3 files changed, 17 insertions, 7 deletions

diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
index 431793e5d484..34baa0eb5d0c 100644
--- a/arch/x86/include/asm/timer.h
+++ b/arch/x86/include/asm/timer.h
@@ -57,14 +57,10 @@ 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);
-        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);
+        ns += mult_frac(cyc, per_cpu(cyc2ns, cpu),
+                        (1UL << CYC2NS_SCALE_FACTOR));
         return ns;
 }
 
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index a62c201c97ec..183c5925a9fe 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -620,7 +620,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 
         if (cpu_khz) {
                 *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
-                *offset = ns_now - (tsc_now * *scale >> CYC2NS_SCALE_FACTOR);
+                *offset = ns_now - mult_frac(tsc_now, *scale,
+                                             (1UL << CYC2NS_SCALE_FACTOR));
         }
 
         sched_clock_idle_wakeup_event(0);
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index e8343422240a..d801acb5e680 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -85,6 +85,19 @@
 }                                                       \
 )
 
+/*
+ * Multiplies an integer by a fraction, while avoiding unnecessary
+ * overflow or loss of precision.
+ */
+#define mult_frac(x, numer, denom)(                     \
+{                                                       \
+        typeof(x) quot = (x) / (denom);                 \
+        typeof(x) rem  = (x) % (denom);                 \
+        (quot * (numer)) + ((rem * (numer)) / (denom)); \
+}                                                       \
+)
+
+
 #define _RET_IP_                (unsigned long)__builtin_return_address(0)
 #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })