clocksource: Improve comments for watchdog skew bounds
Add more detail on the rationale for bounding the clocksource
->uncertainty_margin below at about 500ppm.
Signed-off-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Feng Tang <feng.tang@intel.com>
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index d25ba49..9ca4e8d 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);
