Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754725AbZIAN2q (ORCPT ); Tue, 1 Sep 2009 09:28:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754497AbZIAN2q (ORCPT ); Tue, 1 Sep 2009 09:28:46 -0400 Received: from caramon.arm.linux.org.uk ([78.32.30.218]:39324 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754345AbZIAN2p (ORCPT ); Tue, 1 Sep 2009 09:28:45 -0400 Date: Tue, 1 Sep 2009 14:28:24 +0100 From: Russell King - ARM Linux To: Steven Walter Cc: David Xiao , Laurent Pinchart , Ben Dooks , Hugh Dickins , Robin Holt , "linux-kernel@vger.kernel.org" , v4l2_linux , "linux-arm-kernel@lists.arm.linux.org.uk" Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?") Message-ID: <20090901132824.GN19719@n2100.arm.linux.org.uk> References: <200908061208.22131.laurent.pinchart@ideasonboard.com> <20090806114619.GW2080@trinity.fluff.org> <200908061506.23874.laurent.pinchart@ideasonboard.com> <1249584374.29182.20.camel@david-laptop> <20090806222543.GG31579@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1621 Lines: 35 On Tue, Aug 25, 2009 at 08:53:29AM -0400, 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). You are correct. > 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? I don't think you read what I said (but I've also forgotten what I did say). To put it simply, the kernel does not support DMA direct from userspace pages. Solutions which have been proposed in the past only work with a sub-set of conditions (such as the one above only works with VIPT caches.) -- 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/