bluetooth: fast_pair: fmdn: add restriction for fmdn tx power

Added a restriction on the CONFIG_BT_FAST_PAIR_FMDN_TX_POWER Kconfig
option in the Find My Device Network (FMDN) extension configuration.
This Kconfig option should now be set to 0 at minimum as the Fast Pair
specification now requires that the conducted Bluetooth transmit power
for FMDN advertisements must not be lower than 0 dBm.

Ref: NCSDK-30857

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

Author: kapi-no

doc/nrf/external_comp/bt_fast_pair.rst

@@ -580,6 +580,7 @@ For example, the "It's here" status message is displayed in the "Hot & Cold" exp
 
 You can set the TX power for the FMDN advertising and connections using the :kconfig:option:`CONFIG_BT_FAST_PAIR_FMDN_TX_POWER` Kconfig option.
 The configured value is directly used to set the TX power in the Bluetooth LE controller using an HCI command.
+This Kconfig option must be set to 0 at minimum as the Fast Pair specification requires that the conducted Bluetooth transmit power for FMDN advertisements must not be lower than 0 dBm.
 By default, 0 dBm is used for the FMDN TX power configuration.
 
 You can use the :kconfig:option:`CONFIG_BT_FAST_PAIR_FMDN_TX_POWER_CORRECTION_VAL` Kconfig option to define a correction value that is added to TX power readout from the Bluetooth LE controller (usually equal to the :kconfig:option:`CONFIG_BT_FAST_PAIR_FMDN_TX_POWER` Kconfig option), when calculating the calibrated TX power reported in the Read Beacon Parameters response.

doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst

@@ -588,6 +588,9 @@ Bluetooth libraries and services
 
 * :ref:`bt_fast_pair_readme` library:
 
+  * Added a restriction on the :kconfig:option:`CONFIG_BT_FAST_PAIR_FMDN_TX_POWER` Kconfig option in the Find My Device Network (FMDN) extension configuration.
+    This Kconfig option must now be set to 0 at minimum as the Fast Pair specification now requires that the conducted Bluetooth transmit power for FMDN advertisements must not be lower than 0 dBm.
+
   * Updated the automatically generated ``bt_fast_pair`` partition definition (located in the :file:`subsys/partition_manager/pm.yml.bt_fast_pair`) to work correctly when building with TF-M.
 
 * :ref:`bt_mesh` library:

subsys/bluetooth/services/fast_pair/fmdn/Kconfig

@@ -26,6 +26,10 @@ config BT_FAST_PAIR_FMDN_TX_POWER
 	  procedure of this library. The actual TX power value will be as close to
 	  the desired configuration as possible.
 
+	  This must should be set to zero at minimum. The Fast Pair specification
+	  requires that the conducted Bluetooth transmit power for FMDN advertisements
+	  must be set to at least 0 dBm.
+
 config BT_FAST_PAIR_FMDN_TX_POWER_CORRECTION_VAL
 	int "TX power correction value [dBm]"
 	default 0

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

@@ -98,6 +98,9 @@ enum utp_control_flags_bit_num {
 	UTP_CONTROL_FLAGS_BIT_NUM_RING_AUTH_SKIP = 0,
 };
 
+/* Verify if the conducted Bluetooth TX power for FMDN advertisements is set to at least 0 dBm. */
+BUILD_ASSERT(CONFIG_BT_FAST_PAIR_FMDN_TX_POWER >= 0);
+
 /* Verify if the length of the EIK is consistent with the storage module. */
 BUILD_ASSERT(FP_FMDN_STATE_EIK_LEN == FP_STORAGE_EIK_LEN);