timecounter: keep track of accumulated fractional nanoseconds

The current timecounter implementation will drop a variable amount
of resolution, depending on the magnitude of the time delta. In
other words, reading the clock too often or too close to a time
stamp conversion will introduce errors into the time values. This
patch fixes the issue by introducing a fractional nanosecond field
that accumulates the low order bits.

Reported-by: Janusz Użycki <j.uzycki@elproma.com.pl>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 7 deletions

diff --git a/include/linux/timecounter.h b/include/linux/timecounter.h
index af3dfa4e90f0..74f45496e6d1 100644
--- a/include/linux/timecounter.h
+++ b/include/linux/timecounter.h
@@ -55,27 +55,32 @@ struct cyclecounter {
  * @cycle_last:         most recent cycle counter value seen by
  *                      timecounter_read()
  * @nsec:               continuously increasing count
+ * @mask:               bit mask for maintaining the 'frac' field
+ * @frac:               accumulated fractional nanoseconds
  */
 struct timecounter {
         const struct cyclecounter *cc;
         cycle_t cycle_last;
         u64 nsec;
+        u64 mask;
+        u64 frac;
 };
 
 /**
  * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds
  * @cc:         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.
+ * @mask:       bit mask for maintaining the 'frac' field
+ * @frac:       pointer to storage for the fractional nanoseconds.
  */
 static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
-                                      cycle_t cycles)
+                                      cycle_t cycles, u64 mask, u64 *frac)
 {
-        u64 ret = (u64)cycles;
-        ret = (ret * cc->mult) >> cc->shift;
-        return ret;
+        u64 ns = (u64) cycles;
+
+        ns = (ns * cc->mult) + *frac;
+        *frac = ns & mask;
+        return ns >> cc->shift;
 }
 
 /**