linux-kernel - Carefully chosen CYC2NS_SCALE

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Date:	Wed, 15 Jul 2009 16:03:23 +0200
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Andi Kleen <andi@...stfloor.org>,
	John Stultz <johnstul@...ibm.com>, george@...sta.com
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Carefully chosen CYC2NS_SCALE_FACTOR ??

Hi,

I'm having trouble understanding the rationale behind the choice for
CYC2NS_SCALE_FACTOR in arch/x86/include/asm/timer.h:

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */

Why on Earth is this 10?

AFAICS this constant is the position of the decimal point in the
fixed-point cycle-to-nanoseconds computations. It is computed as
follows:

                *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;

Now, NSEC_PER_MSEC is 10^6, the size of cyc2ns is 32 bits and cpu_khz,
being an integer, cannot be less than 1. So, if I want maximum accuracy,
I should choose the maximium CYC2NS_SCALE_FACTOR such that:

10^6 * 2^CYC2NS_SCALE_FACTOR < 2^32

Which transforms to:

CYC2NS_SCALE_FACTOR = floor(log2(2^32/NSEC_PER_MSEC)) = 12

Did I miss an obvious reason for not using the scale factor of 12?

TIA,
Petr Tesarik
SUSE Linux

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