3 files changed, 70 insertions, 0 deletions

diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index 3b5841016276..4d438b0bc10a 100644
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -130,6 +130,13 @@ extern int clockevents_program_event(struct clock_event_device *dev,
 
 extern void clockevents_handle_noop(struct clock_event_device *dev);
 
+static inline void
+clockevents_calc_mult_shift(struct clock_event_device *ce, u32 freq, u32 minsec)
+{
+        return clocks_calc_mult_shift(&ce->mult, &ce->shift, NSEC_PER_SEC,
+                                      freq, minsec);
+}
+
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
 extern void clockevents_notify(unsigned long reason, void *arg);
 #else
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 83d2fbd81b93..f57f88250526 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -279,6 +279,16 @@ extern void clocksource_resume(void);
 extern struct clocksource * __init __weak clocksource_default_clock(void);
 extern void clocksource_mark_unstable(struct clocksource *cs);
 
+extern void
+clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 minsec);
+
+static inline void
+clocksource_calc_mult_shift(struct clocksource *cs, u32 freq, u32 minsec)
+{
+        return clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
+                                      NSEC_PER_SEC, minsec);
+}
+
 #ifdef CONFIG_GENERIC_TIME_VSYSCALL
 extern void update_vsyscall(struct timespec *ts, struct clocksource *c);
 extern void update_vsyscall_tz(void);
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 5e18c6ab2c6a..407c0894ef37 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -107,6 +107,59 @@ u64 timecounter_cyc2time(struct timecounter *tc,
 }
 EXPORT_SYMBOL(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.