linux-kernel - Re: [PATCHv4 5/8] lib: add rocksoft model crc64

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Date:   Fri, 4 Mar 2022 07:02:57 -0800
From:   Keith Busch <kbusch@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>, "hch@....de" <hch@....de>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCHv4 5/8] lib: add rocksoft model crc64

On Fri, Mar 04, 2022 at 07:53:44AM +0000, David Laight wrote:
> 
> That can be speeded up by using the identity:
> 	table[x ^ y] == table[x] ^ table[y]
> 
> something like:
> 	crc = poly;  /* actually crc(1) */
> 	table[0] = 0;
> 	table[1] = crc;
> 	for (i = 2; i < 8; i++) [
> 		crc = crc & 1 ? (crc >> 1) ^ poly : crc >> 1;
> 		for (j = 0; j < 1u << i; j++)
> 			table[j + (1i << i)] = table[j] ^ crc;
> 	}
> 
> I think the same code can be used for a normal MSB first crc
> provided both the polynomial and crc(1) are passed in.
> 
> OTOH initialisation speed may not matter.

I don't think speed is an issue here. This part is executed just once
for the initial kernel compile, then never again.