[PATCH] nomadik: prevent sched_clock() wraparound v3

[Rabin Vincent]

On Wed, Nov 17, 2010 at 7:15 PM, Linus Walleij
<linus.walleij at stericsson.com> wrote:
> +unsigned long long sched_clock(void)
> +{
> +       u64 cycles;
> +
> +       if (unlikely(!mtu_base))
> +               return 0;
> +
> +       cycles = cnt32_to_63(-readl(mtu_base + MTU_VAL(0)));
> +       cycles &= 0x7FFFFFFFFFFFFFFFLLU;
> +       return (cycles * sched_mult) >> sched_shift;
> +}
> +
> +/* Just kick sched_clock every so often */
> +static void cnt32_to_63_keepwarm(unsigned long data)
> +{
> +       mod_timer(&cnt32_to_63_keepwarm_timer, round_jiffies(jiffies + data));
> +       (void) sched_clock();
> +}
> +
> +/*
> + * Set up a timer to keep sched_clock():s 32_to_63 algorithm warm
> + * once in half a 32bit timer wrap interval.
>  */
> -unsigned long long notrace sched_clock(void)
> +static void __init nmdk_sched_clock_init(unsigned long rate)
>  {
> -       return clocksource_cyc2ns(nmdk_clksrc.read(
> -                                 &nmdk_clksrc),
> -                                 nmdk_clksrc.mult,
> -                                 nmdk_clksrc.shift);
> +       u32 v;
> +       unsigned long delta;
> +       u64 days;
> +
> +       /* Find the apropriate mult and shift factors */
> +       clocks_calc_mult_shift(&sched_mult, &sched_shift,
> +                              rate, NSEC_PER_SEC, SCHED_CLOCK_MIN_WRAP);
> +       /* We need to multiply by atleast 2 to get rid of bit 63 */
> +       BUG_ON(sched_mult < 2);

No, you want to multiply by an even number, not "atleast 2".  Please
also drop the "cycles &= 0x7FFFFFFFFFFFFFFFLLU" in sched_clock().

> +
> +       /* Let's see what we get, take max counter and scale it */
> +       days = 0xFFFFFFFFFFFFFFFFLLU >> sched_shift;
> +       do_div(days, rate);
> +       do_div(days, (3600*24));
> +
> +       pr_debug("sched_clock: using %d bits @ %lu Hz wrap in %lu days\n",
> +               (64 - sched_shift), rate, (unsigned long) days);

This days calculation is wrong because you need to divide by
NSECS_PER_SEC, not rate.