.. include:: /keyword.rst

====================================
|IOT-YOCTO| BSP & Boot Architecture
====================================

This section describes the system booting architecture and the BSP images information at each boot stage.

.. contents:: Sections
    :local:
    :depth: 1

.. toctree::
    :hidden:
    :maxdepth: 1

    ab-part
    part-layout
    secure-boot

The BSP(board support package) has the following booting architecture:

.. figure:: /_asset/sw_yocto_boot-bootflow.png
    :align: center
    :width: 900px

    |IOT-YOCTO| BSP Boot Architecture

MediaTek SoC ROM code
-----------------------

The MediaTek SoC ROM code will try to boot from different media:

* eMMC
* USB
* UFS (MT8395 only)

The |IOT-YOCTO| only allows to boot from eMMC and UFS. USB is only used as part of
the image flashing process but cannot be used alone to boot to a Linux userspace.

Please refer to the functional block and IoT Yocto support status for hardware features that IoT Yocto enables:

* :ref:`MT8365 (Genio 350) <soc-mt8365>`
* :ref:`MT8370 (Genio 510) <soc-mt8370>`
* :ref:`MT8390 (Genio 700) <soc-mt8390>`
* :ref:`MT8395 (Genio 1200) <soc-mt8395>`

.. _boot-emmc:

eMMC Boot
^^^^^^^^^

When booting the SoC will chose which media to boot from. If the eMMC boot
is chosen, the boot ROM will look at the first boot partition to look for a
second stage bootloader.

In case the ROM code fails to find a valid boot partition or valid second
stage bootloader, it will automatically go into :ref:`sw/yocto/boot:USB Boot`.

.. _boot-g1200-ufs:

UFS Boot
^^^^^^^^

MT8395 provides support to boot from UFS storage. However, the prebuilt |G1200-EVK| image
does not provide such support because |G1200-EVK-REF-BOARD| does not have UFS storage.

Please refer to the ``mt8395-evb-ufs`` in 
`meta-mediate-bsp layer <https://gitlab.com/mediatek/aiot/rity/meta-mediatek-bsp/-/blob/dunfell/conf/machine/mt8395-evb-ufs.conf>`_ 
to know how to configure for UFS booting.


USB Boot
^^^^^^^^

In |IOT-YOCTO|, the ROM code USB boot is used only for flashing.


BL2 (TF-A)
----------

The second stage bootloader is based on `Trusted Firmware A (TF-A) <https://www.trustedfirmware.org/projects/tf-a/>`_.
The |IOT-YOCTO| BSP ships pre-built binaries for second stage bootloaders.

BL2 first initializes the hardware:

* Initialize the system timer
* Initialize and turn on the PLLs
* Initialize the PMIC wrap component in order to be able to talk to the PMIC
* Sends a `baseband power-up` signal to the PMIC to notify him of a successful power up.
* Initialize the DDR
* Initialize the eMMC

Once the hardware is initialized, BL2 will read 4MB of memory stored at `bootloaders` partition
of the eMMC. The offset of `bootloaders` partition can be retrieved by TF-A API
:code:`get_partition_entry(partition_name)`.
The API provide the entry of `bootloaders` partition to find the ``fip.bin`` file.
This partition section should contains the ``fip.bin`` file.

Firmware Image Package (FIP) is a packaging format used by TF-A to package
firmware images. In |IOT-YOCTO| the FIP contains BL31, BL32, BL33 and optionally
some certificates when secure boot is enabled.

BL2 will read the FIP package and execute each of the binary it contains.

BL31 (TF-A)
-----------

BL31 is provided by TF-A and provides the `PSCI routines <https://developer.arm.com/architectures/system-architectures/software-standards/psci>`_.
Unlike BL2 which won't get used anymore once BL33 is loaded, BL31 will stay
in DDR until a power off or a reboot.

Once BL31 is finished to initialize, it will jump back to BL2.

Source repository: https://gitlab.com/mediatek/aiot/bsp/trusted-firmware-a

BL32 (OP-TEE)
-------------

BL32 is the secure OS that runs in `TrustZone <https://developer.arm.com/ip-products/security-ip/trustzone>`_.
|IOT-YOCTO| BSP is using by default `OP-TEE <https://www.op-tee.org/>`_ trusted OS.

Like BL31, OP-TEE OS stays in DDR until a power off or a reboot.

Source repository: https://gitlab.com/mediatek/aiot/bsp/optee-os

BL33 (U-Boot)
-------------

U-Boot is the third and last stage bootloader in the |IOT-YOCTO| boot architecture. It
is used to load and boot the kernel from the `kernel` partition.

In |IOT-YOCTO| the kernel must be encapsulated into a Flattened Image Tree (FIT) image.
The FIT image contains the kernel binary, a Device Tree Blob (DTB), and
optionally some Device Tree Blob Overlay (DTBO).

The DTBOs are used to provide optional features to a board, for instance for
cameras, display, or any daughter board. U-Boot looks at its :code:`boot_conf`
environment variable in order to know which (if any) overlay it must load and
merge to the main DTB. To merge a DTBO you need to change :code:`boot_conf` as following format:

.. code-block:: text

        boot_conf=#conf-[platform_dtb_name]#conf-[feature1_dtbo_name]#conf-[feature2_dtbo_name]

For example if you would like to add a DTBO used for DSI panel support to |G350-EVK| (`dtbo` name: :code:`display-dsi.dtbo`).

.. code-block:: text

        boot_conf=#conf-mediatek_mt8365-evk.dtb#conf-display-dsi.dtbo

There are 2 methods that developers can configure :code:`boot_conf` to merge DTBOs by U-Boot in |IOT-YOCTO|.

1. modify it during the Yocto build system by setting the following variable:

.. code-block::

        KERNEL_DEVICETREE_OVERLAYS_AUTOLOAD += "display-dsi.dtbo"

2. modify it with using `genio-tools`

.. code-block::

        genio-flash --load-dtbo display-dsi.dtbo

Besides, it is possible to set the :code:`boot_conf` variable from the U-Boot shell,
target Linux system, or the `u-boot-initial-env` file directly.
There are several ways to modify :code:`boot_conf`.

* Writing to the :code:`u-boot-initial-env` file presented in the deploy image folder before flashing image.
  (Not recommended because it may be overwritten by Bitbake later on)

* In the U-Boot shell:

.. prompt:: auto, =>

        setenv boot_conf "you-boot-env-var"; saveenv

* In the target Linux system:

.. prompt:: auto, $
        
        fw_setenv boot_conf "you-boot-env-var"

U-Boot can also be used to flash (via Fastboot), or to boot the kernel and/or
rootfs using an USB connection. Please refer to the boards documentation in
order to know which features are supported for your specific board.

Boot from Other Storage
^^^^^^^^^^^^^^^^^^^^^^^

In MediaTek's |IOT-YOCTO| platforms, the Boot ROM (BROM) **dose not** natively support booting from removable storage types such as USB, SD Card, or Ethernet.
BROM supports only main storage like eMMC or UFS, depends on the hardware capability of each Genio platforms.
However, it is still possible to boot Linux from these storage types by leveraging the common features provided by U-Boot (BL33).
For more detail, please refer to :doc:`Boot from Other Storage (POC) </sw/yocto/poc/boot-from-other-storage>` chapter.

Source repository: https://gitlab.com/mediatek/aiot/bsp/u-boot

Linux
-----

The `Linux kernel <https://www.kernel.org/>`_ is the main OS running. It will
load the root filesystem (rootfs) and starts running the `init` process.

The following versions of Linux are currently supported on |IOT-YOCTO|:

.. csv-table:: Kernel Version and Branches
   :header: "Kernel version", "Branch name"
   :widths: 15, 15

   "v5.4", ``mtk-v5.4``
   "v5.10", ``mtk-v5.10``
   "v5.15", ``mtk-v5.15-dev``


By default the latest Linux release will be built, if you want to use an older
supported kernel you can set the :code:`PREFERRED_VERSION_linux-mtk` variable
in your `local.conf`.

For example if you want to use the v5.4 branch you can set:

.. code-block::

        PREFERRED_VERSION_linux-mtk = "5.4%"