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

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Date:	Sat, 21 Mar 2009 08:19:26 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ilya Yanok <yanok@...raft.com>,
	"H. Peter Anvin" <h.peter.anvin@...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

On Fri, Mar 20, 2009 at 7:30 PM, H. Peter Anvin <hpa@...or.com> wrote:
> Ilya Yanok wrote:
>>
>> 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.
>
> For any n (which would mean any GF field)?  In that case, that is
> generic scalar-vector multiplication...
>
>> 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.
>
>
> No, you don't.  A and B (and g^{-x}) are scalars, meaning they're the
> same for every element.  This is simpler to do.

Understood.  However this routine also needs to cover the non-generic
and non-constant case where we have a separate coefficient per
element.  I suppose it could scan the coefficient list to see if it
can bypass the 2-dimensional lookup multiply.  At the very least we
need something like the following, because async_pq is really only a
helper routine for async_r6recov.c which knows how to avoid the
synchronous path.

diff --git a/crypto/async_tx/async_pq.c b/crypto/async_tx/async_pq.c
index da47a29..c1087a2 100644
--- a/crypto/async_tx/async_pq.c
+++ b/crypto/async_tx/async_pq.c
@@ -236,6 +236,7 @@ async_pq(struct page **blocks, unsigned int
offset, int src_cnt,
                                 flags, depend_tx, cb_fn, cb_param);
        } else {
                /* run pq synchronously */
+               WARN_ONCE(1, "INFO: async_pq entered synchronous path\n");
                if (!blocks[src_cnt+1]) { /* only p requested, just xor */
                        flags |= ASYNC_TX_XOR_ZERO_DST;
                        return async_xor(blocks[src_cnt], blocks, offset,
@@ -252,7 +253,6 @@ async_pq(struct page **blocks, unsigned int
offset, int src_cnt,

        return tx;
 }
-EXPORT_SYMBOL_GPL(async_pq);

Looking closer, the only other caller, async_pq_zero_sum, can be
deleted because it has no users.  So async_pq can become a static
routine in async_r6recov.

Thanks,
Dan
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