MFC r286701:

  If a specific timecounter has been chosen via sysctl, and a new
  timecounter with higher quality registers (presumably in a module that has
  just been loaded), do not undo the user's choice by switching to the new
  timecounter.

  Document that behavior, and also the fact that there is no way to
  unregister a timecounter (and thus no way to unload a module containing
  one).

1 files changed, 10 insertions, 4 deletions

diff --git a/share/man/man4/timecounters.4 b/share/man/man4/timecounters.4
index 9abb522..52c68d7 100644
--- a/share/man/man4/timecounters.4
+++ b/share/man/man4/timecounters.4
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 12, 2014
+.Dd August 12, 2015
 .Dt TIMECOUNTERS 4
 .Os
 .Sh NAME
@@ -96,10 +96,16 @@ compared to others.
 A negative value means this time counter is broken and should not be used.
 .El
 .Pp
-The time management code of the kernel chooses one time counter from that list.
-The current choice can be read and affected via the
+The time management code of the kernel automatically switches to a
+higher-quality time counter when it registers, unless the
 .Va kern.timecounter.hardware
-tunable/sysctl.
+sysctl has been used to choose a specific device.
+.Pp
+There is no way to unregister a time counter once it has registered
+with the kernel.
+If a dynamically loaded module contains a time counter you will not
+be able to unload that module, even if the time counter it contains
+is not the one currently in use.
 .Sh SEE ALSO
 .Xr attimer 4 ,
 .Xr eventtimers 4 ,