.. _apps_kernel_conf:

Configure an Application's Kernel
#################################

The application's kernel is configured using a set of configuration options
that can be customized for application-specific purposes.
The Zephyr build system takes a configuration option's value
from the first source in which it is specified.
The available sources are (in order):

#. The application's current configuration.
   (i.e. The :file:`.config` file.)

#. The application's default configuration.
   (i.e. The :file:`.conf` file.)

#. The board configuration used by the application.
   (i.e. The board's :file:`.defconfig` file.)

#. The kernel's default configuration.
   (i.e. One of the kernel's :file:`Kconfig` files.)

For information on available kernel configuration options, including
inter-dependencies between options, see the :ref:`configuration`.
Be careful to note if an option is an experimental option
that is not yet fully supported.

.. contents:: Procedures
   :local:
   :depth: 1

Defining the Application's Default Kernel Configuration
=======================================================

An application's :file:`.conf` file defines its default kernel configuration.
The settings in this file override or augment the board configuration settings.

The board configuration settings can be viewed
in :file:`\$ZEPHYR_BASE/boards/ARCHITECTURE/BOARD/BOARD_defconfig`.

.. note::

   When the default board configuration settings are sufficient for your
   application, a :file:`.conf` file is not needed. Skip ahead to
   :ref:`override_kernel_conf`.

Before you begin
----------------

* Ensure you have created an application directory, as described
  in :ref:`apps_structure`.

* Review the kernel configuration options available and know
  which ones you want to set for your application.
  Be aware of any dependencies involving these options.
  See the :ref:`configuration` for a brief description of each option.

Steps
-----

1. Navigate to the :file:`appDir`, and create the :file:`prj.conf` file. Enter:

   .. code-block:: bash

       $ touch prj.conf

  The default name is :file:`prj.conf`. The filename must match
  the ``CONF_FILE`` entry in the application :file:`Makefile`.

2. Edit the file and add the appropriate configuration entries.

   a) Add each configuration entry on a new line.

   b) Begin each entry with ``CONFIG_``.

   c) Ensure that each entry contains no spaces
      (including on either side of the = sign).

   d) Use a # followed by a space to comment a line.

   The example below shows a comment line and a board
   configuration override in the :file:`prj.conf`.

   .. code-block:: c

       # Change the number of IRQs supported by the application
       CONFIG_NUM_IRQS=43

3. Save and close the file.


.. _override_kernel_conf:

Overriding the Application's Default Kernel Configuration
=========================================================

Override the application's default kernel configuration to
temporarily alter the application's configuration, perhaps
to test the effect of a change.

.. note::

   If you want to permanently alter the configuration you
   should revise the :file:`.conf` file.

Configure the kernel options using a menu-driven interface.
While you can add entries manually, using the configuration menu
is a preferred method.

Before you begin
----------------

* Ensure you have created an application directory, as described
  in :ref:`apps_structure`.

* Review the kernel configuration options available and know
  which ones you want to set for your application.
  Be aware of any dependencies involving these options.

* Ensure the Zephyr environment variables are set for each console terminal;
  see :ref:`apps_common_procedures`.

Steps
-----

1.  Run the :command:`make menuconfig` rule to launch the
    menu-driven interface.

 a) In a terminal session, navigate to the application directory
    (:file:`~/appDir`).

 b) Enter the following command:

  .. code-block:: bash

   $ make [BOARD=<type>] menuconfig

  A question-based menu opens that allows you to set individual
  configuration options.

.. image:: figures/app_kernel_conf_1.png
    :width: 400px
    :align: center
    :height: 375px
    :alt: Main Configuration Menu

2.  Set kernel configuration values using the following
    key commands:

   * Use the arrow keys to navigate within any menu or list.

   * Press :kbd:`Enter` to select a menu item.

   * Type an upper case :kbd:`Y` or :kbd:`N` in the
     square brackets :guilabel:`[ ]` to
     enable or disable a kernel configuration option.

   * Type a numerical value in the round brackets :guilabel:`( )`.

   * Press :kbd:`Tab` to navigate the command menu at the
     bottom of the display.

   .. note::

    When a non-default entry is selected for options that
    are nonnumerical, an asterisk :kbd:`*` appears between the
    square brackets in the display. There is nothing added added
    the display when you select the option's default.

3.  For information about any option, select the option and
    tab to :guilabel:`< Help >` and press :kbd:`Enter`.

    Press :kbd:`Enter` to return to the menu.

4.  After configuring the kernel options for your application,
    tab to :guilabel:`< Save >` and press :kbd:`Enter`.

    The following dialog opens with the :guilabel:`< Ok >`
    command selected:

.. image:: figures/app_kernel_conf_2.png
    :width: 400px
    :align: center
    :height: 100px
    :alt: Save Configuration Dialog


5.  Press :kbd:`Enter` to save the kernel configuration options
    to the default file name; alternatively, type a file
    name and press :kbd:`Enter`.

    Typically, you will save to the default file name unless
    you are experimenting with various configuration scenarios.

    An :file:`outdir` directory is created in the application
    directory. The outdir directory contains symbolic links
    to files under :file:`\$ZEPHYR_BASE`.

   .. note::

    At present, only a :file:`.config` file can be built. If
    you have saved files with different file names and want to build
    with one of these, change the file name to :file:`.config`.
    To keep your original :file:`.config`, rename it to something
    other than :file:`.config`.

    Kernel configuration files, such as the :file:`.config`
    file, are saved as hidden files in :file:`outdir`. To list
    all your kernel configuration files, enter :command:`ls -a`
    at the terminal prompt.

    The following dialog opens, displaying the file name the
    configuration was saved to.

.. image:: figures/app_kernel_conf_3.png
    :width: 400px
    :align: center
    :height: 150px
    :alt: Saved Configuration Name Dialog

6.  Press :kbd:`Enter` to return to the options menu.

7.  To load any saved kernel configuration file,
    tab to :guilabel:`< Load >` and press :kbd:`Enter`.

    The following dialog opens with the :guilabel:`< Ok >`
    command selected:

.. image:: figures/app_kernel_conf_4.png
    :width: 400px
    :align: center
    :height: 175px
    :alt: Configuration File Load Dialog

8.  To load the last saved kernel configuration file, press
    :guilabel:`< Ok >`, or to load another saved configuration
    file, type the file name, then select :guilabel:`< Ok >`.

9.  Press :kbd:`Enter` to load the file and return to the main
    menu.

10. To exit the menu configuration, tab to :guilabel:`< Exit >`
    and press :kbd:`Enter`.

    The following confirmation dialog opens with the
    :guilabel:`< Yes >` command selected.

.. image:: figures/app_kernel_conf_5.png
    :width: 400px
    :align: center
    :height: 100px
    :alt: Exit Dialog

11. Press :kbd:`Enter` to retire the menu display and
    return to the console command line.