Re: [PATCH] sched: Support current clocksource handling in fallback

Matthieu CASTET
2009-05-26T15:02:58Z

Linus Walleij a écrit :
> 2009/5/26 Paul Mundt <lethal@linux-sh.org>:
>
>> */
>> unsigned long long __attribute__((weak)) sched_clock(void)
>> {
>> + /*
>> + * Use the current clocksource when it becomes available later in
>> + * the boot process, and ensure that it has a high enough rating
>> + * to make it suitable for general use.
>> + */
>> + if (clock && clock->rating >= 100)
>> + return cyc2ns(clock, clocksource_read(clock));
>> +
>> + /* Otherwise just fall back on jiffies */
>> return (unsigned long long)(jiffies - INITIAL_JIFFIES)
>> * (NSEC_PER_SEC / HZ);
>> }
This is buggy for fast clocksource that wrap often, because sched_clock
is not supposed to wrap.
That why custom implementation use cnt32_to_63 and a smaller shift
(around 8 -10) for conversion for timer unit to ns.

Look for example to arch/arm/mach-pxa/time.c implementation [1]

But we can image you make it generic.

Matthieu

[1]
/*
* This is PXA's sched_clock implementation. This has a resolution
* of at least 308 ns and a maximum value of 208 days.
*
* The return value is guaranteed to be monotonic in that range as
* long as there is always less than 582 seconds between successive
* calls to sched_clock() which should always be the case in practice.
*/
#define OSCR2NS_SCALE_FACTOR 10

static unsigned long oscr2ns_scale;

static void __init set_oscr2ns_scale(unsigned long oscr_rate)
{
unsigned long long v = 1000000000ULL << OSCR2NS_SCALE_FACTOR;
do_div(v, oscr_rate);
oscr2ns_scale = v;
/*
* We want an even value to automatically clear the top bit
* returned by cnt32_to_63() without an additional run time
* instruction. So if the LSB is 1 then round it up.
*/
if (oscr2ns_scale & 1)
oscr2ns_scale++;
}

unsigned long long sched_clock(void)
{
unsigned long long v = cnt32_to_63(OSCR);
return (v * oscr2ns_scale) >> OSCR2NS_SCALE_FACTOR;
}