <1251237768.8877.26.camel@david-laptop> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit Message-Id: <200908260117.27180.laurent.pinchart@ideasonboard.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday 26 August 2009 00:02:48 David Xiao wrote: > On Tue, 2009-08-25 at 05:53 -0700, Steven Walter wrote: > > On Thu, Aug 6, 2009 at 6:25 PM, Russell King - ARM > > Linux wrote: > > [...] > > > > > As far as userspace DMA coherency, the only way you could do it with > > > current kernel APIs is by using get_user_pages(), creating a > > > scatterlist from those, and then passing it to dma_map_sg(). While the > > > device has ownership of the SG, userspace must _not_ touch the buffer > > > until after DMA has completed. > > > > [...] > > > > Would that work on a processor with VIVT caches? It seems not. In > > particular, dma_map_page uses page_address to get a virtual address to > > pass to map_single(). map_single() in turn uses this address to > > perform cache maintenance. Since page_address() returns the kernel > > virtual address, I don't see how any cache-lines for the userspace > > virtual address would get invalidated (for the DMA_FROM_DEVICE case). > > > > If that's true, then what is the correct way to allow DMA to/from a > > userspace buffer with a VIVT cache? If not true, what am I missing? > > page_address() is basically returning page->virtual, which records the > virtual/physical mapping for both user/kernel space; and what only > matters there is highmem or not. I'm not sure to get it. Are you implying that a physical page will then be mapped to the same address in all contexts (kernelspace and userspace processes) ? Is that even possible ? And if not, how could page->virtual store both the initial kernel map and all the userspace mappings ? -- Laurent Pinchart