NRF Connect SDK 1.8.0 for nRF5340 Using Visual Studio Code

Question

I have a working build for the nRF5340 DK that implements a custom service and communicates over BLE. As I add features, the binary for the Application Core changes; the binary for the Network Core remains the same. I have increased the MTU size to 185 to increase BLE throughput. That is also working. However, I am having issues with certain added configuration parameters within prj.conf, specifically CONFIG_BT_L2CAP_DYNAMIC_CHANNEL=y and CONFIG_BT_CTLR_DATA_LENGTH_MAX=251. Apparently, I am missing dependencies for these to become active. I believe there are configuration changes required within the Remote Core build (hci_rpmsg) in order for the configurations with prj.conf to work. There is discussion about a "child_image" folder that might address this, but it is very unclear to me how to add a child_image folder into the project. 
 My CMakeLists.txt has the following entries: 
 # SPDX-License-Identifier: Apache-2.0 
 cmake_minimum_required(VERSION 3.20.0) 
 find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE}) project(veris_chemo_port) 
 target_sources(app PRIVATE src/main.c) 
 # Custom files and folders 
 target_sources(app PRIVATE src/bluetooth/ble_interface.c src/bluetooth/vcp_service.c ) 
 zephyr_library_include_directories(src/bluetooth) zephyr_library_include_directories(${ZEPHYR_BASE}/samples/bluetooth) 
 
 My prj.conf has the following entries: 
 
 # Enable DK LED and Buttons library 
 CONFIG_GPIO =y 
 CONFIG_DK_LIBRARY =y 
 
 # Configure logger 
 #CONFIG_LOG=y 
 CONFIG_USE_SEGGER_RTT =n 
 #CONFIG_LOG_BACKEND_RTT=n 
 #CONFIG_LOG_BACKEND_UART=y 
 #CONFIG_LOG_DEFAULT_LEVEL=3 
 
 # Configure Bluetooth 
 CONFIG_BT =y 
 CONFIG_BT_PERIPHERAL =y 
 CONFIG_BT_DEVICE_NAME_DYNAMIC =y 
 CONFIG_BT_DEVICE_NAME = "Veris Chemo Port" 
 CONFIG_BT_DEVICE_APPEARANCE =1362 
 CONFIG_BT_MAX_CONN =1 
 
 # Config Display Information Service 
 CONFIG_BT_DIS =y 
 CONFIG_BT_DIS_PNP =n 
 CONFIG_BT_DIS_SERIAL_NUMBER =y 
 CONFIG_BT_DIS_SERIAL_NUMBER_STR = "VCP_xxxxxxxxxxxx" 
 CONFIG_BT_DIS_FW_REV =y 
 CONFIG_BT_DIS_FW_REV_STR = "00.01.01" 
 CONFIG_BT_DIS_HW_REV =y 
 CONFIG_BT_DIS_HW_REV_STR = "00.00.00" 
 CONFIG_BT_DIS_MODEL = "VCP MODEL X" 
 CONFIG_BT_DIS_MANUF = "Veris Health" 
 CONFIG_BT_DIS_SETTINGS =y 
 
 #CONFIG_BT_SETTINGS=y 
 CONFIG_SETTINGS_RUNTIME =y 
 CONFIG_SETTINGS =y 
 #CONFIG_SETTINGS_NONE=y 
 
 # Config Battery Service 
 CONFIG_BT_BAS =y 
 
 # Config Data Transfer for larger packet size 
 CONFIG_BT_BUF_ACL_RX_SIZE =251 
 CONFIG_BT_CONN_TX_MAX =10 
 CONFIG_BT_L2CAP_TX_BUF_COUNT =10 
 CONFIG_BT_L2CAP_TX_MTU =185 
 CONFIG_BT_L2CAP_DYNAMIC_CHANNEL =y 
 CONFIG_BT_BUF_ACL_TX_COUNT =10 
 CONFIG_BT_BUF_ACL_TX_SIZE =251 
 CONFIG_BT_CTLR_DATA_LENGTH_MAX =251 
 
 CONFIG_ASSERT =y

Brian Cook · Accepted Answer

I was able to solve my issues. 
 1) If changes to the network core are needed to speed up data transfers, there needs to be child_image subfolder added under the project folder. This new folder needs to include a hci_rpmsg.conf file. This file includes the configurations required to change the network core program. A good example of what the contents of hci_rpmsg.conf should be for improving data throughput can be found in the "throughput" example. CONFIG_BT_CTLR_DATA_LENGTH_MAX belongs in this config file, not the application config file (prj.conf). 
 2) CONFIG_BT_L2CAP_DYNAMIC_CHANNEL does belong in the application config file (prj.conf) but has many dependencies that must be met before it can be activated. CONFIG_BT_SMP=y provides many (if not all) of the dependencies. 
 3) To verify that hci_rpmsg.conf is being used in the build process, CMakeCache.txt within the build subfolder can be examined. If you see something like this: 
 //Extra config fragments for hci_rpmsg child image .......<project folder>/child_image/hci_rpmsg.conf 
 where your child image subfolder contents is being referenced, it was part of the build. 
 By making the configuration modifications outlined in this ticket, I was able to improve the transfer of a 60000 byte data block from 75 seconds to 5 seconds. Knowing how to modify the network core program is a good thing to be able to do.