drivers: timer: Improve the accuracy of the MCUX OS Timer

1. The sys_clock_idle_exit function could be invoked multiple
   times. Hence add code so that is counter is stopped and the
   OS Timer is initialized once.
2. Reset the OS Timer when exiting low power modes where
   the OS Timer loses its state
3. Improve the cycles conversion algorithm. Round to the nearest
   microsecond when converting from ticks to microsecond rather
   than always rounding up to the next highest value.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>

Author: mmahadevan108

drivers/timer/mcux_os_timer.c

@@ -22,6 +22,8 @@
 
 #define CYC_PER_TICK ((uint32_t)((uint64_t)sys_clock_hw_cycles_per_sec()	\
 			      / (uint64_t)CONFIG_SYS_CLOCK_TICKS_PER_SEC))
+#define CYC_PER_US ((uint32_t)((uint64_t)sys_clock_hw_cycles_per_sec()	\
+			      / (uint64_t)USEC_PER_SEC))
 #define MAX_CYC INT_MAX
 #define MAX_TICKS ((MAX_CYC - CYC_PER_TICK) / CYC_PER_TICK)
 #define MIN_DELAY 1000
@@ -37,7 +39,12 @@ static OSTIMER_Type *base;
  */
 static uint64_t cyc_sys_compensated;
 #if DT_NODE_HAS_STATUS_OKAY(DT_NODELABEL(standby)) && CONFIG_PM
+/* This is the counter device used when OS timer is not available in
+ * standby mode.
+ */
 static const struct device *counter_dev;
+/* Indicates if the counter is running. */
+bool counter_running;
 #endif
 
 static uint64_t mcux_lpc_ostick_get_compensated_timer_value(void)
@@ -86,8 +93,7 @@ static uint32_t mcux_lpc_ostick_set_counter_timeout(int32_t curr_timeout)
 		uint32_t timeout;
 		int32_t ticks;
 		struct counter_top_cfg top_cfg = { 0 };
-
-		timeout = k_ticks_to_us_ceil32(curr_timeout);
+		timeout = k_ticks_to_us_near32(curr_timeout);
 
 		ticks = counter_us_to_ticks(counter_dev, timeout);
 		ticks = CLAMP(ticks, 1, counter_get_max_top_value(counter_dev));
@@ -111,17 +117,18 @@ static uint32_t mcux_lpc_ostick_set_counter_timeout(int32_t curr_timeout)
 			}
 		}
 
+		/* Counter is set to wakeup the system after the requested time */
+		if (counter_start(counter_dev) != 0) {
+			ret = 1;
+			goto done;
+		}
+		counter_running = true;
 #if CONFIG_MCUX_OS_TIMER_PM_POWERED_OFF
 		/* Capture the current timer value for cases where it loses its state
 		 * in low power modes.
 		 */
 		cyc_sys_compensated += OSTIMER_GetCurrentTimerValue(base);
 #endif
-
-		/* Counter is set to wakeup the system after the requested time */
-		if (counter_start(counter_dev) != 0) {
-			ret = 1;
-		}
 	} else {
 		ret = 1;
 	}
@@ -137,34 +144,37 @@ static uint32_t mcux_lpc_ostick_set_counter_timeout(int32_t curr_timeout)
  */
 static uint32_t mcux_lpc_ostick_compensate_system_timer(void)
 {
-	uint32_t ret = 0;
-
-	if (counter_dev) {
-		uint32_t slept_time_ticks;
-		uint32_t slept_time_us;
+	uint32_t slept_time_ticks;
+	uint32_t slept_time_us;
 
-		counter_stop(counter_dev);
+	if (!counter_dev) {
+		return 1;
+	}
 
-		counter_get_value(counter_dev, &slept_time_ticks);
+	if (!counter_running) {
+		return 0;
+	}
 
-		if (!(counter_is_counting_up(counter_dev))) {
-			slept_time_ticks = counter_get_top_value(counter_dev) - slept_time_ticks;
-		}
-		slept_time_us = counter_ticks_to_us(counter_dev, slept_time_ticks);
-		cyc_sys_compensated += (k_us_to_ticks_floor32(slept_time_us) * CYC_PER_TICK);
+	counter_stop(counter_dev);
+	counter_running = false;
+	counter_get_value(counter_dev, &slept_time_ticks);
 
+	if (!(counter_is_counting_up(counter_dev))) {
+		slept_time_ticks = counter_get_top_value(counter_dev) -
+				   slept_time_ticks;
+	}
+	slept_time_us = counter_ticks_to_us(counter_dev, slept_time_ticks);
+	cyc_sys_compensated += CYC_PER_US * slept_time_us;
 #if CONFIG_MCUX_OS_TIMER_PM_POWERED_OFF
-		/* Reactivate os_timer for cases where it loses its state */
-		OSTIMER_Init(base);
+	/* Reset the OS Timer to a known state */
+	RESET_PeripheralReset(kOSEVENT_TIMER_RST_SHIFT_RSTn);
+	/* Reactivate os_timer for cases where it loses its state */
+	OSTIMER_Init(base);
 #endif
+	/* Announce the time slept to the kernel*/
+	mcux_lpc_ostick_isr(NULL);
 
-		/* Announce the time slept to the kernel*/
-		mcux_lpc_ostick_isr(NULL);
-	} else {
-		ret = 1;
-	}
-
-	return ret;
+	return 0;
 }
 
 #endif