rtos: add tickless support

Add tickless support to the rtos idle loop. Using low
power ticker and RTOS suspend/resume functionality. We set
the next event according to the kernel, and wake-up within this period.

If lowpower ticker is not available, we fallback to
the previous only sleep function.

Author: 0xc0170

rtos/rtos_idle.c

@@ -22,14 +22,50 @@
 
 #include "rtos/rtos_idle.h"
 #include "platform/mbed_sleep.h"
+#include "mbed_critical.h"
+#include "hal/ticker_api.h"
+#include "hal/lp_ticker_api.h"
+#include "cmsis_os2.h"
+#include <limits.h>
+
+static ticker_event_t idle_loop_event;
+
+void idle_loop_handler(uint32_t id)
+{
+    (void)id;
+}
 
 static void default_idle_hook(void)
 {
-    /* Sleep: ideally, we should put the chip to sleep.
-       Unfortunately, this usually requires disconnecting the interface chip (debugger).
-       This can be done, but it would break the local file system.
-    */
+#if DEVICE_LOWPOWERTIMER
+    uint32_t tick_freq = osKernelGetTickFreq();
+    timestamp_t time_in_sleep = 0UL;
+
+    uint32_t ticks_to_sleep = osKernelSuspend();
+    if (ticks_to_sleep) {
+        core_util_critical_section_enter();
+        uint64_t us_to_sleep = ticks_to_sleep * tick_freq; 
+        // Calculate the maximum period we can sleep and stay within
+        uint64_t max_us_sleep = (UINT_MAX / tick_freq) * tick_freq; 
+        if (us_to_sleep > max_us_sleep) {
+            us_to_sleep = max_us_sleep;
+        }
+
+        const ticker_data_t *lp_ticker_data = get_lp_ticker_data();
+        ticker_remove_event(lp_ticker_data, &idle_loop_event);
+        ticker_set_handler(lp_ticker_data, &idle_loop_handler);
+        timestamp_t start_time = lp_ticker_read();
+        ticker_insert_event_us(lp_ticker_data, &idle_loop_event, us_to_sleep, (uint32_t)&idle_loop_event);
+
+        sleep();
+        core_util_critical_section_exit();
+        // calculate how long we slept
+        time_in_sleep = lp_ticker_read() - start_time;
+    }
+    osKernelResume(time_in_sleep / tick_freq);
+#else
     sleep();
+#endif
 }
 static void (*idle_hook_fptr)(void) = &default_idle_hook;