events: Don't use OsTimer in SysTick builds

Previous fix assumed OsTimer is in use - it is for tickless RTOS builds
and non-RTOS builds, but non-tickless RTOS builds avoid it altogether
and use the SysTick peripheral.

Restore previous behaviour for those builds, and just always read the
tick count - there is no downside to this when ticking.

While in the code, make equeue_tick initialisation explicit. This was
problem if the OsTimer is not yet initialised when th
done while debugging - turns out not to be part of the fix, but it
speeds up the runtime code.

equeue.c

@@ -64,6 +64,7 @@ int equeue_create_inplace(equeue_t *q, size_t size, void *buffer) {
     q->slab.data = q->buffer;
 
     q->queue = 0;
+    equeue_tick_init();
     q->tick = equeue_tick();
     q->generation = 0;
     q->break_requested = false;

equeue_mbed.cpp

@@ -27,7 +27,14 @@ using namespace mbed;
 #include "rtos/Kernel.h"
 #include "platform/mbed_os_timer.h"
 
+void equeue_tick_init() {
+#if defined MBED_TICKLESS || !MBED_CONF_RTOS_PRESENT
+    mbed::internal::init_os_timer();
+#endif
+}
+
 unsigned equeue_tick() {
+#if defined MBED_TICKLESS || !MBED_CONF_RTOS_PRESENT
     // It is not safe to call get_ms_count from ISRs, both
     // because documentation says so, and because it will give
     // a stale value from the RTOS if the interrupt has woken
@@ -42,6 +49,14 @@ unsigned equeue_tick() {
     } else {
         return rtos::Kernel::get_ms_count();
     }
+#else
+    // And this is the legacy behaviour - if running in
+    // non-tickless mode, this works fine, despite Mbed OS
+    // documentation saying no. (Most recent CMSIS-RTOS
+    // permits `ososKernelGetTickCount` from IRQ, and our
+    // `rtos::Kernel` wrapper copes too).
+    return rtos::Kernel::get_ms_count();
+#endif
 }
 
 #else
@@ -57,7 +72,6 @@ unsigned equeue_tick() {
 #define ALIAS_TIMEOUT    Timeout
 #endif
 
-static bool equeue_tick_inited = false;
 static volatile unsigned equeue_minutes = 0;
 static unsigned equeue_timer[
         (sizeof(ALIAS_TIMER)+sizeof(unsigned)-1)/sizeof(unsigned)];
@@ -69,7 +83,7 @@ static void equeue_tick_update() {
     reinterpret_cast<ALIAS_TIMER*>(equeue_timer)->reset();
 }
 
-static void equeue_tick_init() {
+void equeue_tick_init() {
     MBED_STATIC_ASSERT(sizeof(equeue_timer) >= sizeof(ALIAS_TIMER),
             "The equeue_timer buffer must fit the class Timer");
     MBED_STATIC_ASSERT(sizeof(equeue_ticker) >= sizeof(ALIAS_TICKER),
@@ -80,15 +94,9 @@ static void equeue_tick_init() {
     equeue_minutes = 0;
     timer->start();
     ticker->attach_us(equeue_tick_update, 1000 << 16);
-
-    equeue_tick_inited = true;
 }
 
 unsigned equeue_tick() {
-    if (!equeue_tick_inited) {
-        equeue_tick_init();
-    }
-
     unsigned minutes;
     unsigned ms;
 

equeue_platform.h

@@ -60,6 +60,7 @@ extern "C" {
 // limited by the accuracy of this tick.
 //
 // Must intentionally overflow to 0 after 2^32-1
+void equeue_tick_init(void);
 unsigned equeue_tick(void);
 
 

equeue_posix.c

@@ -14,6 +14,9 @@
 
 
 // Tick operations
+void equeue_tick_init(void) {
+}
+
 unsigned equeue_tick(void) {
     struct timeval tv;
     gettimeofday(&tv, 0);