kumquat-buildroot/docs/manual/customize-outside-br.txt

// -*- mode:doc -*- ;

[[outside-br-custom]]
=== Keeping customizations outside Buildroot

The Buildroot community recommends and encourages upstreaming to the
official Buildroot version the packages and board support that are
written by developers. However, it is sometimes not possible or
desirable because some of these packages or board support are highly
specific or proprietary.

In this case, Buildroot users are offered two choices:

 * They can add their packages, board support and configuration files
   directly within the Buildroot tree, and maintain them by using
   branches in a version control system.

 * They can use the +BR2_EXTERNAL+ mechanism, which allows to keep
   package recipes, board support and configuration files outside of
   the Buildroot tree, while still having them nicely integrated in
   the build logic. The following paragraphs give details on how to
   use +BR2_EXTERNAL+.

+BR2_EXTERNAL+ is an environment variable that can be used to point to
a directory that contains Buildroot customizations. It can be passed
to any Buildroot +make+ invocation. It is automatically saved in the
hidden +.br-external+ file in the output directory. By doing this,
there is no need to pass +BR2_EXTERNAL+ at every +make+ invocation. It
can however be changed at any time by passing a new value, and can be
removed by passing an empty value.

*Note:* the +BR2_EXTERNAL+ path can be either an absolute or a relative path,
but if it's passed as a relative path, it is important to note that it
is interpreted relative to the main Buildroot source directory, *not*
the Buildroot output directory.

Some examples:

-----
 buildroot/ $ make BR2_EXTERNAL=/path/to/foobar menuconfig
-----

Starting from now on, external definitions from the +/path/to/foobar+
directory will be used:

-----
 buildroot/ $ make
 buildroot/ $ make legal-info
-----

We can switch to another external definitions directory at any time:

-----
 buildroot/ $ make BR2_EXTERNAL=/where/we/have/barfoo xconfig
-----

Or disable the usage of external definitions:

-----
 buildroot/ $ make BR2_EXTERNAL= xconfig
-----

+BR2_EXTERNAL+ then allows three different things:

 * One can store all the board-specific configuration files there,
   such as the kernel configuration, the root filesystem overlay, or
   any other configuration file for which Buildroot allows to set its
   location. The +BR2_EXTERNAL+ value is available within the
   Buildroot configuration using +$(BR2_EXTERNAL)+. As an example, one
   could set the +BR2_ROOTFS_OVERLAY+ Buildroot option to
   +$(BR2_EXTERNAL)/board/<boardname>/overlay/+ (to specify a root
   filesystem overlay), or the +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+
   Buildroot option to
   +$(BR2_EXTERNAL)/board/<boardname>/kernel.config+ (to specify the
   location of the kernel configuration file). To achieve this, it is
   recommended but not mandatory, to store those details in
   directories called +board/<boardname>/+ under +BR2_EXTERNAL+. This
   matches the directory structure used within Buildroot.

 * One can store package recipes (i.e. +Config.in+ and
   +<packagename>.mk+), or even custom configuration options and make
   logic. Buildroot automatically includes +BR2_EXTERNAL/Config.in+ to
   make it appear in the top-level configuration menu, and includes
   +BR2_EXTERNAL/external.mk+ with the rest of the makefile logic.
   Providing those two files is mandatory, but they can be empty.
+
The main usage of this is to store package recipes. The recommended
   way to do this is to write a +BR2_EXTERNAL/Config.in+ that looks
   like:
+
------
source "$BR2_EXTERNAL/package/package1/Config.in"
source "$BR2_EXTERNAL/package/package2/Config.in"
------
+
Then, have a +BR2_EXTERNAL/external.mk+ file that looks like:
+
------
include $(sort $(wildcard $(BR2_EXTERNAL)/package/*/*.mk))
------
+
And then in +BR2_EXTERNAL/package/package1+ and
   +BR2_EXTERNAL/package/package2+ create normal Buildroot package
   recipes, as explained in xref:adding-packages[].

 * One can store Buildroot defconfigs in the +configs+ subdirectory of
   +BR2_EXTERNAL+. Buildroot will automatically show them in the
   output of +make help+ and allow them to be loaded with the normal
   +make <name>_defconfig+ command. They will be visible under the
   +User-provided configs:+' label in the 'make help' output.

In the end, a typical +BR2_EXTERNAL+ directory organization would
generally be:

-----
$(BR2_EXTERNAL)/
+-- Config.in
+-- external.mk
+-- board/
|   +-- <boardname>/
|       +-- linux.config
|       +-- overlay/
|           +-- etc/
|               +-- <some file>
+-- configs/
|   +-- <boardname>_defconfig
+-- package/
    +-- package1/
    |    +-- Config.in
    |    +-- package1.mk
    +-- package2/
        +-- Config.in
        +-- package2.mk
------