U-Boot

.. SPDX-License-Identifier: GPL-2.0-or-later

Booting from TPL/SPL
====================

The main U-Boot binary may be too large to be loaded directly by the Boot ROM.
This was the original driver for splitting up U-Boot into multiple boot stages.

U-Boot typically goes through the following boot phases where TPL, VPL, and SPL
are optional. While many boards use SPL only few use TPL.

TPL
   Tertiary Program Loader. Very early init, as tiny as possible. This loads SPL
   (or VPL if enabled).

VPL
   Verifying Program Loader. Optional verification step, which can select one of
   several SPL binaries, if A/B verified boot is enabled. Implementation of the
   VPL logic is work-in-progress. For now it just boots into SPL.

SPL
   Secondary Program Loader. Sets up SDRAM and loads U-Boot proper. It may also
   load other firmware components.

U-Boot
   U-Boot proper. This is the only stage containing command. It also implements
   logic to load an operating system, e.g. via UEFI.

.. note::

   The naming convention on the PowerPC architecture deviates from the other
   archtitectures. Here the boot sequence is SPL->TPL->U-Boot.

Further usages for U-Boot SPL comprise:

* launching BL31 of ARM Trusted Firmware which invokes U-Boot as BL33
* launching EDK II
* launching Linux, e.g. :doc:`Falcon Mode <../develop/falcon>`
* launching RISC-V OpenSBI which invokes main U-Boot

Target binaries
---------------

Binaries loaded by SPL/TPL can be:

* raw binaries where the entry address equals the start address. This is the
  only binary format supported by TPL.
* :doc:`FIT <fit/index>` images
* legacy U-Boot images

Configuration
~~~~~~~~~~~~~

Raw images are only supported in SPL if CONFIG_SPL_RAW_IMAGE_SUPPORT=y.

CONFIG_SPL_FIT=y and CONFIG_SPL_LOAD_FIT=y are needed to load FIT images.

CONFIG_SPL_LEGACY_IMAGE_FORMAT=y is needed to load legacy U-Boot images.
CONFIG_SPL_LEGACY_IMAGE_CRC_CHECK=y enables checking the CRC32 of legacy U-Boot
images.

Image load methods
------------------

The image boot methods available for a board must be defined in two places:

The board code implements a function board_boot_order() enumerating up to
five boot methods and the sequence in which they are tried. (The maximum
number of boot methods is currently hard coded as variable spl_boot_list[]).
If there is only one boot method function, spl_boot_device() may be implemented
instead.

The configuration controls which of these boot methods are actually available.

Loading from block devices
~~~~~~~~~~~~~~~~~~~~~~~~~~

MMC1, MMC2, MMC2_2
    These methods read an image from SD card or eMMC. The first digit after
    'MMC' indicates the device number. Required configuration settings include:

    * CONFIG_SPL_MMC=y or CONFIG_TPL_MMC=y

    To use a PCI connected MMC controller you need to additionally specify:

    * CONFIG_SPL_PCI=y

    * CONFIG_SPL_PCI_PNP=y

    * CONFIG_MMC=y

    * CONFIG_MMC_PCI=y

    * CONFIG_MMC_SDHCI=y

    To load from a file system use:

    * CONFIG_SPL_FS_FAT=y or CONFIG_SPL_FS_EXT=y

    * CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="<filepath>"

NVMe
    This methods load the image from an NVMe drive.
    Required configuration settings include:

    * CONFIG_SPL_PCI=y

    * CONFIG_SPL_PCI_PNP=y

    * CONFIG_SPL_NVME=y

    * CONFIG_SPL_NVME_PCI=y

    * CONFIG_SPL_NVME_BOOT_DEVICE (number of the NVMe device)

    * CONFIG_SYS_NVME_BOOT_PARTITION (partition to read from)

    To load from a file system use:

    * CONFIG_SPL_FS_FAT=y or CONFIG_SPL_FS_EXT=y

    * CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="<filepath>"

SATA
    This method reads an image from a SATA drive.
    Required configuration settings include:

    * CONFIG_SPL_SATA=y or CONFIG_TPL_SATA=y

    To use a PCIe connecte SATA controller you additionally need:

    * CONFIG_SPL_PCI=y

    * CONFIG_SPL_SATA=y

    * CONFIG_SPL_AHCI_PCI=y

    * CONFIG_SPL_PCI_PNP=y

    To load from a file system use:

    * CONFIG_SPL_FS_FAT=y

    * CONFIG_SYS_SATA_FAT_BOOT_PARTITION=<partition number>

    * CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="<filepath>"

USB
    The USB method loads an image from a USB block device.
    Required configuration settings include:

    * CONFIG_SPL_USB_HOST=y

    * CONFIG_SPL_USB_STORAGE=y

    To use a PCI connected USB 3.0 controller you additionally need:

    * CONFIG_SPL_FS_FAT=y

    * CONFIG_SPL_PCI=y

    * CONFIG_SPL_PCI_PNP=y

    * CONFIG_USB=y

    * CONFIG_USB_XHCI_HCD=y

    * CONFIG_USB_XHCI_PCI=y

    To load from a file system use:

    * CONFIG_SPL_FS_FAT=y or CONFIG_SPL_FS_EXT=y

    * CONFIG_SYS_USB_FAT_BOOT_PARTITION=<partition number>

    * CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="<filepath>"

Loading from raw flash devices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NAND
    This method loads the image from NAND flash. To read from raw NAND the
    following configuration settings are required:

    * CONFIG_SPL_NAND_SUPPORT=y or CONFIG_TPL_NAND_SUPPORT=y

    If CONFIG_SPL_NAND_RAW_ONLY=y only raw images can be loaded.

    For using UBI (Unsorted Block Images) volumes to read from NAND the
    following configuration settings are required:

    * CONFIG_SPL_UBI=y or CONFIG_TPL_UBI=y

    The UBI volume to read can either be specified

    * by name using CONFIG_SPL_UBI_LOAD_BY_VOLNAME or

    * by number using CONFIG_SPL_UBI_LOAD_MONITOR_ID.

NOR
    This method loads the image from NOR flash.
    Required configuration settings include:

    * CONFIG_SPL_NOR_SUPPORT=y or CONFIG_TPL_NOR_SUPPORT=y

OneNAND
    This methods loads the image from a OneNAND device. To read from raw OneNAND
    the following configuration settings are required:

    * CONFIG_SPL_ONENAND_SUPPORT=y or CONFIG_TPL_ONENAND_SUPPORT=y

    For using the Ubi file system to read from NAND the following configuration
    settings are required:

    * CONFIG_SPL_UBI=y or CONFIG_TPL_UBI=y

SPI
    This method loads an image form SPI NOR flash.
    Required configuration settings include:

    * CONFIG_SPL_DM_SPI=y

    * CONFIG_SPL_SPI_FLASH=y

    * CONFIG_SPI_LOAD=y or CONFIG_TPL_SPI_LOAD=y


Sunxi SPI
    This method which is specific to Allwinner SoCs loads an image form SPI NOR
    flash. Required configuration settings include:

    * CONFIG_SPL_SPI_SUNXI=y

Loading from other devices
~~~~~~~~~~~~~~~~~~~~~~~~~~

BOOTROM
    The binary is loaded by the boot ROM.
    Required configuration settings include:

    * CONFIG_SPL_BOOTROM_SUPPORT=y or CONFIG_TPL_BOOTROM_SUPPORT=y

DFU
    :doc:`Device Firmware Upgrade <dfu>` is used to load the binary into RAM.
    Required configuration settings include:

    * CONFIG_DFU=y

    * CONFIG_SPL_RAM_SUPPORT=y or CONFIG TPL_RAM_SUPPORT=y

Ethernet
    This method loads an image over Ethernet. The BOOTP protocol is used to find
    a TFTP server and binary name. The binary is downloaded via the TFTP
    protocol. Required configuration settings include:

    * CONFIG_SPL_NET=y or CONFIG_TPL_NET=y

    * CONFIG_SPL_ETH_DEVICE=y or CONFIG_DM_USB_GADGET=y

FEL
    This method does not actually load an image for U-Boot.
    FEL is a routine contained in the boot ROM of Allwinner SoCs which serves
    for the initial programming or recovery via USB

RAM
    This method uses an image preloaded into RAM.
    Required configuration settings include:

    * CONFIG_SPL_RAM_SUPPORT=y or CONFIG_TPL_RAM_SUPPORT=y

    * CONFIG_RAM_DEVICE=y

Sandbox file
    On the sandbox this method loads an image from the host file system.

Sandbox image
    On the sandbox this method loads an image from the host file system.

Semihosting
    When running in an ARM or RISC-V virtual machine the semihosting method can
    be used to load an image from the host file system.
    Required configuration settings include:

    * CONFIG_SPL_SEMIHOSTING=y

    * CONFIG_SPL_SEMIHOSTING_FALLBACK=y

    * CONFIG_SPL_FS_LOAD_PAYLOAD_NAME=<path to file>

UART
    This method loads an image via the Y-Modem protocol from the UART.
    Required configuration settings include:

    * CONFIG_SPL_YMODEM_SUPPORT=y or CONFIG_TPL_YMODEM_SUPPORT=y

USB SDP
    This method loads the image using the Serial Download Protocol as
    implemented by the boot ROM of the i.MX family of SoCs.

    Required configuration settings include:

    * CONFIG_SPL_SERIAL=y

    * CONFIG_SPL_USB_SDP_SUPPORT=y or CONFIG_TPL_USB_SDP_SUPPORT

VBE Simple
    This method is used by the VPL stage to extract the next stage image from
    the loaded image.

    Required configuration settings include:

    * CONFIG_VPL=y

    * CONFIG_SPL_BOOTMETH_VBE_SIMPLE_FW=y or CONFIG_TPL_BOOTMETH_VBE_SIMPLE_FW=y

XIP
    This method executes an image in place.

    Required configuration settings include:

    * CONFIG_SPL_XIP_SUPPORT