platforms: use CLOCK_MONOTONIC

For the non-lockstep case we want to use CLOCK_MONOTONIC if possible.

platforms/posix/src/px4_layer/px4_sem.cpp

@@ -58,6 +58,12 @@ int px4_sem_init(px4_sem_t *s, int pshared, unsigned value)
 	pthread_cond_init(&(s->wait), nullptr);
 	pthread_mutex_init(&(s->lock), nullptr);
 
+	// We want to use CLOCK_MONOTONIC if possible.
+	pthread_condattr_t attr;
+	pthread_condattr_init(&attr);
+	pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
+	pthread_cond_init(&(s->wait), &attr);
+
 	return 0;
 }
 

src/lib/cdev/posix/cdev_platform.cpp

@@ -379,7 +379,8 @@ extern "C" {
 
 				// Get the current time
 				struct timespec ts;
-				// px4_sem_timedwait is implemented using CLOCK_MONOTONIC.
+				// px4_sem_timedwait is implemented using CLOCK_MONOTONIC,
+				// at least for lockstep, on Qurt and on Linux.
 				px4_clock_gettime(CLOCK_MONOTONIC, &ts);
 
 				// Calculate an absolute time in the future