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authorRichard Cochran <richardcochran@gmail.com>2014-12-21 21:47:06 +0300
committerDavid S. Miller <davem@davemloft.net>2014-12-31 02:29:27 +0300
commit2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0 (patch)
treec4b176d1e40eecb2caa00952eae95ecb2b2a046b /include/linux
parentf25a30be359d0535fb1c7c1619cabb0ad17cfbf1 (diff)
downloadlinux-2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0.tar.xz
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>
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/timecounter.h19
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;
}
/**