Bazel Rules for Raspberry Pi Pico

This repository contains experimental rules for building the Raspberry Pi Pico SDK with bazel. This is mostly for my own benefit since it made it easier to integrate with some of my other projects which already use bazel.

Originally, the intention was to use the excellent rules_foreign_cc library to bridge between bazel and cmake but this ended up being difficult due to the need for cross compilation and the extensive use of cmake interface libraries in the Pico C/C++ SDK and I ended up writing bazel native build rules.

To use this in a project, just add the following to your WORKSPACE file:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "rules_pico",
    url = "https://github.com/dfr/rules_pico/archive/refs/heads/main.zip",
    # sha256 = "...",
    strip_prefix = "rules_pico-main",
)

load("@rules_pico//pico:repositories.bzl", "rules_pico_dependencies", "rules_pico_toolchains")

rules_pico_dependencies()
rules_pico_toolchains()

A bazel toolchain is included which allows the use of the standard arm-none-eabi toolchain for building the Pico libraries. To make use of this, select the pico plaform in your .bazelrc:

build --incompatible_enable_cc_toolchain_resolution
build --platforms=@rules_pico//:pico

The approach is to download the Pico SDK sources from github using bazel's new_git_repository rule and inject a BUILD file which handles most of the build, following the Pico SDK library structure as closely as possible. Linking binaries for the Pico is handled using bazel macros which include the necessary boot code and memory layout for compatiblity with Pico. The SDK libraries can be added as dependencies in the usual way and the result is an ELF format binary which can be further transformed to UF2:

load("//pico:defs.bzl", "pico_binary")
load("//pico:defs.bzl", "pico_add_uf2_output")

pico_binary(
    name = "hello.elf",
    srcs = [
        "hello.c",
    ],
    deps = [
        "@rules_pico//pico:pico_stdlib",
    ],
)

pico_add_uf2_output(
    name = "hello_serial.uf2",
    input = "hello_serial.elf",
)

A very small subset of Pico SDK configuration options are defined in @rules_pico/pico/config and these can be overridden on the bazel command line or via .bazelrc. Right now, this is limited to selecting the board name and selecting output options for stdio. A very experimental rule, pico_build_with_config, can be used to override the config settings at build time:

load("//pico:defs.bzl", "pico_add_uf2_output")

pico_build_with_config(
    name = "hello_usb.elf",
    input = "hello.elf",
    stdio_uart = False,
    stdio_usb = True,
)