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

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Message-ID: <20150401075325.GV27490@worktop.programming.kicks-ass.net>
Date:	Wed, 1 Apr 2015 09:53:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
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 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.

> +	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.
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