rho-kconfig

The configuration bridge. It maps what a Rho project declares into the Kconfig,
sdkconfig and FreeRTOSConfig.h that the toolchains around it read.

rho kconfig values            # what this project declared
rho kconfig list              # every symbol, its value, and where it came from
rho kconfig set CONFIG_SPI_MASTER n
rho kconfig merge --dialect espidf <fragment>...

What it is for

Embedded projects are configured by the tools that build them: ESP-IDF reads
sdkconfig, a kernel build reads .config and include/generated/autoconf.h,
and a FreeRTOS project reads FreeRTOSConfig.h. The same settings, spelled three
ways, in directories that must be exactly right.

So a project declares its configuration once, in its own terms, and this
component writes it in the form the toolchain it targets reads. The project does
not carry a sdkconfig it has to keep in step with its own code.

A project declares it

# module.yml
build:
  language: c

config:
  dialect: espidf
  kconfig_fragments:
    - kconfig/board.Kconfig
  values:
    CONFIG_SPI_MASTER: "y"
    CONFIG_UART_BAUD: "115200"
  module: config.rho

The fragments define the symbols, values set them, and module is a
Rho module of constants for a project that would rather write its configuration
as the code it is.

The build resolves it

rho build does not resolve the configuration. It generates a project that
does, in its own CMake:

include(rho-config.cmake)
add_executable(app main.c app.c runtime.c)
# every source depends on the resolved .config

CMake runs rho kconfig merge before compiling, and the generated sources
include the header it wrote. That is what keeps a value change from needing a
regeneration, and it is written in the build file rather than the generator so a
person can read what a build does before it does it.

Precedence

Later wins: defaults, then kconfig/rho-defconfig, then module.yml values,
then config.rho. A value for a symbol no fragment declares is refused rather
than written, because a value nothing reads is a setting that silently did
nothing.

Dialects

Each writes where its reader looks, which is part of the contract: a header in
the wrong directory is a header nothing includes.

Reading it from Rho

A program asks its own configuration through the rho.kconfig library, supplied
by the kconfig port the way logging is supplied by logemit:

import rho.kconfig

fn init() {
    if !kconfig_enabled("CONFIG_SPI_MASTER") {
        info("board", "no SPI bus");
    }
}

Extending it

A plugin that fails is reported and skipped: one broken extension must not cost a
project its configuration.