[rtems commit] timecounter: Merge FreeBSD change r286701

[Sebastian Huber]

Module:    rtems
Branch:    master
Commit:    f1463c8bdd6c967501365ca37e662d763864167b
Changeset: http://git.rtems.org/rtems/commit/?id=f1463c8bdd6c967501365ca37e662d763864167b

Author:    Ian Lepore <ian at FreeBSD.org>
Date:      Wed Aug 12 20:50:20 2015 +0000

timecounter: Merge FreeBSD change 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).

Update #3175.

---

 cpukit/score/src/kern_tc.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/cpukit/score/src/kern_tc.c b/cpukit/score/src/kern_tc.c
index bb8a3f1..e6e952f 100644
--- a/cpukit/score/src/kern_tc.c
+++ b/cpukit/score/src/kern_tc.c
@@ -242,6 +242,8 @@ SYSCTL_PROC(_kern_timecounter, OID_AUTO, alloweddeviation,
     CTLTYPE_INT | CTLFLAG_RWTUN | CTLFLAG_MPSAFE, 0, 0,
     sysctl_kern_timecounter_adjprecision, "I",
     "Allowed time interval deviation in percents");
+
+static int tc_chosen;	/* Non-zero if a specific tc was chosen via sysctl. */
 #endif /* __rtems__ */
 
 static void tc_windup(void);
@@ -1344,10 +1346,13 @@ tc_init(struct timecounter *tc)
 	    "quality", CTLFLAG_RD, &(tc->tc_quality), 0,
 	    "goodness of time counter");
 	/*
-	 * Never automatically use a timecounter with negative quality.
+	 * Do not automatically switch if the current tc was specifically
+	 * chosen.  Never automatically use a timecounter with negative quality.
 	 * Even though we run on the dummy counter, switching here may be
-	 * worse since this timecounter may not be monotonous.
+	 * worse since this timecounter may not be monotonic.
 	 */
+	if (tc_chosen)
+		return;
 	if (tc->tc_quality < 0)
 		return;
 	if (tc->tc_quality < timecounter->tc_quality)
@@ -1630,9 +1635,12 @@ sysctl_kern_timecounter_hardware(SYSCTL_HANDLER_ARGS)
 	strlcpy(newname, tc->tc_name, sizeof(newname));
 
 	error = sysctl_handle_string(oidp, &newname[0], sizeof(newname), req);
-	if (error != 0 || req->newptr == NULL ||
-	    strcmp(newname, tc->tc_name) == 0)
+	if (error != 0 || req->newptr == NULL)
 		return (error);
+	/* Record that the tc in use now was specifically chosen. */
+	tc_chosen = 1;
+	if (strcmp(newname, tc->tc_name) == 0)
+		return (0);
 	for (newtc = timecounters; newtc != NULL; newtc = newtc->tc_next) {
 		if (strcmp(newname, newtc->tc_name) != 0)
 			continue;
@@ -1661,7 +1669,7 @@ SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware, CTLTYPE_STRING | CTLFLAG_RW,
     "Timecounter hardware selected");
 
 
-/* Report or change the active timecounter hardware. */
+/* Report the available timecounter hardware. */
 static int
 sysctl_kern_timecounter_choice(SYSCTL_HANDLER_ARGS)
 {