My new (WiP) orchestrator for building test image for testing my #kernel tree is fully implemented with GNU make:

# SPDX-License-Identifier: MIT

ROOT			:= $(dir $(abspath $(firstword $(MAKEFILE_LIST))))
BUILDROOT_VERSION	:= 2023.11
OUTPUT			:= $(ROOT)output
BUILDROOT_URL		:= https://buildroot.org/downloads/buildroot-$(BUILDROOT_VERSION).tar.gz
EXTERNAL_URL		:= https://github.com/jarkkojs/tpmdd-buildroot-external/tarball/main

define make-buildroot
	make -C "$(OUTPUT)/buildroot" BR2_EXTERNAL="$(OUTPUT)/external" O="$(OUTPUT)/build" $(1)
endef

define download-package
	mkdir -p $(2)
	curl -sL "$(1)" | tar -zxv -C "$(2)" --strip-components=1
endef

all: buildroot

.PHONY: buildroot
buildroot: $(OUTPUT)/download-stamp
	$(call make-buildroot,tpmdd_qemu_x86_64_defconfig)
	$(call make-buildroot,all)

.PHONY: buildroot-menuconfig
buildroot-menuconfig: $(OUTPUT)/download-stamp
	$(call make-buildroot,tpmdd_qemu_x86_64_defconfig)
	$(call make-buildroot,menuconfig)
	$(call make-buildroot,savedefconfig)

.PHONY: linux-menuconfig
linux-menuconfig: $(OUTPUT)/download-stamp
	$(call make-buildroot,tpmdd_qemu_x86_64_defconfig)
	$(call make-buildroot,linux-menuconfig)
	$(call make-buildroot,linux-savedefconfig)

$(OUTPUT)/download-stamp: 
	$(call download-package,"$(BUILDROOT_URL)","$(OUTPUT)/buildroot")
	$(call download-package,"$(EXTERNAL_URL)","$(OUTPUT)/external")
	touch $@

.PHONY: clean
clean:
	rm -rf "$(OUTPUT)"

It is pretty robust structure because I can e.g. easily add packages (like maybe host swtpm) in a robust manner to buildroot.

#buildroot #linux

After some experimentation it is best the define boards in this context as fairly self-contained packages to the specific test devices at hand and not have anything shared e.g. by CPU architecture even at the cost of some redundancy:

$ tree
.
├── Config.in
├── LICENSE
├── board
│   └── tpmdd_qemu_x86_64
│       ├── linux.config
│       ├── post-build.sh
│       ├── post-image.sh
│       └── start-qemu.sh.in
├── configs
│   └── tpmdd_qemu_x86_64_defconfig
├── external.desc
└── external.mk

4 directories, 9 files

E.g. similarly I’ll add tpmdd_raspberrypi3b target and so forth. For instance, I would not share post-image.sh and similar scripts between boards even if they were 1:1. It kills robustness.

The orchestrator itself has a flat repository:

$ tree
.
├── LICENSE
├── Makefile
└── README.md

I licensed external with GPL2 for the sake of upstream compatibility and orchestrator with MIT. This is overally pretty usable structure to use Buildroot.