bluetooth: fast_pair: fmdn: add uptime workaround to the clock module

Added the kernel uptime workaround to the clock module in the FMDN
extension that is part of the Fast Pair library.

Applied the workaround for the kernel uptime persistence after a system
reset. The FMDN clock module assumes that the kernel uptime starts from
zero after the system reboot.

Activated the workaround only for devices from the nRF54L series that
use the GRTC timer as the system clock source (the default
configuration). For this configuration, the kernel uptime persists
after a system reset if the reset reason is of a specific type, such as
a software reset (for example, triggered by the sys_reboot API).

Ref: NCSDK-32268

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>

Author: kapi-no

subsys/bluetooth/services/fast_pair/fmdn/Kconfig

@@ -237,6 +237,24 @@ config BT_FAST_PAIR_FMDN_CLOCK_NVM_UPDATE_RETRY_TIME
 	  determine the next storage attempt in case of a storage operation
 	  failure.
 
+config BT_FAST_PAIR_FMDN_CLOCK_UPTIME_PERSISTENCE
+	bool
+	default y if NRF_GRTC_TIMER && SOC_SERIES_NRF54LX
+	help
+	  Apply the workaround for the kernel uptime persistence after a system reset.
+	  The FMDN clock module assumes that the kernel uptime starts from zero after
+	  the system reboot.
+
+	  Activate the workaround only for devices from the nRF54L series that use the
+	  GRTC timer as the system clock source (the default configuration). For this
+	  configuration, the kernel uptime persists after a system reset if the reset
+	  reason is of a specific type, such as a software reset (for example, triggered
+	  by the sys_reboot API).
+
+	  The kernel uptime does not persist for devices from the nRF54H series even
+	  though they use the GRTC timer as the system clock source. The workaround
+	  is not applied for this device series.
+
 endif # BT_FAST_PAIR_FMDN_CLOCK
 
 config BT_FAST_PAIR_FMDN_DULT

subsys/bluetooth/services/fast_pair/fmdn/clock.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2024 Nordic Semiconductor ASA
+ * Copyright (c) 2024-2025 Nordic Semiconductor ASA
  *
  * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
  */
@@ -17,6 +17,7 @@ LOG_MODULE_REGISTER(fp_fmdn_clock, CONFIG_BT_FAST_PAIR_LOG_LEVEL);
 #include "fp_storage_clock.h"
 
 static uint32_t storage_clock_boot_checkpoint;
+static int64_t uptime_on_boot;
 
 static void fmdn_clock_storage_work_handle(struct k_work *work);
 
@@ -33,6 +34,14 @@ static uint32_t fmdn_clock_read(void)
 	/* Calculate elapsed time since bootup. */
 	sys_uptime = k_uptime_get();
 
+	/* Ensure that uptime starts from zero after a system reset by subtracting
+	 * the timestamp during the system initialization.
+	 */
+	if (IS_ENABLED(CONFIG_BT_FAST_PAIR_FMDN_CLOCK_UPTIME_PERSISTENCE)) {
+		__ASSERT_NO_MSG(sys_uptime >= uptime_on_boot);
+		sys_uptime -= uptime_on_boot;
+	}
+
 	/* Convert from milliseconds to seconds. */
 	sys_uptime /= MSEC_PER_SEC;
 
@@ -156,3 +165,18 @@ FP_ACTIVATION_MODULE_REGISTER(fp_fmdn_clock,
 			      FP_ACTIVATION_INIT_PRIORITY_DEFAULT,
 			      fp_fmdn_clock_init,
 			      fp_fmdn_clock_uninit);
+
+#if defined(CONFIG_BT_FAST_PAIR_FMDN_CLOCK_UPTIME_PERSISTENCE)
+static int bt_fast_pair_fmdn_clock_uptime_persistence_workaround_init(void)
+{
+	uptime_on_boot = k_uptime_get();
+
+	return 0;
+}
+
+/* Define the system initialization hook with the priority level that is executed as closely
+ * as possible to the kernel initialization. The kernel uptime value should be read as soon
+ * as possible after the kernel starts to increase accuracy of the FMDN clock.
+ */
+SYS_INIT(bt_fast_pair_fmdn_clock_uptime_persistence_workaround_init, POST_KERNEL, 0);
+#endif /* defined(CONFIG_BT_FAST_PAIR_FMDN_CLOCK_UPTIME_PERSISTENCE) */