1 files changed, 22 insertions, 1 deletions

diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
index fa7b9176b76c..431793e5d484 100644
--- a/arch/x86/include/asm/timer.h
+++ b/arch/x86/include/asm/timer.h
@@ -32,6 +32,22 @@ extern int no_timer_check;
  *  (mathieu.desnoyers@polymtl.ca)
  *
  *                      -johnstul@us.ibm.com "math is hard, lets go shopping!"
+ *
+ * In:
+ *
+ * ns = cycles * cyc2ns_scale / SC
+ *
+ * Although we may still have enough bits to store the value of ns,
+ * in some cases, we may not have enough bits to store cycles * cyc2ns_scale,
+ * leading to an incorrect result.
+ *
+ * To avoid this, we can decompose 'cycles' into quotient and remainder
+ * of division by SC.  Then,
+ *
+ * ns = (quot * SC + rem) * cyc2ns_scale / SC
+ *    = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC
+ *
+ *                      - sqazi@google.com
  */
 
 DECLARE_PER_CPU(unsigned long, cyc2ns);
@@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);
 
 static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
 {
+        unsigned long long quot;
+        unsigned long long rem;
         int cpu = smp_processor_id();
         unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
-        ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR;
+        quot = (cyc >> CYC2NS_SCALE_FACTOR);
+        rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
+        ns += quot * per_cpu(cyc2ns, cpu) +
+                ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
         return ns;
 }