s390/time: fix sched_clock() overflow

[~andy/linux] / arch / s390 / include / asm / timex.h

diff --git a/arch/s390/include/asm/timex.h b/arch/s390/include/asm/timex.h
index fba4d66788a247c94adfba5c13ae750e19929496..4c060bb5b8eafbfc5b816411dc91f9dea1201daf 100644 (file)
--- a/arch/s390/include/asm/timex.h
+++ b/arch/s390/include/asm/timex.h
@@ -128,4 +128,32 @@ static inline unsigned long long get_clock_monotonic(void)
        return get_clock_xt() - sched_clock_base_cc;
 }
 
+/**
+ * tod_to_ns - convert a TOD format value to nanoseconds
+ * @todval: to be converted TOD format value
+ * Returns: number of nanoseconds that correspond to the TOD format value
+ *
+ * Converting a 64 Bit TOD format value to nanoseconds means that the value
+ * must be divided by 4.096. In order to achieve that we multiply with 125
+ * and divide by 512:
+ *
+ *    ns = (todval * 125) >> 9;
+ *
+ * In order to avoid an overflow with the multiplication we can rewrite this.
+ * With a split todval == 2^32 * th + tl (th upper 32 bits, tl lower 32 bits)
+ * we end up with
+ *
+ *    ns = ((2^32 * th + tl) * 125 ) >> 9;
+ * -> ns = (2^23 * th * 125) + ((tl * 125) >> 9);
+ *
+ */
+static inline unsigned long long tod_to_ns(unsigned long long todval)
+{
+       unsigned long long ns;
+
+       ns = ((todval >> 32) << 23) * 125;
+       ns += ((todval & 0xffffffff) * 125) >> 9;
+       return ns;
+}
+
 #endif