s390/timex: micro optimization for tod_to_ns

The conversion of a TOD value to nano-seconds currently uses a 32/32 bit
split with the calculation for "nsecs = (TOD * 125) >> 9". Using a
55/9 bit split saves an instruction.

Signed-off-by: Martin Schwidefsky <[email protected]>

Author: Martin Schwidefsky

arch/s390/include/asm/timex.h

@@ -206,20 +206,16 @@ static inline unsigned long long get_tod_clock_monotonic(void)
  *    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)
+ * With a split todval == 2^9 * th + tl (th upper 55 bits, tl lower 9 bits)
  * we end up with
  *
- *    ns = ((2^32 * th + tl) * 125 ) >> 9;
- * -> ns = (2^23 * th * 125) + ((tl * 125) >> 9);
+ *    ns = ((2^9 * th + tl) * 125 ) >> 9;
+ * -> ns = (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;
+	return ((todval >> 9) * 125) + (((todval & 0x1ff) * 125) >> 9);
 }
 
 #endif