1 files changed, 22 insertions, 13 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index f86453801..6ada9d131 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -169,21 +169,20 @@ static void cyc2ns_write_end(int cpu, struct cyc2ns_data *data)
  *              ns = cycles * cyc2ns_scale / SC
  *
  *      And since SC is a constant power of two, we can convert the div
- *  into a shift.
+ *  into a shift. The larger SC is, the more accurate the conversion, but
+ *  cyc2ns_scale needs to be a 32-bit value so that 32-bit multiplication
+ *  (64-bit result) can be used.
  *
- *  We can use khz divisor instead of mhz to keep a better precision, since
- *  cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
+ *  We can use khz divisor instead of mhz to keep a better precision.
  *  (mathieu.desnoyers@polymtl.ca)
  *
  *                      -johnstul@us.ibm.com "math is hard, lets go shopping!"
  */
 
-#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
-
 static void cyc2ns_data_init(struct cyc2ns_data *data)
 {
 	data->cyc2ns_mul = 0;
-	data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
+	data->cyc2ns_shift = 0;
 	data->cyc2ns_offset = 0;
 	data->__count = 0;
 }
@@ -221,14 +220,14 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
 
 	if (likely(data == tail)) {
 		ns = data->cyc2ns_offset;
-		ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, CYC2NS_SCALE_FACTOR);
+		ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, data->cyc2ns_shift);
 	} else {
 		data->__count++;
 
 		barrier();
 
 		ns = data->cyc2ns_offset;
-		ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, CYC2NS_SCALE_FACTOR);
+		ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, data->cyc2ns_shift);
 
 		barrier();
 
@@ -262,12 +261,22 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 	 * time function is continuous; see the comment near struct
 	 * cyc2ns_data.
 	 */
-	data->cyc2ns_mul =
-		DIV_ROUND_CLOSEST(NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR,
-				  cpu_khz);
-	data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
+	clocks_calc_mult_shift(&data->cyc2ns_mul, &data->cyc2ns_shift, cpu_khz,
+			       NSEC_PER_MSEC, 0);
+
+	/*
+	 * cyc2ns_shift is exported via arch_perf_update_userpage() where it is
+	 * not expected to be greater than 31 due to the original published
+	 * conversion algorithm shifting a 32-bit value (now specifies a 64-bit
+	 * value) - refer perf_event_mmap_page documentation in perf_event.h.
+	 */
+	if (data->cyc2ns_shift == 32) {
+		data->cyc2ns_shift = 31;
+		data->cyc2ns_mul >>= 1;
+	}
+
 	data->cyc2ns_offset = ns_now -
-		mul_u64_u32_shr(tsc_now, data->cyc2ns_mul, CYC2NS_SCALE_FACTOR);
+		mul_u64_u32_shr(tsc_now, data->cyc2ns_mul, data->cyc2ns_shift);
 
 	cyc2ns_write_end(cpu, data);