The protocol that is used over USB and UDP is described in .
The current implementation supports the following standard commands:
set_active(only a stub implementation which always succeeds)
The following OEM commands are supported (if enabled):
oem format- this executes
gpt write mmc %x $partitions
oem partconf- this executes
mmc partconf %x <arg> 0to configure eMMC with <arg> = boot_ack boot_partition
oem bootbus- this executes
mmc bootbus %x %sto configure eMMC
Support for both eMMC and NAND devices is included.
The counterpart to this is the fastboot client which can be found in
platform/system/core repository in the fastboot
folder. It runs on Windows, Linux and OSX. The fastboot client is
part of the Android SDK Platform-Tools and can be downloaded from .
The fastboot gadget relies on the USB download gadget, so the following options must be configured:
CONFIG_USB_GADGET_DOWNLOAD CONFIG_USB_GADGET_VENDOR_NUM CONFIG_USB_GADGET_PRODUCT_NUM CONFIG_USB_GADGET_MANUFACTURER
CONFIG_USB_GADGET_VENDOR_NUM must be one of the numbers
supported by the fastboot client. The list of vendor IDs supported can
be found in the fastboot client source code.
The fastboot protocol requires a large memory buffer for
downloads. This buffer should be as large as possible for a
platform. The location of the buffer and size are set with
may be overridden on the fastboot command line using
Fastboot environment variables¶
Fastboot partition aliases can also be defined for devices where GPT
limitations prevent user-friendly partition names such as
cache. Or, where the actual partition name doesn’t match a standard
partition name used commonly with fastboot.
The current implementation checks aliases when accessing partitions by name (flash_write and erase functions). To define a partition alias add an environment variable similar to:
fastboot_partition_alias_<alias partition name>=<actual partition name>
Raw partition descriptors¶
In cases where no partition table is present, a raw partition descriptor can be defined, specifying the offset, size, and optionally the MMC hardware partition number for a given partition name.
This is useful when using fastboot to flash files (e.g. SPL or U-Boot) to a specific offset in the eMMC boot partition, without having to update the entire boot partition.
To define a raw partition descriptor, add an environment variable similar to:
fastboot_raw_partition_<raw partition name>=<offset> <size> [mmcpart <num>]
fastboot_raw_partition_boot=0x100 0x1f00 mmcpart 1
Variables retrived through
getvar can be overridden by defining
environment variables of the form
fastboot.<variable>. These are
looked up first so can be used to override values which would
otherwise be returned. Using this mechanism you can also return types
for NAND filesystems, as the fully parameterised variable is looked
When executing the fastboot
boot command, if
fastboot_bootcmd is set
then that will be executed in place of
The Fastboot implementation in U-Boot allows to write images into disk partitions. Target partitions are referred on the host computer by their names.
For GPT/EFI the respective partition name is used.
For MBR the partitions are referred by generic names according to the following schema:
<device type><device index letter><partition index>
The device type is as follows:
- IDE, ATAPI and SATA disks:
- SCSI disks:
- USB media:
- MMC and SD cards:
- Disk on chip:
The device index starts from
a and refers to the interface (e.g. USB
controller, SD/MMC controller) or disk index. The partition index starts
1 and describes the partition number on the particular device.
Alternatively, partition types may be specified using U-Boot’s partition
syntax. This allows specifying partitions like
:3. The interface is always
Writing Partition Table¶
Fastboot also allows to write the partition table to the media. This can be done by writing the respective partition table image to a special target “gpt” or “mbr”. These names can be customized by defining the following configuration options:
Enter into fastboot by executing the fastboot command in U-Boot for either USB:
=> fastboot usb 0
=> fastboot udp link up on port 0, speed 100, full duplex Using ethernet@4a100000 device Listening for fastboot command on 192.168.0.102
On the client side you can fetch the bootloader version for instance:
$ fastboot getvar version-bootloader version-bootloader: U-Boot 2019.07-rc4-00240-g00c9f2a2ec Finished. Total time: 0.005s
or initiate a reboot:
$ fastboot reboot
and once the client comes back, the board should reset.
You can also specify a kernel image to boot. You have to either specify the an image in Android format or pass a binary kernel and let the fastboot client wrap the Android suite around it. On OMAP for instance you take zImage kernel and pass it to the fastboot client:
$ fastboot -b 0x80000000 -c "console=ttyO2 earlyprintk root=/dev/ram0 mem=128M" boot zImage creating boot image... creating boot image - 1847296 bytes downloading 'boot.img'... OKAY [ 2.766s] booting... OKAY [ -0.000s] finished. total time: 2.766s
and on the U-Boot side you should see:
Starting download of 1847296 bytes ........................................................ downloading of 1847296 bytes finished Booting kernel.. ## Booting Android Image at 0x81000000 ... Kernel load addr 0x80008000 size 1801 KiB Kernel command line: console=ttyO2 earlyprintk root=/dev/ram0 mem=128M Loading Kernel Image ... OK OK Starting kernel ...