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

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Date:	Sat, 17 May 2014 21:51:45 -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 9:12 PM, Joe Perches <joe@...ches.com> wrote:
> On Sat, 2014-05-17 at 21:00 -0700, David Matlack wrote:
> []
>> diff --git a/drivers/staging/slicoss/slicoss.c b/drivers/staging/slicoss/slicoss.c
> []
>> +static inline u16 __reduce(u32 checksum)
>> +{
>> +     u16 lower_16 = checksum & 0xFFFF;
>> +     u16 upper_16 = (checksum >> 16) & 0xFFFF;
>> +
>> +     checksum = lower_16 + upper_16;
>> +
>> +     if (checksum > 65535)
>> +             checksum -= 65535;
>> +
>> +     return checksum;
>> +}
>
> 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. The eeprom on this device is
small (0x80 bytes max, not enough to trigger overflow) and I have no
documentation, so I don't know how to test :(
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