.. _ConfiguringTheBootloader-57:

Configuring the Bootloader
==========================

Change the Device Tree
----------------------

By default, U-boot loads a device tree binary (dtb) file named *oftree* during boot. The *oftree* file is a copy of *am5728-phytec-pcm-948-40300111I.dtb*. This portion of the guide will instruct how to change U-boot to load different device trees.

* Power on the board and press any key to stop autoboot when prompted by the bootloader.

  .. note::
    ``help`` is a useful tool in U-Boot to show available commands and usage.

* Use the following command to verify that all of the environment variables are set as intended:

  .. code-block:: none
    :caption: Target (U-Boot)

    printenv

* Use the following command to list the available file's within the root filesystem's /boot directory:

  .. code-block:: none
    :caption: Target (U-Boot)

    ls mmc 0:2 boot

  .. code-block:: none
    :caption: Example Output

    => ls mmc 0:2 boot
    <DIR>       4096 .
    <DIR>       4096 ..
              214503 am5728-phytec-pcm-948-40300111I.dtb
              214503 am5728-phytec-pcm-948-50500111I.dtb
              214503 am5729-phytec-pcm-948-10306111I.dtb
              208683 am5749-phytec-pcm-948-32302111I.dtb
                1538 am57xx-phytec-pcm-948-gpio-fan.dtbo
                3409 am57xx-phytec-pcm-948-lcd-017.dtbo
                3924 am57xx-phytec-pcm-948-lcd-018.dtbo
                3755 am57xx-phytec-pcm-948-vm-009.dtbo
                3235 am57xx-phytec-pcm-948-vm-011.dtbo
                3615 am57xx-phytec-pcm-948-vm-016.dtbo
                3454 am57xx-phytec-pcm-948-wlan-wilink8.dtbo
                 370 am57xx-phytec-pcm-948-x28-spidev.dtbo
              205388 am57xx-phytec-pcm-948.dtb
              205388 oftree
    <SYM>         15 zImage
             5366272 zImage-5.10.168


* We can check which file is the active device tree like so:

  .. code-block:: none
    :caption: Target (U-Boot)

    printenv fdtfile

* Change the default device tree.

  .. code-block:: none
   :caption: Target (U-Boot)

   setenv fdtfile <dtb name>
   saveenv
   boot

U-boot will now load the dtb file named '<dtb name>' automatically during subsequent boots (just replace it with the appropriate file name when you run the command).

Working with Overlays
---------------------

The PHYTEC BSP also comes with some device tree overlays to help evaluate certain interfaces and accessory hardware. The table :ref:`Linux Device Tree Summary <releasenotes-57>` in the Release Notes shows an overview of all available overlays.

Enable an Overlay
~~~~~~~~~~~~~~~~~

* U-boot has an extra environment variable called "overlays" to apply device tree overlay files. We can enable device tree overlays with it like so:

.. code-block:: none
   :caption: Target (U-boot)

   setenv overlays am57xx-phytec-pcm-948-vm-016.dtbo
   saveenv
   boot

* Moreover, it's possible to enable multiple overlays by separating them with a space.

.. code-block:: none
   :caption: Target (U-boot)

   setenv overlays am57xx-phytec-pcm-948-vm-016.dtbo am57xx-phytec-pcm-948-x28-spidev.dtbo
   saveenv
   boot

The *saveenv* command stores the boot environment to non-volatile memory. If you would like temporarily adjust the boot environment, you can omit the ``saveenv`` command and the boot environment will go back to the previously saved settings on the next boot.


Reset the U-Boot Environment to Default Settings
------------------------------------------------

* In order to revert the U-Boot environment back to its original settings, run the following:

.. code-block:: none
    :caption: Target (U-Boot)

    env default -f -a
    saveenv
    boot