Messages in this thread | | | Subject | Carefully chosen CYC2NS_SCALE_FACTOR ?? | From | Petr Tesarik <> | Date | Wed, 15 Jul 2009 16:03:23 +0200 |
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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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