1 files changed, 50 insertions, 15 deletions

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 38959c866789..662c5798a685 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -33,29 +33,64 @@ struct ce_unbind {
         int res;
 };
 
-/**
- * clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
- * @latch:      value to convert
- * @evt:        pointer to clock event device descriptor
- *
- * Math helper, returns latch value converted to nanoseconds (bound checked)
- */
-u64 clockevent_delta2ns(unsigned long latch, struct clock_event_device *evt)
+static u64 cev_delta2ns(unsigned long latch, struct clock_event_device *evt,
+                        bool ismax)
 {
         u64 clc = (u64) latch << evt->shift;
+        u64 rnd;
 
         if (unlikely(!evt->mult)) {
                 evt->mult = 1;
                 WARN_ON(1);
         }
+        rnd = (u64) evt->mult - 1;
+
+        /*
+         * Upper bound sanity check. If the backwards conversion is
+         * not equal latch, we know that the above shift overflowed.
+         */
+        if ((clc >> evt->shift) != (u64)latch)
+                clc = ~0ULL;
+
+        /*
+         * Scaled math oddities:
+         *
+         * For mult <= (1 << shift) we can safely add mult - 1 to
+         * prevent integer rounding loss. So the backwards conversion
+         * from nsec to device ticks will be correct.
+         *
+         * For mult > (1 << shift), i.e. device frequency is > 1GHz we
+         * need to be careful. Adding mult - 1 will result in a value
+         * which when converted back to device ticks can be larger
+         * than latch by up to (mult - 1) >> shift. For the min_delta
+         * calculation we still want to apply this in order to stay
+         * above the minimum device ticks limit. For the upper limit
+         * we would end up with a latch value larger than the upper
+         * limit of the device, so we omit the add to stay below the
+         * device upper boundary.
+         *
+         * Also omit the add if it would overflow the u64 boundary.
+         */
+        if ((~0ULL - clc > rnd) &&
+            (!ismax || evt->mult <= (1U << evt->shift)))
+                clc += rnd;
 
         do_div(clc, evt->mult);
-        if (clc < 1000)
-                clc = 1000;
-        if (clc > KTIME_MAX)
-                clc = KTIME_MAX;
 
-        return clc;
+        /* Deltas less than 1usec are pointless noise */
+        return clc > 1000 ? clc : 1000;
+}
+
+/**
+ * clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
+ * @latch:      value to convert
+ * @evt:        pointer to clock event device descriptor
+ *
+ * Math helper, returns latch value converted to nanoseconds (bound checked)
+ */
+u64 clockevent_delta2ns(unsigned long latch, struct clock_event_device *evt)
+{
+        return cev_delta2ns(latch, evt, false);
 }
 EXPORT_SYMBOL_GPL(clockevent_delta2ns);
 
@@ -380,8 +415,8 @@ void clockevents_config(struct clock_event_device *dev, u32 freq)
                 sec = 600;
 
         clockevents_calc_mult_shift(dev, freq, sec);
-        dev->min_delta_ns = clockevent_delta2ns(dev->min_delta_ticks, dev);
-        dev->max_delta_ns = clockevent_delta2ns(dev->max_delta_ticks, dev);
+        dev->min_delta_ns = cev_delta2ns(dev->min_delta_ticks, dev, false);
+        dev->max_delta_ns = cev_delta2ns(dev->max_delta_ticks, dev, true);
 }
 
 /**