diff options
| -rw-r--r-- | kernel/time/clocksource.c | 28 |
1 files changed, 20 insertions, 8 deletions
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index d0538a75f4c6..581cdbb53844 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -125,6 +125,13 @@ static u64 suspend_start; * * The default of 500 parts per million is based on NTP's limits. * If a clocksource is good enough for NTP, it is good enough for us! + * + * In other words, by default, even if a clocksource is extremely + * precise (for example, with a sub-nanosecond period), the maximum + * permissible skew between the clocksource watchdog and the clocksource + * under test is not permitted to go below the 500ppm minimum defined + * by MAX_SKEW_USEC. This 500ppm minimum may be overridden using the + * CLOCKSOURCE_WATCHDOG_MAX_SKEW_US Kconfig option. */ #ifdef CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US #define MAX_SKEW_USEC CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US @@ -1146,14 +1153,19 @@ void __clocksource_update_freq_scale(struct clocksource *cs, u32 scale, u32 freq } /* - * If the uncertainty margin is not specified, calculate it. - * If both scale and freq are non-zero, calculate the clock - * period, but bound below at 2*WATCHDOG_MAX_SKEW. However, - * if either of scale or freq is zero, be very conservative and - * take the tens-of-milliseconds WATCHDOG_THRESHOLD value for the - * uncertainty margin. Allow stupidly small uncertainty margins - * to be specified by the caller for testing purposes, but warn - * to discourage production use of this capability. + * If the uncertainty margin is not specified, calculate it. If + * both scale and freq are non-zero, calculate the clock period, but + * bound below at 2*WATCHDOG_MAX_SKEW, that is, 500ppm by default. + * However, if either of scale or freq is zero, be very conservative + * and take the tens-of-milliseconds WATCHDOG_THRESHOLD value + * for the uncertainty margin. Allow stupidly small uncertainty + * margins to be specified by the caller for testing purposes, + * but warn to discourage production use of this capability. + * + * Bottom line: The sum of the uncertainty margins of the + * watchdog clocksource and the clocksource under test will be at + * least 500ppm by default. For more information, please see the + * comment preceding CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US above. */ if (scale && freq && !cs->uncertainty_margin) { cs->uncertainty_margin = NSEC_PER_SEC / (scale * freq); |