Confused about CLOCK_TICK_RATE

[Paul Mackerras]

Timur Tabi writes:

> Can someone explain CLOCK_TICK_RATE to me?  It's defined in
> arch/powerpc/include/asm/timex.h as such:
> 
> #define CLOCK_TICK_RATE	1024000 /* Underlying HZ */
> 
> Every architecture defines this, but some use the better comment
> "Underlying frequency of the HZ timer".
> 
> My question is: why is this a constant? Shouldn't it be a variable,
> perhaps something that is based on tb_ticks_per_usec?

It's used essentially for making small adjustments to ACTHZ and
TICK_NSEC in include/linux/jiffies.h.  The value was chosen so that
ACTHZ == HZ and TICK_NSEC = 1e9 for all common HZ values, since we
can't say at compile time what the decrementer frequency will be.
I believe it's all pretty moot since we changed over to using the
generic time framework anyway.

Paul.