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>
diff --git a/include/linux/timecounter.h b/include/linux/timecounter.h
index af3dfa4..74f4549 100644
--- a/include/linux/timecounter.h
+++ b/include/linux/timecounter.h
@@ -55,27 +55,32 @@
* @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;
}
/**