[PATCH 17/27] Make kern.timecounter.hardware tunable

[Moyano, Gabriel]

From: Konstantin Belousov <kib at FreeBSD.org>

Noted and reviewed by:	kevans
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D29122
---
 cpukit/score/src/kern_tc.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/cpukit/score/src/kern_tc.c b/cpukit/score/src/kern_tc.c
index 6fbb59038a..5e0b6fa227 100644
--- a/cpukit/score/src/kern_tc.c
+++ b/cpukit/score/src/kern_tc.c
@@ -273,6 +273,7 @@ SYSCTL_PROC(_kern_timecounter, OID_AUTO, alloweddeviation,
 volatile int rtc_generation = 1;
 
 static int tc_chosen;	/* Non-zero if a specific tc was chosen via sysctl. */
+static char tc_from_tunable[16];
 #endif /* __rtems__ */
 
 static void tc_windup(struct bintime *new_boottimebin);
@@ -1383,17 +1384,26 @@ tc_init(struct timecounter *tc)
 		return;
 	if (tc->tc_quality < 0)
 		return;
-#endif /* __rtems__ */
+	if (tc_from_tunable[0] != '\0' &&
+	    strcmp(tc->tc_name, tc_from_tunable) == 0) {
+		tc_chosen = 1;
+		tc_from_tunable[0] = '\0';
+	} else {
+		if (tc->tc_quality < timecounter->tc_quality)
+			return;
+		if (tc->tc_quality == timecounter->tc_quality &&
+		    tc->tc_frequency < timecounter->tc_frequency)
+			return;
+	}
+	(void)tc->tc_get_timecount(tc);
+	timecounter = tc;
+#else /* __rtems__ */
 	if (tc->tc_quality < timecounter->tc_quality)
 		return;
 	if (tc->tc_quality == timecounter->tc_quality &&
 	    tc->tc_frequency < timecounter->tc_frequency)
 		return;
-#ifndef __rtems__
-	(void)tc->tc_get_timecount(tc);
-#endif /* __rtems__ */
 	timecounter = tc;
-#ifdef __rtems__
 	tc_windup(NULL);
 #endif /* __rtems__ */
 }
@@ -2224,6 +2234,9 @@ inittimehands(void *dummy)
 	for (i = 1, thp = &ths[0]; i < timehands_count;  thp = &ths[i++])
 		thp->th_next = &ths[i];
 	thp->th_next = &ths[0];
+
+	TUNABLE_STR_FETCH("kern.timecounter.hardware", tc_from_tunable,
+	    sizeof(tc_from_tunable));
 }
 SYSINIT(timehands, SI_SUB_TUNABLES, SI_ORDER_ANY, inittimehands, NULL);
 
-- 
2.17.1