linux-kernel - Re: [PATCH 2/8] firmware: Sigma: Skip header during CRC generation

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Message-ID: <4ED33EB1.6030200@metafoo.de>
Date:	Mon, 28 Nov 2011 08:56:33 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Mike Frysinger <vapier@...too.org>
CC:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Liam Girdwood <lrg@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, alsa-devel@...a-project.org,
	drivers@...log.com, stable@...nel.org
Subject: Re: [PATCH 2/8] firmware: Sigma: Skip header during CRC generation

On 11/25/2011 09:00 PM, Mike Frysinger wrote:
> On Friday 25 November 2011 03:55:42 Lars-Peter Clausen wrote:
>> On 11/24/2011 06:21 PM, Mike Frysinger wrote:
>>> On Thursday 24 November 2011 07:48:21 Lars-Peter Clausen wrote:
>>>> The firmware header is not part of the CRC, so skip it. Otherwise the
>>>> firmware will be rejected due to non-matching CRCs.
>>>
>>> that's because you didn't compare to the right value ;).  include the CRC
>>> -> compare to 0.  omit the CRC -> compare to the CRC value.
>>
>> Does this really work if the CRC is inserted somewhere in the middle of the
>> bytestream?
> 
> i don't think the position matters to the CRC algorithm used by sigmadsp.  
> math principle: a ^ b ^ c is the same thing as b ^ a ^ c and c ^ b ^ a.

If CRC algorithms  were commutative they would be pretty weak, I guess ;)
> 
> i could be wrong as to the CRC algo used though.  simple enough for you to 
> check -- i implemented this firmware code based on a spec i wrote up for the 
> sigmadsp peeps; i never actually had real firmware to test with.
> -mike

It does not work.

- Lars
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