[PATCH] MSEC_TO_JIFFIES consolidation

From: Ingo Molnar <mingo@elte.hu>

We have various different implementations of MSEC[S]_TO_JIFFIES and
JIFFIES_TO_MSEC[S].  We recently had a compile-time clash in USB.

Fix all that up.

- The SCTP version was very inefficient.  Hopefully this version is accurate
  enough.

- Optimise for the HZ=100 and HZ=1000 cases

- This version does round-up, so sleep(9 milliseconds) works OK on 100HZ.

- We still have lots of jiffies_to_msec and msec_to_jiffies implementations.

From: William Lee Irwin III <wli@holomorphy.com>

  Optimize the cases where HZ is a divisor of 1000 or vice-versa in
  JIFFIES_TO_MSECS() and MSECS_TO_JIFFIES() by allowing the nonvanishing(!)
  integral ratios to appear as a parenthesized expressions eligible for
  constant folding optimizations.

From: me

  Use typesafe inlines for the jiffies-to-millisecond conversion functions.

  This means that milliseconds officially takes the type `unsigned int'.
  All current callers seem to be OK with that.

  Drivers need to be fixed up to use this instead of their private versions.

1 files changed, 31 insertions, 0 deletions

diff --git a/include/linux/time.h b/include/linux/time.h
index b85d596949a3..9a92850cab4b 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -177,6 +177,37 @@ struct timezone {
 	(SH_DIV((MAX_JIFFY_OFFSET >> SEC_JIFFIE_SC) * TICK_NSEC, NSEC_PER_SEC, 1) - 1)
 
 #endif
+
+/*
+ * Convert jiffies to milliseconds and back.
+ *
+ * Avoid unnecessary multiplications/divisions in the
+ * two most common HZ cases:
+ */
+static inline unsigned int jiffies_to_msecs(unsigned long j)
+{
+#if HZ <= 1000 && !(1000 % HZ)
+	return (1000 / HZ) * j;
+#elif HZ > 1000 && !(HZ % 1000)
+	return (j + (HZ / 1000) - 1)/(HZ / 1000);
+#else
+	return (j * 1000) / HZ;
+#endif
+}
+static inline unsigned long msecs_to_jiffies(unsigned int m)
+{
+#if HZ <= 1000 && !(1000 % HZ)
+	return (m + (1000 / HZ) - 1) / (1000 / HZ);
+#elif HZ > 1000 && !(HZ % 1000)
+	return m * (HZ / 1000);
+#else
+	return (m * HZ + 999) / 1000;
+#endif
+}
+
+#define JIFFIES_TO_MSECS(j)	jiffies_to_msecs(j)
+#define MSECS_TO_JIFFIES(m)	msecs_to_jiffies(m)
+
 /*
  * The TICK_NSEC - 1 rounds up the value to the next resolution.  Note
  * that a remainder subtract here would not do the right thing as the