Add HCI Zephyr vendor commands to read and write TX power

Create platforms/zephyr/hci.py with definitions of vendor HCI commands
to read and write TX power.

Add documentation for how to prepare an nRF52840 dongle with a Zephyr
HCI USB firmware application that includes dynamic TX power support and
how to send a write TX power vendor HCI command from Bumble.

docs/mkdocs/mkdocs.yml

@@ -64,6 +64,7 @@ nav:
     - Linux: platforms/linux.md
     - Windows: platforms/windows.md
     - Android: platforms/android.md
+    - Zephyr: platforms/zephyr.md
   - Examples:
     - Overview: examples/index.md
 

docs/mkdocs/src/platforms/index.md

@@ -9,3 +9,4 @@ For platform-specific information, see the following pages:
   * :material-linux: Linux - see the [Linux platform page](linux.md)
   * :material-microsoft-windows: Windows - see the [Windows platform page](windows.md)
   * :material-android: Android - see the [Android platform page](android.md)
+  * :material-memory: Zephyr - see the [Zephyr platform page](zephyr.md)

docs/mkdocs/src/platforms/zephyr.md

@@ -0,0 +1,51 @@
+:material-memory: ZEPHYR PLATFORM
+=================================
+
+Set TX Power on nRF52840
+------------------------
+
+The Nordic nRF52840 supports Zephyr's vendor specific HCI command for setting TX
+power during advertising, connection, or scanning. With the example [HCI
+USB](https://docs.zephyrproject.org/latest/samples/bluetooth/hci_usb/README.html)
+application, an [nRF52840
+dongle](https://www.nordicsemi.com/Products/Development-
+hardware/nRF52840-Dongle) can be used as a Bumble controller.
+
+To add dynamic TX power support to the HCI USB application, add the following to
+`zephyr/samples/bluetooth/hci_usb/prj.conf` and build.
+
+```
+CONFIG_BT_CTLR_ADVANCED_FEATURES=y
+CONFIG_BT_CTLR_CONN_RSSI=y
+CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL=y
+```
+
+Alternatively, a prebuilt firmware application can be downloaded here:
+[hci_usb.zip](../downloads/zephyr/hci_usb.zip).
+
+Put the nRF52840 dongle into bootloader mode by pressing the RESET button. The
+LED should pulse red. Load the firmware application with the `nrfutil` tool:
+
+```
+nrfutil dfu usb-serial -pkg hci_usb.zip -p /dev/ttyACM0
+```
+
+The vendor specific HCI commands to read and write TX power are defined in
+`bumble/vendor/zephyr/hci.py` and may be used as such:
+
+```python
+from bumble.vendor.zephyr.hci import HCI_Write_Tx_Power_Level_Command
+
+# set advertising power to -4 dB
+response = await host.send_command(
+    HCI_Write_Tx_Power_Level_Command(
+        handle_type=HCI_Write_Tx_Power_Level_Command.TX_POWER_HANDLE_TYPE_ADV,
+        connection_handle=0,
+        tx_power_level=-4,
+    )
+)
+
+if response.return_parameters.status == HCI_SUCCESS:
+    print(f"TX power set to {response.return_parameters.selected_tx_power_level}")
+
+```