Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765308AbcJaKVM (ORCPT ); Mon, 31 Oct 2016 06:21:12 -0400 Received: from pandora.armlinux.org.uk ([78.32.30.218]:56542 "EHLO pandora.armlinux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756059AbcJaKVK (ORCPT ); Mon, 31 Oct 2016 06:21:10 -0400 Date: Mon, 31 Oct 2016 10:20:57 +0000 From: Russell King - ARM Linux To: Alexander Duyck Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, brouer@redhat.com, davem@davemloft.net Subject: Re: [net-next PATCH RFC 04/26] arch/arm: Add option to skip sync on DMA map and unmap Message-ID: <20161031102057.GZ1041@n2100.armlinux.org.uk> References: <20161024115737.16276.71059.stgit@ahduyck-blue-test.jf.intel.com> <20161024120447.16276.50401.stgit@ahduyck-blue-test.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161024120447.16276.50401.stgit@ahduyck-blue-test.jf.intel.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 981 Lines: 22 On Mon, Oct 24, 2016 at 08:04:47AM -0400, Alexander Duyck wrote: > The use of DMA_ATTR_SKIP_CPU_SYNC was not consistent across all of the DMA > APIs in the arch/arm folder. This change is meant to correct that so that > we get consistent behavior. I'm really not convinced that this is anywhere close to correct behaviour. If we're DMA-ing to a buffer, and we unmap it or sync_for_cpu, then we will want to access the DMA'd data - especially in the sync_for_cpu case, it's pointless to call sync_for_cpu if we're not going to access the data. So the idea of skipping the CPU copy when DMA_ATTR_SKIP_CPU_SYNC is set seems to be completely wrong - it means we end up reading the stale data that was in the buffer, completely ignoring whatever was DMA'd to it. What's the use case for DMA_ATTR_SKIP_CPU_SYNC ? -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.