From: Dan Williams Subject: Re: [PATCH 0/8] ARM: mvebu: Add support for RAID6 PQ offloading Date: Wed, 13 May 2015 09:00:46 -0700 Message-ID: References: <1431445063-20226-1-git-send-email-maxime.ripard@free-electrons.com> <20150513091733.GW10961@lukather> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: Vinod Koul , Gregory Clement , Jason Cooper , Andrew Lunn , Sebastian Hesselbarth , "dmaengine@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , linux-crypto@vger.kernel.org, Lior Amsalem , Thomas Petazzoni , Herbert Xu , "David S. Miller" To: Maxime Ripard Return-path: Received: from mail-wg0-f52.google.com ([74.125.82.52]:34411 "EHLO mail-wg0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965055AbbEMQAr (ORCPT ); Wed, 13 May 2015 12:00:47 -0400 Received: by wgic8 with SMTP id c8so48466408wgi.1 for ; Wed, 13 May 2015 09:00:46 -0700 (PDT) In-Reply-To: <20150513091733.GW10961@lukather> Sender: linux-crypto-owner@vger.kernel.org List-ID: On Wed, May 13, 2015 at 2:17 AM, Maxime Ripard wrote: > Hi Dan, > > On Tue, May 12, 2015 at 09:05:41AM -0700, Dan Williams wrote: >> On Tue, May 12, 2015 at 8:37 AM, Maxime Ripard >> wrote: >> > Hi, >> > >> > This serie refactors the mv_xor in order to support the latest Armada >> > 38x features, including the PQ support in order to offload the RAID6 >> > PQ operations. >> > >> > Not all the PQ operations are supported by the XOR engine, so we had >> > to introduce new async_tx flags in the process to identify >> > un-supported operations. >> > >> > Please note that this is currently not usable because of a possible >> > regression in the RAID stack in 4.1 that is being discussed at the >> > moment here: https://lkml.org/lkml/2015/5/7/527 >> >> This is problematic as async_tx is a wart on the dmaengine subsystem >> and needs to be deprecated, I just have yet to find the time to do >> that work. It turns out it was a mistake to hide the device details >> from md, it should be explicitly managing the dma channels, not >> relying on a abstraction api. The async_tx api usage of the >> dma-mapping api is broken in that it relies on overlapping mappings of >> the same address. This happens to work on x86, but on arm it needs >> explicit non-overlapping mappings. I started the work to reference >> count dma-mappings in 3.13, and we need to teach md to use >> dmaengine_unmap_data explicitly. Yielding dma channel management to >> md also results in a more efficient implementation as we can dma_map() >> the stripe cache once rather than per-io. The "async_tx_ack()" >> disaster can also go away when md is explicitly handling channel >> switching. > > Even though I'd be very much in favor of deprecating / removing > async_tx, is it something likely to happen soon? Not unless someone else takes it on, I'm actively asking for help. > I remember discussing this with Vinod at Plumbers back in October, but > haven't seen anything since then. Right, "help!" :) > If not, I think that we shouldn't really hold back patches to > async_tx, even though we know than in a year from now, it's going to > be gone. We definitely should block new usages, because they make a bad situation worse. Russell already warned that the dma_mapping api abuse could lead to data corruption on ARM (speculative pre-fetching). We need to mark ASYNC_TX_DMA as "depends on !ARM" or even "depends on BROKEN" until we can get this resolved.