buildroot/docs/manual/customize-configuration.txt

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[[customize-store-buildroot-config]]
=== Storing the Buildroot configuration

The Buildroot configuration can be stored using the command
 +make savedefconfig+.

This strips the Buildroot configuration down by removing configuration
options that are at their default value. The result is stored in a file
called +defconfig+. If you want to save it in another place, change the
+BR2_DEFCONFIG+ option in the Buildroot configuration itself, or call
make with +make savedefconfig BR2_DEFCONFIG=<path-to-defconfig>+.

The recommended place to store this defconfig is
+configs/<boardname>_defconfig+. If you follow this recommendation, the
configuration will be listed in +make help+ and can be set again by
running +make <boardname>_defconfig+.

Alternatively, you can copy the file to any other place and rebuild with
+make defconfig BR2_DEFCONFIG=<path-to-defconfig-file>+.

[[customize-store-package-config]]
=== Storing the configuration of other components

The configuration files for BusyBox, the Linux kernel, Barebox, U-Boot
and uClibc should be stored as well if changed. For each of these
components, a Buildroot configuration option exists to point to an input
configuration file, e.g. +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+. To store
their configuration, set these configuration options to a path where you
want to save the configuration files, and then use the helper targets
described below to actually store the configuration.

As explained in xref:customize-dir-structure[], the recommended path to
store these configuration files is
+board/<company>/<boardname>/foo.config+.

Make sure that you create a configuration file 'before' changing
the +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+ etc. options. Otherwise,
Buildroot will try to access this config file, which doesn't exist
yet, and will fail. You can create the configuration file by running
+make linux-menuconfig+ etc.

Buildroot provides a few helper targets to make the saving of
configuration files easier.

* +make linux-update-defconfig+ saves the linux configuration to the
  path specified by +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+. It
  simplifies the config file by removing default values. However,
  this only works with kernels starting from 2.6.33. For earlier
  kernels, use +make linux-update-config+.
* +make busybox-update-config+ saves the busybox configuration to the
  path specified by +BR2_PACKAGE_BUSYBOX_CONFIG+.
* +make uclibc-update-config+ saves the uClibc configuration to the
  path specified by +BR2_UCLIBC_CONFIG+.
* +make barebox-update-defconfig+ saves the barebox configuration to the
  path specified by +BR2_TARGET_BAREBOX_CUSTOM_CONFIG_FILE+.
* +make uboot-update-defconfig+ saves the U-Boot configuration to the
  path specified by +BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE+.
* For at91bootstrap3, no helper exists so you have to copy the config
  file manually to +BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_CONFIG_FILE+.