[PATCH 3/3 V2] time: update msecs_to_jiffies doc and move to kernel-doc

[Nicholas Mc Guire]

update the documentation of msecs_to_jiffies and move to kernel-doc format

Signed-off-by: Nicholas Mc Guire <hofrat@xxxxxxxxx>
---

V2: reformatting of documentation for the Hz range dependent 
    _msecs_to_jiffies() helpers

Patch is against 4.0-rc7 (localversion-next is -next-20150410)

 include/linux/jiffies.h |   39 +++++++++++++++++++++++++++++++++++++++
 kernel/time/time.c      |   25 ++++++++++++++++---------
 2 files changed, 55 insertions(+), 9 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 273b093..bb8d0d7 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -291,11 +291,22 @@ static inline u64 jiffies_to_nsecs(const unsigned long j)
 
 extern unsigned long __msecs_to_jiffies(const unsigned int m);
 #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
+/*
+ * HZ is equal to or smaller than 1000, and 1000 is a nice round
+ * multiple of HZ, divide with the factor between them, but round
+ * upwards:
+ */
 static inline unsigned long _msecs_to_jiffies(const unsigned int m)
 {
 		return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
 }
 #elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
+/*
+ * HZ is larger than 1000, and HZ is a nice round multiple of 1000 -
+ * simply multiply with the factor between them.
+ *
+ * But first make sure the multiplication result cannot overflow:
+ */
 static inline unsigned long _msecs_to_jiffies(const unsigned int m)
 {
 		if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
@@ -303,6 +314,10 @@ static inline unsigned long _msecs_to_jiffies(const unsigned int m)
 		return m * (HZ / MSEC_PER_SEC);
 }
 #else
+/*
+ * Generic case - multiply, round and divide. But first check that if
+ * we are doing a net multiplication, that we wouldn't overflow:
+ */
 static inline unsigned long _msecs_to_jiffies(const unsigned int m)
 {
 		if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
@@ -312,6 +327,30 @@ static inline unsigned long _msecs_to_jiffies(const unsigned int m)
 			>> MSEC_TO_HZ_SHR32;
 }
 #endif
+/**
+ * msecs_to_jiffies: - convert milliseconds to jiffies
+ * @m:	time in milliseconds
+ *
+ * conversion is done as follows:
+ *
+ * - negative values mean 'infinite timeout' (MAX_JIFFY_OFFSET)
+ *
+ * - 'too large' values [that would result in larger than
+ *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
+ *
+ * - all other values are converted to jiffies by either multiplying
+ *   the input value by a factor or dividing it with a factor and
+ *   handling any 32-bit overflows.
+ *   for the details see __msecs_to_jiffies()
+ *
+ * msecs_to_jiffies() checks for the passed in value being a constant
+ * via __builtin_constant_p() allowing gcc to eliminate most of the
+ * code, __msecs_to_jiffies() is called if the value passed does not
+ * allow constant folding and the actual conversion must be done at
+ * runtime.
+ * the HZ range specific helpers _msecs_to_jiffies() are used for both
+ * cases
+ */
 static inline unsigned long msecs_to_jiffies(const unsigned int m)
 {
 	if (__builtin_constant_p(m)) {
diff --git a/kernel/time/time.c b/kernel/time/time.c
index f5196aa..884bb8e 100644
--- a/kernel/time/time.c
+++ b/kernel/time/time.c
@@ -483,22 +483,29 @@ struct timespec64 ns_to_timespec64(const s64 nsec)
 }
 EXPORT_SYMBOL(ns_to_timespec64);
 #endif
-/*
- * When we convert to jiffies then we interpret incoming values
- * the following way:
+/**
+ * msecs_to_jiffies: - convert milliseconds to jiffies
+ * @m:	time in milliseconds
+ *
+ * conversion is done as follows:
  *
  * - negative values mean 'infinite timeout' (MAX_JIFFY_OFFSET)
- *   negative values are handled in msecs_to_jiffies in
- *   include/linux/jiffies.h
  *
  * - 'too large' values [that would result in larger than
  *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
  *
  * - all other values are converted to jiffies by either multiplying
- *   the input value by a factor or dividing it with a factor
- *
- * We must also be careful about 32-bit overflows.
- *
+ *   the input value by a factor or dividing it with a factor and
+ *   handling any 32-bit overflows.
+ *   for the details see __msecs_to_jiffies()
+ *
+ * msecs_to_jiffies() checks for the passed in value being a constant
+ * via __builtin_constant_p() allowing gcc to eliminate most of the
+ * code, __msecs_to_jiffies() is called if the value passed does not
+ * allow constant folding and the actual conversion must be done at
+ * runtime.
+ * the _msecs_to_jiffies helpers are the HZ dependent conversion
+ * routines found in include/linux/jiffies.h
  */
 unsigned long __msecs_to_jiffies(const unsigned int m)
 {
-- 
1.7.10.4

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