> -----Original Message-----
> From: Eugene Surovegin [mailto:ebs at ebshome.net]
> Sent: Tuesday, August 09, 2005 13:57
> > and it would be much better to just
> > use the clockrate given to the kernel than have to hardcode
> it into the
> > kernel.
>
> Nothing prevents you from defining this macro as a function call for
> example. If this is needed for your CPU/board port. Probably common
> case will be a constant, though.
Problem is that time_nsec is not directly set to CLOCK_TICK_RATE but
goes through some calculations I'd rather not have to lots of times in
runtime (maybe I'm not seeing how it could be done)
Anyways, the #define chain leading to time_nsec is:
unsigned long tick_nsec = TICK_NSEC; /* ACTHZ period (nsec)
*/ kernel/time.c:562
#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */
asm-ppc/timex.h:14
Include/linux/jiffies.h:45 and out
/* LATCH is used in the interval timer and ftape setup. */
#define LATCH ((CLOCK_TICK_RATE + HZ/2) / HZ) /* For divider */
/* Suppose we want to devide two numbers NOM and DEN: NOM/DEN, the we
can
* improve accuracy by shifting LSH bits, hence calculating:
* (NOM << LSH) / DEN
* This however means trouble for large NOM, because (NOM << LSH) may no
* longer fit in 32 bits. The following way of calculating this gives us
* some slack, under the following conditions:
* - (NOM / DEN) fits in (32 - LSH) bits.
* - (NOM % DEN) fits in (32 - LSH) bits.
*/
#define SH_DIV(NOM,DEN,LSH) ( ((NOM / DEN) << LSH)
\
+ (((NOM % DEN) << LSH) + DEN / 2) / DEN)
/* HZ is the requested value. ACTHZ is actual HZ ("<< 8" is for
accuracy) */
#define ACTHZ (SH_DIV (CLOCK_TICK_RATE, LATCH, 8))
/* TICK_NSEC is the time between ticks in nsec assuming real ACTHZ */
#define TICK_NSEC (SH_DIV (1000000UL * 1000, ACTHZ, 8))
I cannot se a clean easy way of changing time_nsec or TIME_NSEC runtime
without messing with code that probably should not be messed with.
Of course I am probably wrong.
