linux-kernel - Re: [PATCH 0/8] ARM: mvebu: Add support for RAID6 PQ offloading

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Date:	Wed, 13 May 2015 11:17:33 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Vinod Koul <vinod.koul@...el.com>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Jason Cooper <jason@...edaemon.net>,
	Andrew Lunn <andrew@...n.ch>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org, Lior Amsalem <alior@...vell.com>,
	Thomas Petazzoni <thomas@...e-electrons.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 0/8] ARM: mvebu: Add support for RAID6 PQ offloading

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
> <maxime.ripard@...e-electrons.com> 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?

I remember discussing this with Vinod at Plumbers back in October, but
haven't seen anything since then.

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.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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