---
title: "Kconfig"
canonical_url: https://developer.repebble.com/pebbleos/docs/development/build_system/kconfig/
last_updated: 2026-10-21
sdk_version: 4.33.1
---
# Kconfig

Source: https://developer.repebble.com/pebbleos/docs/development/build_system/kconfig/

Everything that can be left out of the firmware is a Kconfig symbol: SoCs, drivers, subsystems, services, debug aids and their parameters. The configuration is resolved with [Kconfiglib](https://github.com/ulfalizer/Kconfiglib) when the build directory is configured, and the result drives both the C sources and the CMake build.

## Configuration tree

The root `Kconfig` sources the rest of the tree:

- `soc/*/Kconfig.defconfig` and `subsys/bluetooth/Kconfig.defconfig` first, so their defaults win over the ones given with the definitions;

- `soc/Kconfig`, with the SoC and CPU symbols;

- the board’s files, only for the selected board and revision: `boards/<board>/Kconfig`, `Kconfig.<revision>`, `Kconfig.defconfig` and `Kconfig.defconfig.<revision>`;

- `lib/c`, `kernel`, `subsys`, `drivers`, `fw` and `third_party`, each with a `Kconfig` that sources its own subdirectories.

A symbol is defined next to the code it enables: a driver class’s `Kconfig` lives in `drivers/<class>/`, a subsystem’s in `subsys/<name>/`. [Kconfig options](https://developer.repebble.com/pebbleos/docs/reference/kconfig.md) lists every option, with its dependencies, defaults and location.

## Configuration fragments

A build’s configuration is resolved from these fragments, in this order:

| 

Fragment

 | 

Purpose

 |
| --- | --- |
| 

`boards/<board>/defconfig`

 | 

What the board has: SoC, platform, drivers and their parameters

 |
| 

`boards/<board>/defconfig.<revision>`

 | 

What a revision changes

 |
| 

`boards/<board>/defconfig.<board variant>`

 | 

What a board variant (`native_emery/wasm`) changes

 |
| 

`fw/prj.conf`

 | 

Services and features every firmware enables

 |
| 

`fw/prj_<variant>.conf`

 | 

What the `normal` or `prf` variant adds or removes

 |
| 

`<build>/menuconfig.conf`

 | 

Changes made with [pbl menuconfig](https://developer.repebble.com/pebbleos/docs/pbl/menuconfig.md)

 |
| 

`-DCONFIG_<SYMBOL>=<value>`

 | 

Overrides given when configuring

 |

Overrides are passed to [pbl configure](https://developer.repebble.com/pebbleos/docs/pbl/configure.md) or to CMake:

```
pbl configure --board obelix@pvt -DCONFIG_RELEASE=y
```

The variant is selected with `--variant` (`-DVARIANT=`), not by setting `CONFIG_RECOVERY_FW` directly; `prj_prf.conf` sets it, and the build stops if the two disagree.

The fragments add to each other: a symbol can be assigned in only one of them, and a later fragment cannot change what an earlier one set. To change a value for one build, override it with `-D`.

Configuration warnings are errors. The build stops when:

- a fragment or an override names a symbol that does not exist;

- a fragment assigns a symbol an earlier fragment already assigned;

- a value cannot be applied because of the symbol’s dependencies; the error names the unsatisfied ones.

## Using the configuration

Every symbol reaches all three consumers:

- **C sources** : `autoconf.h` is force-included into every compilation, so symbols are plain macros. An enabled `bool` symbol is defined to `1` and a disabled one is not defined at all, so test it with `#ifdef CONFIG_TOUCH`.

- **CMake** : `config.cmake` sets one variable per enabled symbol, used by the `_ifdef` helpers; see [CMake](https://developer.repebble.com/pebbleos/docs/development/build_system/cmake.md).

- **Linker script** : preprocessed with `autoconf.h`, so memory layout and sections can depend on the configuration.

`.config` in the build directory holds the resolved configuration.

## Writing Kconfig
Driver classes

A class is a `menuconfig` symbol, with one symbol per driver inside its `if` block. A driver that only exists on one SoC defaults to `y` there (`default y if SOC_QEMU`); a part on the board is enabled by the board’s defconfig.

Capabilities

What hardware or drivers provide is a hidden symbol (no prompt) that the providers `select`, such as `BACKLIGHT_HAS_COLOR` or `CPU_HAS_FPU`. Code and other symbols depend on the capability, not on a list of providers.

Optional libraries

A library that is not always needed is gated behind a symbol, and its consumers `select` (or `imply`) it: `BT` selects `MBEDTLS`, and `SERVICE_VOICE` selects `SPEEX`.

Board and SoC defaults

A default that depends on the board or SoC goes in its `Kconfig.defconfig`, which is read before the definitions:

```
config FW_FLASH_SIZEdefault 0xf8000
```

Log levels

A module’s log level comes from a template:

```
module = TASK_WDT
module-str = Task watchdogsource "subsys/logging/Kconfig.template.log\_level"
```

It defines `CONFIG_TASK_WDT_LOG_LEVEL` and its choice; see [Logging](https://developer.repebble.com/pebbleos/docs/subsystems/logging.md).

Generated from PebbleOS [3a7b71785](https://github.com/coredevices/pebbleos/commit/3a7b71785fd439666c4a8bc116f561140e8dec01). This page is maintained in the pebbleos repository: [docs/development/build\_system/kconfig.md](https://github.com/coredevices/pebbleos/blob/3a7b71785fd439666c4a8bc116f561140e8dec01/docs/development/build_system/kconfig.md).