buildroot/docs/manual/customize-directory-structure.adoc

// -*- mode:doc; -*-
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[[customize-dir-structure]]
=== Recommended directory structure

When customizing Buildroot for your project, you will be creating one or
more project-specific files that need to be stored somewhere. While most
of these files could be placed in _any_ location as their path is to be
specified in the Buildroot configuration, the Buildroot developers
recommend a specific directory structure which is described in this
section.

Orthogonal to this directory structure, you can choose _where_ you place
this structure itself: either inside the Buildroot tree, or outside of
it using a br2-external tree. Both options are valid, the choice is up
to you.

----
+-- board/
|   +-- <company>/
|       +-- <boardname>/
|           +-- linux.config
|           +-- busybox.config
|           +-- <other configuration files>
|           +-- post_build.sh
|           +-- post_image.sh
|           +-- rootfs_overlay/
|           |   +-- etc/
|           |   +-- <some files>
|           +-- patches/
|               +-- foo/
|               |   +-- <some patches>
|               +-- libbar/
|                   +-- <some other patches>
|
+-- configs/
|   +-- <boardname>_defconfig
|
+-- package/
|   +-- <company>/
|       +-- Config.in (if not using a br2-external tree)
|       +-- <company>.mk (if not using a br2-external tree)
|       +-- package1/
|       |    +-- Config.in
|       |    +-- package1.mk
|       +-- package2/
|           +-- Config.in
|           +-- package2.mk
|
+-- Config.in (if using a br2-external tree)
+-- Makefile (if using a custom top makefile)
+-- external.mk (if using a br2-external tree)
+-- external.desc (if using a br2-external tree)
----

Details on the files shown above are given further in this chapter.

Note: if you choose to place this structure outside of the Buildroot
tree but in a br2-external tree, the <company> and possibly <boardname>
components may be superfluous and can be left out.

==== Implementing layered customizations

It is quite common for a user to have several related projects that partly
need the same customizations. Instead of duplicating these
customizations for each project, it is recommended to use a layered
customization approach, as explained in this section.

Almost all of the customization methods available in Buildroot, like
post-build scripts and root filesystem overlays, accept a
space-separated list of items. The specified items are always treated in
order, from left to right. By creating more than one such item, one for
the common customizations and another one for the really
project-specific customizations, you can avoid unnecessary duplication.
Each layer is typically embodied by a separate directory inside
+board/<company>/+. Depending on your projects, you could even introduce
more than two layers.

An example directory structure for where a user has two customization
layers 'common' and 'fooboard' is:

----
+-- board/
    +-- <company>/
        +-- common/
        |   +-- post_build.sh
        |   +-- rootfs_overlay/
        |   |   +-- ...
        |   +-- patches/
        |       +-- ...
        |
        +-- fooboard/
            +-- linux.config
            +-- busybox.config
            +-- <other configuration files>
            +-- post_build.sh
            +-- rootfs_overlay/
            |   +-- ...
            +-- patches/
                +-- ...
----

For example, if the user has the +BR2_GLOBAL_PATCH_DIR+ configuration
option set as:

----
BR2_GLOBAL_PATCH_DIR="board/<company>/common/patches board/<company>/fooboard/patches"
----

then first the patches from the 'common' layer would be applied,
followed by the patches from the 'fooboard' layer.

==== Custom top Makefile

You normally launch Buildroot from the buildroot source directory, pointing
+BR2_EXTERNAL+ and +O+ to the right places for the build you want to make.
You can simplify this by adding a Makefile to your br2-external that sets
these variables and calls into buildroot.

You can add additional, custom rules to this Makefile for various tasks you
need to perform, e.g. integrate multiple configurations into a single image,
upload to a release server or to a test device, include multiple
br2-external configurations, etc.

A basic Makefile looks like this. It assumes the buildroot source is available
(e.g. as a git submodule) in the +buildroot+ subdirectory. It makes sure that
the download directory is shared between different builds, and it organizes
the output directories in a structure under +outputs/+.

----
# SPDX-License-Identifier: GPL-2.0

# Avoid surprises by disabling default rules
MAKEFLAGS += --no-builtin-rules
.SUFFIXES:

THIS_EXTERNAL_PATH := $(abspath $(dir $(lastword $(MAKEFILE_LIST))))

# Put downloads in this directory instead of in the Buildroot directory
ifeq ($(BR2_DL_DIR),)
BR2_DL_DIR = $(THIS_EXTERNAL_PATH)/dl
endif

OUTPUT_BASEDIR = $(THIS_EXTERNAL_PATH)/output
OUTPUT_DIR = $(OUTPUT_BASEDIR)/$(patsubst %_defconfig,%,$@)

MAKE_BUILDROOT = $(MAKE) -C $(THIS_EXTERNAL_PATH)/buildroot BR2_EXTERNAL=$(THIS_EXTERNAL_PATH)

%: $(THIS_EXTERNAL_PATH)/configs/%
	$(MAKE_BUILDROOT) O=$(OUTPUT_DIR) $@
	sed -i /^BR2_DL_DIR=.*/s%%BR2_DL_DIR=$(BR2_DL_DIR)% $(OUTPUT_DIR)/.config
----