nanostack-hal: timer: conditionalize the use of high pri event thread

nanostack-hal.critical-section-usable-from-interrupt -tunable was
previously added to optionally make critical section code interrupt safe.
The IRQ safe critical section is a prequisite for interrupt safe timer
callbacks.

The same flag can be used to enable calling of the timer callbacks
directly from the timer interrupt context, without bouncing them via
event thread. This removes the code and RAM consumed by EventQueue
and the thread serving the high priority events.

If the system does not have any dependencies on mbed_shared_queues,
by setting this flag the static RAM usage is now further reduced
by ~1600 bytes and code size by 4KB.

Note: the default behavior is not changed, one needs to override the
"nanostack-hal.critical-section-usable-from-interrupt" to have "true".

Author: TeroJaasko

features/FEATURE_COMMON_PAL/nanostack-hal-mbed-cmsis-rtos/arm_hal_timer.cpp

@@ -12,15 +12,26 @@
 
 static Timer timer;
 static Timeout timeout;
+
+// If critical sections are implemented using mutexes, timers must be called in thread context, and
+// we use the high-priority event queue for this.
+// If critical sections disable interrupts, we can call timers directly from interrupt. Avoiding the
+// event queue can save ~1600B of RAM if the rest of the system is not using the event queue either.
+// Caveats of this tunable are listed on arm_hal_interrupt.c.
+#if !MBED_CONF_NANOSTACK_HAL_CRITICAL_SECTION_USABLE_FROM_INTERRUPT
 static EventQueue *equeue;
+#endif
+
 static uint32_t due;
 static void (*arm_hal_callback)(void);
 
 // Called once at boot
 void platform_timer_enable(void)
 {
+#if !MBED_CONF_NANOSTACK_HAL_CRITICAL_SECTION_USABLE_FROM_INTERRUPT
     equeue = mbed_highprio_event_queue();
     MBED_ASSERT(equeue != NULL);
+#endif
 }
 
 // Actually cancels a timer, not the opposite of enable
@@ -38,7 +49,14 @@ void platform_timer_set_cb(void (*new_fp)(void))
 static void timer_callback(void)
 {
     due = 0;
+
+#if MBED_CONF_NANOSTACK_HAL_CRITICAL_SECTION_USABLE_FROM_INTERRUPT
+    // Callback is interrupt safe so it can be called directly without
+    // bouncing via event queue thread.
+    arm_hal_callback();
+#else
     equeue->call(arm_hal_callback);
+#endif
 }
 
 // This is called from inside platform_enter_critical - IRQs can't happen