Re: [patch] Re: alpha - generic_init_pit - why using RTC for calibration?

["Oleg I. Vdovikin" <vdovikin@jscc.ru>]

Jeff,

    The things are pretty simple (HZ - in real should be "Hz", cause HZ are
fixed for Alpha, HZ = 1024 Hz) :
        Hz = cc / calibration_time

    cc == rpcc() at end - rpcc() at begin
    calibration_time = (CLOCK_TICK_RATE / CALIBRATE_LATCH).

    So there is nothing wrong - clock ticks with CLOCK_TICK_RATE and we've a
divisor equal to CALIBRATE_LATCH. So the time interval elapses after
(CLOCK_TICK_RATE / CALIBRATE_LATCH) seconds and the CPU performs 'cc'
cycles.

    That's all. And these really works. ;-))

    About x86 style code - don't forget what LATCH defined as follows in
linux/timex.h:

#define LATCH  ((CLOCK_TICK_RATE + HZ/2) / HZ) /* For divider */

Regards,
    Oleg.

> Oleg,
>
> How is this relative to HZ, when you remove all references to HZ?
>
> > -#define CALIBRATE_LATCH        (52 * LATCH)
> > -#define CALIBRATE_TIME (52 * 1000020 / HZ)
> > +#define CALIBRATE_LATCH        0xffff
> [...]
> > +       /* and the final result in HZ */
> > +       return ((unsigned long)cc * CLOCK_TICK_RATE) / CALIBRATE_LATCH;
>
> and in asm-alpha/timex.h,
> > #define CLOCK_TICK_RATE 1193180 /* Underlying HZ */
>
> --
> Jeff Garzik      | Thalidomide, eh?
> Building 1024    | So you're saying the eggplant has an accomplice?
> MandrakeSoft     |
>