linux-kernel - Re: [RFC] lib: crc8: add new library module providing crc8

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Date:	23 May 2011 19:38:35 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	arend@...adcom.com, linux@...izon.com
Cc:	johannes@...solutions.net, linux-kernel@...r.kernel.org
Subject: Re: [RFC] lib: crc8: add new library module providing crc8

>> Thanks. However, your code example is confusing.

>>> You can fill in a CRC table for an arbitrary polynomial with
>>>
>>> #define POLY 0xAB	/* 1 + x^2 + x^4 + x^6 + x^7 (+ x^8) */
>>> typedef uint8_t crc_type;	/* Must be an unsigned type */
>>>
>>> void
>>> crc_le(crc_type const table[256], crc_type crc, u8 const *buf, size_t len)
>>> {
>>> 	while (len--)
>>> 		crc = (crc>>  8) ^ table[(crc ^ *buf++)&  0xff];
> 
>> Here is where my confusion starts. Shifting crc by 8 bits basically 
>> means 0 ^ table[], right?

> I think I understand the code a bit better. Your code is generic for any 
> given crc_type.

Yes, exactly.  The crc_type has to be at least as wide as the CRC
being computed.  If the crc_type is only 8 bits, the shift by 8 does
indeed get optimized away.
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