2 files changed, 177 insertions, 0 deletions

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index f88d32f8ff7c..573819ef4cc0 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -22,8 +22,109 @@ typedef u64 cycle_t;
 struct clocksource;
 
 /**
+ * struct cyclecounter - hardware abstraction for a free running counter
+ *      Provides completely state-free accessors to the underlying hardware.
+ *      Depending on which hardware it reads, the cycle counter may wrap
+ *      around quickly. Locking rules (if necessary) have to be defined
+ *      by the implementor and user of specific instances of this API.
+ *
+ * @read:               returns the current cycle value
+ * @mask:               bitmask for two's complement
+ *                      subtraction of non 64 bit counters,
+ *                      see CLOCKSOURCE_MASK() helper macro
+ * @mult:               cycle to nanosecond multiplier
+ * @shift:              cycle to nanosecond divisor (power of two)
+ */
+struct cyclecounter {
+        cycle_t (*read)(const struct cyclecounter *cc);
+        cycle_t mask;
+        u32 mult;
+        u32 shift;
+};
+
+/**
+ * struct timecounter - layer above a %struct cyclecounter which counts nanoseconds
+ *      Contains the state needed by timecounter_read() to detect
+ *      cycle counter wrap around. Initialize with
+ *      timecounter_init(). Also used to convert cycle counts into the
+ *      corresponding nanosecond counts with timecounter_cyc2time(). Users
+ *      of this code are responsible for initializing the underlying
+ *      cycle counter hardware, locking issues and reading the time
+ *      more often than the cycle counter wraps around. The nanosecond
+ *      counter will only wrap around after ~585 years.
+ *
+ * @cc:                 the cycle counter used by this instance
+ * @cycle_last:         most recent cycle counter value seen by
+ *                      timecounter_read()
+ * @nsec:               continuously increasing count
+ */
+struct timecounter {
+        const struct cyclecounter *cc;
+        cycle_t cycle_last;
+        u64 nsec;
+};
+
+/**
+ * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds
+ * @tc:         Pointer to cycle counter.
+ * @cycles:     Cycles
+ *
+ * XXX - This could use some mult_lxl_ll() asm optimization. Same code
+ * as in cyc2ns, but with unsigned result.
+ */
+static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
+                                      cycle_t cycles)
+{
+        u64 ret = (u64)cycles;
+        ret = (ret * cc->mult) >> cc->shift;
+        return ret;
+}
+
+/**
+ * timecounter_init - initialize a time counter
+ * @tc:                 Pointer to time counter which is to be initialized/reset
+ * @cc:                 A cycle counter, ready to be used.
+ * @start_tstamp:       Arbitrary initial time stamp.
+ *
+ * After this call the current cycle register (roughly) corresponds to
+ * the initial time stamp. Every call to timecounter_read() increments
+ * the time stamp counter by the number of elapsed nanoseconds.
+ */
+extern void timecounter_init(struct timecounter *tc,
+                             const struct cyclecounter *cc,
+                             u64 start_tstamp);
+
+/**
+ * timecounter_read - return nanoseconds elapsed since timecounter_init()
+ *                    plus the initial time stamp
+ * @tc:          Pointer to time counter.
+ *
+ * In other words, keeps track of time since the same epoch as
+ * the function which generated the initial time stamp.
+ */
+extern u64 timecounter_read(struct timecounter *tc);
+
+/**
+ * timecounter_cyc2time - convert a cycle counter to same
+ *                        time base as values returned by
+ *                        timecounter_read()
+ * @tc:         Pointer to time counter.
+ * @cycle:      a value returned by tc->cc->read()
+ *
+ * Cycle counts that are converted correctly as long as they
+ * fall into the interval [-1/2 max cycle count, +1/2 max cycle count],
+ * with "max cycle count" == cs->mask+1.
+ *
+ * This allows conversion of cycle counter values which were generated
+ * in the past.
+ */
+extern u64 timecounter_cyc2time(struct timecounter *tc,
+                                cycle_t cycle_tstamp);
+
+/**
  * struct clocksource - hardware abstraction for a free running counter
  *      Provides mostly state-free accessors to the underlying hardware.
+ *      This is the structure used for system time.
  *
  * @name:               ptr to clocksource name
  * @list:               list head for registration
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index ca89e1593f08..c46c931a7fe7 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -31,6 +31,82 @@
 #include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
 #include <linux/tick.h>
 
+void timecounter_init(struct timecounter *tc,
+                      const struct cyclecounter *cc,
+                      u64 start_tstamp)
+{
+        tc->cc = cc;
+        tc->cycle_last = cc->read(cc);
+        tc->nsec = start_tstamp;
+}
+EXPORT_SYMBOL(timecounter_init);
+
+/**
+ * timecounter_read_delta - get nanoseconds since last call of this function
+ * @tc:         Pointer to time counter
+ *
+ * When the underlying cycle counter runs over, this will be handled
+ * correctly as long as it does not run over more than once between
+ * calls.
+ *
+ * The first call to this function for a new time counter initializes
+ * the time tracking and returns an undefined result.
+ */
+static u64 timecounter_read_delta(struct timecounter *tc)
+{
+        cycle_t cycle_now, cycle_delta;
+        u64 ns_offset;
+
+        /* read cycle counter: */
+        cycle_now = tc->cc->read(tc->cc);
+
+        /* calculate the delta since the last timecounter_read_delta(): */
+        cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;
+
+        /* convert to nanoseconds: */
+        ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta);
+
+        /* update time stamp of timecounter_read_delta() call: */
+        tc->cycle_last = cycle_now;
+
+        return ns_offset;
+}
+
+u64 timecounter_read(struct timecounter *tc)
+{
+        u64 nsec;
+
+        /* increment time by nanoseconds since last call */
+        nsec = timecounter_read_delta(tc);
+        nsec += tc->nsec;
+        tc->nsec = nsec;
+
+        return nsec;
+}
+EXPORT_SYMBOL(timecounter_read);
+
+u64 timecounter_cyc2time(struct timecounter *tc,
+                         cycle_t cycle_tstamp)
+{
+        u64 cycle_delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
+        u64 nsec;
+
+        /*
+         * Instead of always treating cycle_tstamp as more recent
+         * than tc->cycle_last, detect when it is too far in the
+         * future and treat it as old time stamp instead.
+         */
+        if (cycle_delta > tc->cc->mask / 2) {
+                cycle_delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
+                nsec = tc->nsec - cyclecounter_cyc2ns(tc->cc, cycle_delta);
+        } else {
+                nsec = cyclecounter_cyc2ns(tc->cc, cycle_delta) + tc->nsec;
+        }
+
+        return nsec;
+}
+EXPORT_SYMBOL(timecounter_cyc2time);
+
 /* XXX - Would like a better way for initializing curr_clocksource */
 extern struct clocksource clocksource_jiffies;