linux-kernel - Re: [PATCH] lfsr: a simple binary Galois linear feedback shift register

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Message-ID: <551BFD9F.2030403@hp.com>
Date:	Wed, 01 Apr 2015 10:15:59 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Shuah Khan <shuahkh@....samsung.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH] lfsr: a simple binary Galois linear feedback shift register

On 04/01/2015 03:53 AM, Peter Zijlstra wrote:
> On Tue, Mar 31, 2015 at 01:28:09PM -0400, Waiman Long wrote:
>> +static __always_inline u32 lfsr_taps(int bits)
>> +static inline u32 lfsr(u32 val, int bits)
>> +{
>> +	u32 bit = val&  1;
>> +
>> +	/*
>> +	 * LFSR doesn't work with a start state of 0, so force it to a
>> +	 * non-zero value (bits) as the next state.
>> +	 */
>> +	if (val == 0)
>> +		return bits;
> Arguably this should be a debug/warn instead of a silent modification.

Since it is used in conjunction with hashing, it is possible that 
hashing can produce a value of 0. Do we really want to have a warning 
for that? Alternatively, we can pass in some flag to decide if a warning 
should be issued.

>> +	val>>= 1;
>> +	if (bit)
>> +		val ^= lfsr_taps(bits);
>> +	return val;
>> +}
> I was also thinking that if we modify the hash to be dynamically signed
> we cannot use the compile time tap selection and need to change the
> interface slightly.

So you mean having another argument for the caller to pass in the tap 
value instead of using the default. Right?

-Longman
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