Don't reschedule ticker while dispatching

Wait until dispatching is finished before scheduling the next ticker
interrupt. This prevents unnecissary calls to set_interrupt from
periodic elements being added back.

This is particularly useful for the low power ticker on devices with
LPTICKER_DELAY_TICKS set to a non-zero value. This is because the low
power ticker cannot be reschduled immediately and needs to fall back
onto the microsecond ticker which temporarily locks deep sleep.

hal/mbed_ticker_api.c

@@ -69,6 +69,7 @@ static void initialize(const ticker_data_t *ticker)
     ticker->queue->max_delta = max_delta;
     ticker->queue->max_delta_us = max_delta_us;
     ticker->queue->present_time = 0;
+    ticker->queue->dispatching = false;
     ticker->queue->initialized = true;
 
     update_present_time(ticker);
@@ -229,6 +230,12 @@ int _ticker_match_interval_passed(timestamp_t prev_tick, timestamp_t cur_tick, t
 static void schedule_interrupt(const ticker_data_t *const ticker)
 {
     ticker_event_queue_t *queue = ticker->queue;
+    if (ticker->queue->dispatching) {
+        // Don't schedule the next interrupt until dispatching is
+        // finished. This prevents repeated calls to interface->set_interrupt
+        return;
+    }
+
     update_present_time(ticker);
 
     if (ticker->queue->head) {
@@ -280,6 +287,7 @@ void ticker_irq_handler(const ticker_data_t *const ticker)
     ticker->interface->clear_interrupt();
 
     /* Go through all the pending TimerEvents */
+    ticker->queue->dispatching = true;
     while (1) {
         if (ticker->queue->head == NULL) {
             break;
@@ -302,6 +310,7 @@ void ticker_irq_handler(const ticker_data_t *const ticker)
             break;
         }
     }
+    ticker->queue->dispatching = false;
 
     schedule_interrupt(ticker);
 

hal/ticker_api.h

@@ -80,6 +80,7 @@ typedef struct {
     uint64_t tick_remainder;            /**< Ticks that have not been added to base_time */
     us_timestamp_t present_time;        /**< Store the timestamp used for present time */
     bool initialized;                   /**< Indicate if the instance is initialized */
+    bool dispatching;                   /**< The function ticker_irq_handler is dispatching */
     uint8_t frequency_shifts;           /**< If frequency is a value of 2^n, this is n, otherwise 0 */
 } ticker_event_queue_t;