From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757195AbZHGII2 (ORCPT ); Fri, 7 Aug 2009 04:08:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757076AbZHGII1 (ORCPT ); Fri, 7 Aug 2009 04:08:27 -0400 Received: from caramon.arm.linux.org.uk ([78.32.30.218]:41862 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754982AbZHGIIY (ORCPT ); Fri, 7 Aug 2009 04:08:24 -0400 Date: Fri, 7 Aug 2009 09:08:11 +0100 From: Russell King - ARM Linux To: David Xiao Cc: 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: <20090807080811.GA18343@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> <1249624766.32621.61.camel@david-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1249624766.32621.61.camel@david-laptop> 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 On Thu, Aug 06, 2009 at 10:59:26PM -0700, David Xiao wrote: > The V7 speculative prefetching will then probably apply to DMA coherency > issue in general, both kernel and user space DMAs. Could this be > addressed by inside the dma_unmap_sg/single() calling dma_cache_maint() > when the direction is DMA_FROM_DEVICE/DMA_BIDIRECTIONAL, to basically > invalidate the related cache lines in case any filled by prefetching? > Assuming dma_unmap_sg/single() is called after each DMA operation is > completed. It's something that I was going to look at, and it's probably going to have to be something I do blind - I currently have no MPCore platform, and even if my Realview EB worked, it doesn't use DMA at all. However, it's not trivial - the unmap functions don't have all the necessary information. dma_unmap_single() has the DMA address, which we can convert to the original virtual address via dma_to_virt(). However, dma_unmap_page() can't translate back to a virtual page since we're missing some information there. It bugs me that the DMA API is restrictive in the information which architectures can retain across a mapping which makes this non-trivial. Had I known of these issues when the DMA API was originally being discussed, I'd have suggested that we have an arch-specific dma_map struct which could contain whatever information was required, rather than requiring the driver to maintain the handle/size/direction/etc between each of the calls. That would mean we could retain the virtual address/struct page rather than having to work it back in some way.