Reduce tickless interrupt latency

Add the function osKernelCanSleep which indicates if it is safe to
call sleep (while the kernel is suspended). This patch also replaces
the svcRtx* calls with os* calls since they no longer need to be
called from a critical section.

Author: c1728p9

rtos/TARGET_CORTEX/mbed_rtx_idle.cpp

@@ -177,22 +177,28 @@ static void default_idle_hook(void)
 {
     uint32_t elapsed_ticks = 0;
 
-    core_util_critical_section_enter();
-    uint32_t ticks_to_sleep = svcRtxKernelSuspend();
-    MBED_ASSERT(os_timer->get_tick() == svcRtxKernelGetTickCount());
+
+    uint32_t ticks_to_sleep = osKernelSuspend();
+    MBED_ASSERT(os_timer->get_tick() == osKernelGetTickCount());
     if (ticks_to_sleep) {
+
         os_timer->schedule_tick(ticks_to_sleep);
 
+        core_util_critical_section_enter();
         sleep_manager_lock_deep_sleep();
-        sleep();
+        if (osKernelCanSleep()) {
+            sleep();
+        }
         sleep_manager_unlock_deep_sleep();
+        core_util_critical_section_exit();
 
         os_timer->cancel_tick();
         // calculate how long we slept
         elapsed_ticks = os_timer->update_tick();
+
     }
-    svcRtxKernelResume(elapsed_ticks);
-    core_util_critical_section_exit();
+    osKernelResume(elapsed_ticks);
+
 }
 
 #else

rtos/TARGET_CORTEX/rtx5/cmsis_os2.h

@@ -350,7 +350,11 @@ uint32_t osKernelGetSysTimerCount (void);
 /// Get the RTOS kernel system timer frequency.
 /// \return frequency of the system timer.
 uint32_t osKernelGetSysTimerFreq (void);
- 
+
+/// Check if it is safe to sleep.
+/// \return osOK if it is safe to sleep, otherwise osErrorTimeout.
+osStatus_t osKernelCanSleep (void);
+
 
 //  ==== Thread Management Functions ====
 

rtos/TARGET_CORTEX/rtx5/rtx_kernel.c

@@ -641,3 +641,13 @@ uint32_t osKernelGetSysTimerFreq (void) {
     return  __svcKernelGetSysTimerFreq();
   }
 }
+
+/// Check if it is safe to sleep.
+osStatus_t osKernelCanSleep (void) {
+  if (osRtxInfo.kernel.pendSV) {
+    return osErrorTimeout;
+  } else {
+    return osOK;
+  }
+}
+