linux-kernel - Re: [PATCH 1/2] staging: slicoss: rewrite eeprom checksum code

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Message-ID: <CANoiuGK-ys5Sf5Stw5cxRJ1y8SrH4RWYteZdhZwNxchoV4Hbvw@mail.gmail.com>
Date:	Sun, 18 May 2014 15:43:10 -0700
From:	David Matlack <matlackdavid@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	devel@...verdev.osuosl.org, Lior Dotan <liodot@...il.com>,
	charrer@...critech.com
Subject: Re: [PATCH 1/2] staging: slicoss: rewrite eeprom checksum code

On Sat, May 17, 2014 at 9:51 PM, David Matlack <matlackdavid@...il.com> wrote:
> On Sat, May 17, 2014 at 9:12 PM, Joe Perches <joe@...ches.com> wrote:
>> The if seems unnecessary.
>>
>> Perhaps declare a u16 return var or use
>>
>>         return lower_16 + upper_16;
>
> I agree it's fishy... but using overflow doesn't produce the same result:
>
>          (u16) 65536   == 0
>          65536 - 65535 == 1
>
> Now which is the correct result, I have no idea.

I think the checksum algorithm being used here is RFC 1071 [1]. Which means the
if is correct and just accounting for double overflow.

> The eeprom on this device is
> small (0x80 bytes max, not enough to trigger overflow) and I have no

Sorry, I was wrong about this. I was thinking in terms of summing bytes,
but the checksum is summing words. Overflow _does_ get triggered.


I think I'll go over this patch again while looking at the RFC to make sure
everything is ok. Thanks!

[1] http://tools.ietf.org/html/rfc1071
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