linux-kernel - Re: [PATCH 06/13] async_tx: add support for asynchronous GF multiplication

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Message-ID: <49C42F99.5010700@emcraft.com>
Date:	Sat, 21 Mar 2009 03:06:49 +0300
From:	Ilya Yanok <yanok@...raft.com>
To:	"H. Peter Anvin" <h.peter.anvin@...el.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"neilb@...e.de" <neilb@...e.de>,
	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>,
	Yuri Tikhonov <yur@...raft.com>
Subject: Re: [PATCH 06/13] async_tx: add support for asynchronous GF 	multiplication

Hi H. Peter,

H. Peter Anvin wrote:
>> We defined async_pq() function as hardware can do GF multiplication of
>> arbitrary elements and we want to take advantage of this fact. But by
>> the design of ASYNC_TX API we have to provide synchronous version of
>> that function too.
>>
>
> I don't think that makes sense since the algorithm doesn't use
> vector-vector multiplications and there is no need for them.  As such,
> you're actively excluding support for hardware which only provides
> scalar-vector multiplication, even though that is all we need.
>
> In fact, vectorizing the constant doesn't make much sense.

Hm... I have to admit I can't understand your
vector-vector/scalar-vector terminology... What vector space are you
talking about?

async_gen_syndrome() takes element of GF^n(256) and returns result of
scalar multiplication of it with constant ({01}, {02}, {02}^2, ...,
{02}^n) vector.
async_pq() takes two vectors from GF^n(256) and returns their scalar
multiplication.

We need async_pq() function if we want to offload D_x = A * (P + P_{xy})
+ B * (Q + Q_{xy}) part of DD recovery and D_x = (Q + Q_x) * g^{-x} part
of DP recovery.

Regards, Ilya.

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