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authorLinus Torvalds <torvalds@linux-foundation.org>2009-12-08 19:27:08 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2009-12-08 19:27:08 -0800
commit60d8ce2cd6c283132928c11f3fd57ff4187287e0 (patch)
tree36d08a2ead7a7d8c3c081d484215ccca00bf6aab /kernel/time/clocksource.c
parent849e8dea099aafa56db9e74b580b0d858b956533 (diff)
parentfeae3203d711db0a9965300ee6d592257fdaae4f (diff)
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Merge branch 'timers-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip
* 'timers-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip: timers, init: Limit the number of per cpu calibration bootup messages posix-cpu-timers: optimize and document timer_create callback clockevents: Add missing include to pacify sparse x86: vmiclock: Fix printk format x86: Fix printk format due to variable type change sparc: fix printk for change of variable type clocksource/events: Fix fallout of generic code changes nohz: Allow 32-bit machines to sleep for more than 2.15 seconds nohz: Track last do_timer() cpu nohz: Prevent clocksource wrapping during idle nohz: Type cast printk argument mips: Use generic mult/shift factor calculation for clocks clocksource: Provide a generic mult/shift factor calculation clockevents: Use u32 for mult and shift factors nohz: Introduce arch_needs_cpu nohz: Reuse ktime in sub-functions of tick_check_idle. time: Remove xtime_cache time: Implement logarithmic time accumulation
Diffstat (limited to 'kernel/time/clocksource.c')
-rw-r--r--kernel/time/clocksource.c97
1 files changed, 97 insertions, 0 deletions
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 4a31090..d422c7b 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -107,6 +107,59 @@ u64 timecounter_cyc2time(struct timecounter *tc,
}
EXPORT_SYMBOL_GPL(timecounter_cyc2time);
+/**
+ * clocks_calc_mult_shift - calculate mult/shift factors for scaled math of clocks
+ * @mult: pointer to mult variable
+ * @shift: pointer to shift variable
+ * @from: frequency to convert from
+ * @to: frequency to convert to
+ * @minsec: guaranteed runtime conversion range in seconds
+ *
+ * The function evaluates the shift/mult pair for the scaled math
+ * operations of clocksources and clockevents.
+ *
+ * @to and @from are frequency values in HZ. For clock sources @to is
+ * NSEC_PER_SEC == 1GHz and @from is the counter frequency. For clock
+ * event @to is the counter frequency and @from is NSEC_PER_SEC.
+ *
+ * The @minsec conversion range argument controls the time frame in
+ * seconds which must be covered by the runtime conversion with the
+ * calculated mult and shift factors. This guarantees that no 64bit
+ * overflow happens when the input value of the conversion is
+ * multiplied with the calculated mult factor. Larger ranges may
+ * reduce the conversion accuracy by chosing smaller mult and shift
+ * factors.
+ */
+void
+clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 minsec)
+{
+ u64 tmp;
+ u32 sft, sftacc= 32;
+
+ /*
+ * Calculate the shift factor which is limiting the conversion
+ * range:
+ */
+ tmp = ((u64)minsec * from) >> 32;
+ while (tmp) {
+ tmp >>=1;
+ sftacc--;
+ }
+
+ /*
+ * Find the conversion shift/mult pair which has the best
+ * accuracy and fits the maxsec conversion range:
+ */
+ for (sft = 32; sft > 0; sft--) {
+ tmp = (u64) to << sft;
+ do_div(tmp, from);
+ if ((tmp >> sftacc) == 0)
+ break;
+ }
+ *mult = tmp;
+ *shift = sft;
+}
+
/*[Clocksource internal variables]---------
* curr_clocksource:
* currently selected clocksource.
@@ -413,6 +466,47 @@ void clocksource_touch_watchdog(void)
clocksource_resume_watchdog();
}
+/**
+ * clocksource_max_deferment - Returns max time the clocksource can be deferred
+ * @cs: Pointer to clocksource
+ *
+ */
+static u64 clocksource_max_deferment(struct clocksource *cs)
+{
+ u64 max_nsecs, max_cycles;
+
+ /*
+ * Calculate the maximum number of cycles that we can pass to the
+ * cyc2ns function without overflowing a 64-bit signed result. The
+ * maximum number of cycles is equal to ULLONG_MAX/cs->mult which
+ * is equivalent to the below.
+ * max_cycles < (2^63)/cs->mult
+ * max_cycles < 2^(log2((2^63)/cs->mult))
+ * max_cycles < 2^(log2(2^63) - log2(cs->mult))
+ * max_cycles < 2^(63 - log2(cs->mult))
+ * max_cycles < 1 << (63 - log2(cs->mult))
+ * Please note that we add 1 to the result of the log2 to account for
+ * any rounding errors, ensure the above inequality is satisfied and
+ * no overflow will occur.
+ */
+ max_cycles = 1ULL << (63 - (ilog2(cs->mult) + 1));
+
+ /*
+ * The actual maximum number of cycles we can defer the clocksource is
+ * determined by the minimum of max_cycles and cs->mask.
+ */
+ max_cycles = min_t(u64, max_cycles, (u64) cs->mask);
+ max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult, cs->shift);
+
+ /*
+ * To ensure that the clocksource does not wrap whilst we are idle,
+ * limit the time the clocksource can be deferred by 12.5%. Please
+ * note a margin of 12.5% is used because this can be computed with
+ * a shift, versus say 10% which would require division.
+ */
+ return max_nsecs - (max_nsecs >> 5);
+}
+
#ifdef CONFIG_GENERIC_TIME
/**
@@ -511,6 +605,9 @@ static void clocksource_enqueue(struct clocksource *cs)
*/
int clocksource_register(struct clocksource *cs)
{
+ /* calculate max idle time permitted for this clocksource */
+ cs->max_idle_ns = clocksource_max_deferment(cs);
+
mutex_lock(&clocksource_mutex);
clocksource_enqueue(cs);
clocksource_select();
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