Why does prj.conf not take precedence over a local boards/.conf file?

[droberson-ds]

Hello,

I am new to Zephyr and stumbled onto an issue I cannot wrap my brain around. To preface:

• nRF Connect SDK 2.9.0 (Using nRF Connect VS Code extension)
• PC: Windows 11

My understanding was that prj.conf takes precedence over any other configuration files. In practice, it seems that a local .conf file in my boards directory takes precedence.

I have the following project structure:

And CONFIG_HEAP_MEM_POOL_SIZE is defined in both of the following files:

• app/boards/nrf9151dk_nrf9151_ns.conf
• app/prj.conf

The issue is that whatever I set CONFIG_HEAP_MEM_POOL_SIZE to in prj.conf, it is ignored and defaults to the value assigned in nrf9151dk_nrf9151_ns.conf.

Question: Is this expected behavior? If so, where is this detailed in the Zephyr documentation? I can't seem to find it.

Note: I already asked Nordic this question and they couldn't point me to any clear documentation that states how this is supposed to work. The only stated that the nrf9151dk_nrf9151_ns.conf file takes precedence.

For reference: https://devzone.nordicsemi.com/f/nordic-q-a/119413/unable-to-change-config_heap_mem_pool_size-in-prj-conf

Thanks for any clarification. I really want to understand why it works the way it does.

Derek

[kartben]

Question: Is this expected behavior? If so, where is this detailed in the Zephyr documentation? I can't seem to find it.

I believe you're looking for: https://docs.zephyrproject.org/latest/build/kconfig/setting.html#the-initial-configuration
It's totally expected that the board's conf would take precedence over prj.conf, since one usually wants to "override" default configuration with whathever is needed by or specific to their board.

[droberson-ds]

Hey Benjamin,

Thanks for the quick response and the link. I have read this link prior, and item number 2 states the following:

"Otherwise, if boards/.conf exists in the application configuration directory, the result of merging it with prj.conf is used."

In regard to "result of merging it with prj.conf is used.", this doesn't say anything about what happens when both files contain the same Kconfig item, nor which has precedence.

Further down in the page linked, it states:

"If a symbol is assigned both in _defconfig and in the application configuration, the value set in the application configuration takes precedence."

By "application configuration", this is understood to mean prj.conf as described here: Application Configuration Directory , which states "Application configuration options are usually set in prj.conf in the application directory"

I understand this is referring to _defconfig files, but one could misunderstand this to think it also applies to 'boards/.conf' files since it doesn't explicitly state which one takes precedence in this case as mentioned above.

Thanks,

Derek

[kartben]

@droberson-ds yes I believe you are right, it looks like some clarification would be needed with regards to basically what actual merge strategy is being applied -- I am hoping @nordicjm or @tejlmand can maybe chime in.

[nordicjm]

In regard to "result of merging it with prj.conf is used.", this doesn't say anything about what happens when both files contain the same Kconfig item, nor which has precedence.

prj.conf is the base, the board Kconfig fragment is applied on top. If they contain the same options, it will still be applied but is a bit moot. Easiest way to see is configure an application and see the cmake output e.g.:

Parsing /tmp/aa/zephyr/Kconfig
Loaded configuration '/tmp/aa/zephyr/boards/nordic/nrf52840dk/nrf52840dk_nrf52840_defconfig'
Merged configuration '/tmp/aa/zephyr/samples/hello_world/prj.conf'
Merged configuration '/tmp/aa/zephyr/samples/hello_world/boards/nrf52840dk_nrf52840.conf'
Merged configuration '/tmp/aa/zephyr/samples/hello_world/_AA/hello_world/zephyr/.config.sysbuild'
Configuration saved to '/tmp/aa/zephyr/samples/hello_world/_AA/hello_world/zephyr/.config'

board defconfig -> project configuration -> board Kconfig fragment -> sysbuild

[droberson-ds]

Hey @nordicjm ,

prj.conf is the base, the board Kconfig fragment is applied on top

Based on the cmake output you provided, it seems that prj.conf can only override the defconfig files in the zephyr/boards/ folder. All the other Kconfig fragments override the prj.conf file, as you mentioned.

This is good to know, thanks for the additional clarification! The Zephyr documentation could lead one to believe otherwise, as highlighted above. Especially for someone new to cmake, Zephyr, etc, such as myself.

Thanks,

Derek

[yashi]

What is nrf9151dk_nrf9151_ns.conf?

❯ cd zephyr
❯ git describe
v4.1.0-rc3-147-gfb4bb5d7688f
❯ git ls-files | grep nrf9151dk_nrf9151_ns.conf
❯

Without nrf9151dk_nrf9151_ns.conf, this is what I see:

❯ cd zephyr
❯ git describe
v4.1.0-rc3-147-gfb4bb5d7688f
❯ git status
On branch main
Your branch is up to date with 'origin/main'.

nothing to commit, working tree clean

west build -b nrf9151dk samples/basic/blinky: CONFIG_HEAP_MEM_POOL_SIZE=0

❯ rm -rf build && west build -b nrf9151dk samples/basic/blinky && grep MEM_POOL_SIZE build/zephyr/.config
-- west build: generating a build system
Loading Zephyr default modules (Zephyr base).
-- Application: /home/yashi/src/zephyr-project/zephyr/samples/basic/blinky
-- CMake version: 3.31.6
-- Found Python3: /home/yashi/src/zephyr-project/.venv/bin/python (found suitable version "3.13.2", minimum required is "3.10") found components: Interpreter
-- Cache files will be written to: /home/yashi/.cache/zephyr
-- Zephyr version: 4.1.0-rc3 (/home/yashi/src/zephyr-project/zephyr)
-- Found west (found suitable version "1.3.0", minimum required is "0.14.0")
-- Board: nrf9151dk, qualifiers: nrf9151
-- ZEPHYR_TOOLCHAIN_VARIANT not set, trying to locate Zephyr SDK
-- Found host-tools: zephyr 0.17.0 (/home/yashi/zephyr-sdk-0.17.0)
-- Found toolchain: zephyr 0.17.0 (/home/yashi/zephyr-sdk-0.17.0)
-- Found Dtc: /usr/bin/dtc (found suitable version "1.7.2", minimum required is "1.4.6")
-- Found BOARD.dts: /home/yashi/src/zephyr-project/zephyr/boards/nordic/nrf9151dk/nrf9151dk_nrf9151.dts
-- Generated zephyr.dts: /home/yashi/src/zephyr-project/zephyr/build/zephyr/zephyr.dts
-- Generated pickled edt: /home/yashi/src/zephyr-project/zephyr/build/zephyr/edt.pickle
-- Generated devicetree_generated.h: /home/yashi/src/zephyr-project/zephyr/build/zephyr/include/generated/zephyr/devicetree_generated.h
-- Including generated dts.cmake file: /home/yashi/src/zephyr-project/zephyr/build/zephyr/dts.cmake
Parsing /home/yashi/src/zephyr-project/zephyr/Kconfig
Loaded configuration '/home/yashi/src/zephyr-project/zephyr/boards/nordic/nrf9151dk/nrf9151dk_nrf9151_defconfig'
Merged configuration '/home/yashi/src/zephyr-project/zephyr/samples/basic/blinky/prj.conf'
Configuration saved to '/home/yashi/src/zephyr-project/zephyr/build/zephyr/.config'
Kconfig header saved to '/home/yashi/src/zephyr-project/zephyr/build/zephyr/include/generated/zephyr/autoconf.h'
-- Found GnuLd: /home/yashi/zephyr-sdk-0.17.0/arm-zephyr-eabi/arm-zephyr-eabi/bin/ld.bfd (found version "2.38")
-- The C compiler identification is GNU 12.2.0
-- The CXX compiler identification is GNU 12.2.0
-- The ASM compiler identification is GNU
-- Found assembler: /home/yashi/zephyr-sdk-0.17.0/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc
-- Using ccache: /usr/bin/ccache
-- Configuring done (3.3s)
-- Generating done (0.1s)
-- Build files have been written to: /home/yashi/src/zephyr-project/zephyr/build
-- west build: building application
[1/143] Preparing syscall dependency handling

[3/143] Generating include/generated/zephyr/version.h
-- Zephyr version: 4.1.0-rc3 (/home/yashi/src/zephyr-project/zephyr), build: v4.1.0-rc3-147-gfb4bb5d7688f
[143/143] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       18488 B         1 MB      1.76%
             RAM:        4256 B        88 KB      4.72%
        IDT_LIST:          0 GB        32 KB      0.00%
Generating files from /home/yashi/src/zephyr-project/zephyr/build/zephyr/zephyr.elf for board: nrf9151dk
CONFIG_HEAP_MEM_POOL_SIZE=0

west build -b nrf9151dk samples/basic/blinky -- -DCONFIG_HEAP_MEM_POOL_SIZE=2048: CONFIG_HEAP_MEM_POOL_SIZE=2048

❯ rm -rf build && west build -b nrf9151dk samples/basic/blinky -- -DCONFIG_HEAP_MEM_POOL_SIZE=2048 && grep MEM_POOL_SIZE build/zephyr/.config
-- west build: generating a build system
Loading Zephyr default modules (Zephyr base).
-- Application: /home/yashi/src/zephyr-project/zephyr/samples/basic/blinky
-- CMake version: 3.31.6
-- Found Python3: /home/yashi/src/zephyr-project/.venv/bin/python (found suitable version "3.13.2", minimum required is "3.10") found components: Interpreter
-- Cache files will be written to: /home/yashi/.cache/zephyr
-- Zephyr version: 4.1.0-rc3 (/home/yashi/src/zephyr-project/zephyr)
-- Found west (found suitable version "1.3.0", minimum required is "0.14.0")
-- Board: nrf9151dk, qualifiers: nrf9151
-- ZEPHYR_TOOLCHAIN_VARIANT not set, trying to locate Zephyr SDK
-- Found host-tools: zephyr 0.17.0 (/home/yashi/zephyr-sdk-0.17.0)
-- Found toolchain: zephyr 0.17.0 (/home/yashi/zephyr-sdk-0.17.0)
-- Found Dtc: /usr/bin/dtc (found suitable version "1.7.2", minimum required is "1.4.6")
-- Found BOARD.dts: /home/yashi/src/zephyr-project/zephyr/boards/nordic/nrf9151dk/nrf9151dk_nrf9151.dts
-- Generated zephyr.dts: /home/yashi/src/zephyr-project/zephyr/build/zephyr/zephyr.dts
-- Generated pickled edt: /home/yashi/src/zephyr-project/zephyr/build/zephyr/edt.pickle
-- Generated devicetree_generated.h: /home/yashi/src/zephyr-project/zephyr/build/zephyr/include/generated/zephyr/devicetree_generated.h
-- Including generated dts.cmake file: /home/yashi/src/zephyr-project/zephyr/build/zephyr/dts.cmake
Parsing /home/yashi/src/zephyr-project/zephyr/Kconfig
Loaded configuration '/home/yashi/src/zephyr-project/zephyr/boards/nordic/nrf9151dk/nrf9151dk_nrf9151_defconfig'
Merged configuration '/home/yashi/src/zephyr-project/zephyr/samples/basic/blinky/prj.conf'
Merged configuration '/home/yashi/src/zephyr-project/zephyr/build/zephyr/misc/generated/extra_kconfig_options.conf'
Configuration saved to '/home/yashi/src/zephyr-project/zephyr/build/zephyr/.config'
Kconfig header saved to '/home/yashi/src/zephyr-project/zephyr/build/zephyr/include/generated/zephyr/autoconf.h'
-- Found GnuLd: /home/yashi/zephyr-sdk-0.17.0/arm-zephyr-eabi/arm-zephyr-eabi/bin/ld.bfd (found version "2.38")
-- The C compiler identification is GNU 12.2.0
-- The CXX compiler identification is GNU 12.2.0
-- The ASM compiler identification is GNU
-- Found assembler: /home/yashi/zephyr-sdk-0.17.0/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc
-- Using ccache: /usr/bin/ccache
-- Configuring done (3.3s)
-- Generating done (0.1s)
-- Build files have been written to: /home/yashi/src/zephyr-project/zephyr/build
-- west build: building application
[1/143] Preparing syscall dependency handling

[3/143] Generating include/generated/zephyr/version.h
-- Zephyr version: 4.1.0-rc3 (/home/yashi/src/zephyr-project/zephyr), build: v4.1.0-rc3-147-gfb4bb5d7688f
[143/143] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       18588 B         1 MB      1.77%
             RAM:        6328 B        88 KB      7.02%
        IDT_LIST:          0 GB        32 KB      0.00%
Generating files from /home/yashi/src/zephyr-project/zephyr/build/zephyr/zephyr.elf for board: nrf9151dk
CONFIG_HEAP_MEM_POOL_SIZE=2048

echo 'CONFIG_HEAP_MEM_POOL_SIZE=1024' >> samples/basic/blinky/prj.conf: CONFIG_HEAP_MEM_POOL_SIZE=1024

❯ rm -rf build && echo 'CONFIG_HEAP_MEM_POOL_SIZE=1024' >> samples/basic/blinky/prj.conf && west build -b nrf9151dk samples/basic/blinky && grep MEM_POOL_SIZE build/zephyr/.config
-- west build: generating a build system
Loading Zephyr default modules (Zephyr base).
-- Application: /home/yashi/src/zephyr-project/zephyr/samples/basic/blinky
-- CMake version: 3.31.6
-- Found Python3: /home/yashi/src/zephyr-project/.venv/bin/python (found suitable version "3.13.2", minimum required is "3.10") found components: Interpreter
-- Cache files will be written to: /home/yashi/.cache/zephyr
-- Zephyr version: 4.1.0-rc3 (/home/yashi/src/zephyr-project/zephyr)
-- Found west (found suitable version "1.3.0", minimum required is "0.14.0")
-- Board: nrf9151dk, qualifiers: nrf9151
-- ZEPHYR_TOOLCHAIN_VARIANT not set, trying to locate Zephyr SDK
-- Found host-tools: zephyr 0.17.0 (/home/yashi/zephyr-sdk-0.17.0)
-- Found toolchain: zephyr 0.17.0 (/home/yashi/zephyr-sdk-0.17.0)
-- Found Dtc: /usr/bin/dtc (found suitable version "1.7.2", minimum required is "1.4.6")
-- Found BOARD.dts: /home/yashi/src/zephyr-project/zephyr/boards/nordic/nrf9151dk/nrf9151dk_nrf9151.dts
-- Generated zephyr.dts: /home/yashi/src/zephyr-project/zephyr/build/zephyr/zephyr.dts
-- Generated pickled edt: /home/yashi/src/zephyr-project/zephyr/build/zephyr/edt.pickle
-- Generated devicetree_generated.h: /home/yashi/src/zephyr-project/zephyr/build/zephyr/include/generated/zephyr/devicetree_generated.h
-- Including generated dts.cmake file: /home/yashi/src/zephyr-project/zephyr/build/zephyr/dts.cmake
Parsing /home/yashi/src/zephyr-project/zephyr/Kconfig
Loaded configuration '/home/yashi/src/zephyr-project/zephyr/boards/nordic/nrf9151dk/nrf9151dk_nrf9151_defconfig'
Merged configuration '/home/yashi/src/zephyr-project/zephyr/samples/basic/blinky/prj.conf'
Configuration saved to '/home/yashi/src/zephyr-project/zephyr/build/zephyr/.config'
Kconfig header saved to '/home/yashi/src/zephyr-project/zephyr/build/zephyr/include/generated/zephyr/autoconf.h'
-- Found GnuLd: /home/yashi/zephyr-sdk-0.17.0/arm-zephyr-eabi/arm-zephyr-eabi/bin/ld.bfd (found version "2.38")
-- The C compiler identification is GNU 12.2.0
-- The CXX compiler identification is GNU 12.2.0
-- The ASM compiler identification is GNU
-- Found assembler: /home/yashi/zephyr-sdk-0.17.0/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc
-- Using ccache: /usr/bin/ccache
-- Configuring done (3.3s)
-- Generating done (0.1s)
-- Build files have been written to: /home/yashi/src/zephyr-project/zephyr/build
-- west build: building application
[1/143] Preparing syscall dependency handling

[3/143] Generating include/generated/zephyr/version.h
-- Zephyr version: 4.1.0-rc3 (/home/yashi/src/zephyr-project/zephyr), build: v4.1.0-rc3-147-gfb4bb5d7688f
[143/143] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       18588 B         1 MB      1.77%
             RAM:        5304 B        88 KB      5.89%
        IDT_LIST:          0 GB        32 KB      0.00%
Generating files from /home/yashi/src/zephyr-project/zephyr/build/zephyr/zephyr.elf for board: nrf9151dk
CONFIG_HEAP_MEM_POOL_SIZE=1024

[droberson-ds]

Hey @yashi ,

What is nrf9151dk_nrf9151_ns.conf?

It is a board configuration file for the nRF9151 development kit provided by Nordic in the HTTPS Client example in their SDK. Its contents are as so:

#
# Copyright (c) 2024 Nordic Semiconductor ASA
#
# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
#

# Heap and stacks
CONFIG_HEAP_MEM_POOL_SIZE=4096
CONFIG_MAIN_STACK_SIZE=4096
CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=4096

# LED control
CONFIG_DK_LIBRARY=y

# Disable Duplicate Address Detection (DAD)
# due to not being properly implemented for offloaded interfaces.
CONFIG_NET_IPV6_NBR_CACHE=n
CONFIG_NET_IPV6_MLD=n

# Connection Manager and Connectivity layer
CONFIG_NRF_MODEM_LIB_NET_IF=y

# AT Host
CONFIG_UART_INTERRUPT_DRIVEN=y
CONFIG_AT_HOST_LIBRARY=y

# Modem key management
CONFIG_MODEM_KEY_MGMT=y

In my opinion, CONFIG_HEAP_MEM_POOL_SIZE, CONFIG_MAIN_STACK_SIZE, and CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE should not be defined the the board config file because they can't be overridden by prj.conf, but I don't know what the Zephyr documentation states regarding this.

Oddly, the HTTPS Client example defines CONFIG_HEAP_MEM_POOL_SIZE and CONFIG_MAIN_STACK_SIZE in both the nrf9151dk_nrf9151_ns.conf and prj.conf files. Not sure why this is done since the value cannot be changed in prj.conf given that nrf9151dk_nrf9151_ns.conf overrides it.

prj.conf:

#
# Copyright (c) 2023 Nordic Semiconductor ASA
#
# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
#

# General
CONFIG_HEAP_MEM_POOL_SIZE=1024
CONFIG_MAIN_STACK_SIZE=4096

# Logging
CONFIG_LOG=y

# Network
CONFIG_NETWORKING=y
CONFIG_NET_SOCKETS=y
CONFIG_POSIX_API=y
CONFIG_NET_IPV4=y
CONFIG_NET_IPV6=y
CONFIG_NET_CONNECTION_MANAGER=y
CONFIG_NET_CONNECTION_MANAGER_MONITOR_STACK_SIZE=1024

Is there not a way to do something like this in the nrf9151dk_nrf9151_ns.conf file:

#ifndef CONFIG_HEAP_MEM_POOL_SIZE
CONFIG_HEAP_MEM_POOL_SIZE=4096
#endif

So that the prj.conf file can override the heap configuration in the nrf9151dk_nrf9151_ns.conf file?

Should CONFIG_HEAP_MEM_POOL_SIZE even be in the nrf9151dk_nrf9151_ns.conf file? What is the recommended practice?

Thanks,

Derek