drivers: timer: nrf_grtc_timer: add last_count initialization

The GRTC counter is not cleared at startup, therefore the
`last_count` variable needs to be initialized accordingly.
This change:
- Prevents overflow of the `sys_clock_announce()` int32_t parameter
- Ensures the correct uptime value, which should be reset during
  initialization

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>

Author: adamkondraciuk

drivers/timer/nrf_grtc_timer.c

@@ -70,6 +70,7 @@ static struct k_spinlock lock;
 static uint64_t last_count; /* Time (SYSCOUNTER value) @last sys_clock_announce() */
 static atomic_t int_mask;
 static uint8_t ext_channels_allocated;
+static uint64_t grtc_start_value;
 static nrfx_grtc_channel_t system_clock_channel_data = {
 	.handler = sys_clock_timeout_handler,
 	.p_context = NULL,
@@ -362,6 +363,11 @@ int z_nrf_grtc_timer_capture_read(int32_t chan, uint64_t *captured_time)
 	return 0;
 }
 
+uint64_t z_nrf_grtc_timer_startup_value_get(void)
+{
+	return grtc_start_value;
+}
+
 #if defined(CONFIG_POWEROFF) && defined(CONFIG_NRF_GRTC_START_SYSCOUNTER)
 int z_nrf_grtc_wakeup_prepare(uint64_t wake_time_us)
 {
@@ -489,6 +495,8 @@ static int sys_clock_driver_init(void)
 	}
 #endif /* CONFIG_NRF_GRTC_START_SYSCOUNTER */
 
+	last_count = (counter() / CYC_PER_TICK) * CYC_PER_TICK;
+	grtc_start_value = last_count;
 	int_mask = NRFX_GRTC_CONFIG_ALLOWED_CC_CHANNELS_MASK;
 	if (!IS_ENABLED(CONFIG_TICKLESS_KERNEL)) {
 		system_timeout_set_relative(CYC_PER_TICK);

include/zephyr/drivers/timer/nrf_grtc_timer.h

@@ -189,6 +189,17 @@ int z_nrf_grtc_timer_capture_read(int32_t chan, uint64_t *captured_time);
  */
 int z_nrf_grtc_wakeup_prepare(uint64_t wake_time_us);
 
+/** @brief Get the GRTC counter value latched at startup.
+ *
+ * @note The GRTC timer is not cleared by software at startup,
+ *	 while the system tick starts counting from zero.
+ *	 In some cases, it may be necessary to compare the system tick
+ *	 with the GRTC value — in such situations, this offset can be useful.
+ *
+ * @return GRTC value latched during system clock initialization.
+ */
+uint64_t z_nrf_grtc_timer_startup_value_get(void);
+
 /**
  * @brief       Initialize the GRTC clock timer driver from an application-
  *              defined function.