[PATCH 18/27] kern: clarify boot time

[Moyano, Gabriel]

From: Warner Losh <imp at FreeBSD.org>

In FreeBSD, the current time is computed from uptime + boottime. Uptime
is a continuous, smooth function that's monotonically increasing. To
effect changes to the current time, boottime is adjusted.  boottime is
mutable and shouldn't be cached against future need. Document the
current implementation, with the caveat that we may stop stepping
boottime on resume in the future and will step uptime instead (noted in
the commit message, but not in the code).

Sponsored by:		Netflix
Reviewed by:		phk, rpokala
Differential Revision:	https://reviews.freebsd.org/D30116
---
 cpukit/score/src/kern_tc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/cpukit/score/src/kern_tc.c b/cpukit/score/src/kern_tc.c
index 5e0b6fa227..076139ebfb 100644
--- a/cpukit/score/src/kern_tc.c
+++ b/cpukit/score/src/kern_tc.c
@@ -235,11 +235,17 @@ volatile int32_t time_uptime = 1;
 #endif /* __rtems__ */
 
 #ifndef __rtems__
+/*
+ * The system time is always computed by summing the estimated boot time and the
+ * system uptime. The timehands track boot time, but it changes when the system
+ * time is set by the user, stepped by ntpd or adjusted when resuming. It
+ * is set to new_time - uptime.
+ */
 static int sysctl_kern_boottime(SYSCTL_HANDLER_ARGS);
 SYSCTL_PROC(_kern, KERN_BOOTTIME, boottime,
     CTLTYPE_STRUCT | CTLFLAG_RD | CTLFLAG_MPSAFE, NULL, 0,
     sysctl_kern_boottime, "S,timeval",
-    "System boottime");
+    "Estimated system boottime");
 
 SYSCTL_NODE(_kern, OID_AUTO, timecounter, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
     "");
-- 
2.17.1