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.. SPDX-License-Identifier: GPL-2.0+
FastBoot Version 0.4
====================
The fastboot protocol is a mechanism for communicating with bootloaders
over USB. It is designed to be very straightforward to implement, to
allow it to be used across a wide range of devices and from hosts running
Linux, Windows, or OSX.
Basic Requirements
------------------
* Two bulk endpoints (in, out) are required
* Max packet size must be 64 bytes for full-speed and 512 bytes for
high-speed USB
* The protocol is entirely host-driven and synchronous (unlike the
multi-channel, bi-directional, asynchronous ADB protocol)
Transport and Framing
---------------------
1. Host sends a command, which is an ascii string in a single
packet no greater than 64 bytes.
2. Client response with a single packet no greater than 64 bytes.
The first four bytes of the response are "OKAY", "FAIL", "DATA",
or "INFO". Additional bytes may contain an (ascii) informative
message.
a. INFO -> the remaining 60 bytes are an informative message
(providing progress or diagnostic messages). They should
be displayed and then step #2 repeats
b. FAIL -> the requested command failed. The remaining 60 bytes
of the response (if present) provide a textual failure message
to present to the user. Stop.
c. OKAY -> the requested command completed successfully. Go to #5
d. DATA -> the requested command is ready for the data phase.
A DATA response packet will be 12 bytes long, in the form of
DATA00000000 where the 8 digit hexidecimal number represents
the total data size to transfer.
3. Data phase. Depending on the command, the host or client will
send the indicated amount of data. Short packets are always
acceptable and zero-length packets are ignored. This phase continues
until the client has sent or received the number of bytes indicated
in the "DATA" response above.
4. Client responds with a single packet no greater than 64 bytes.
The first four bytes of the response are "OKAY", "FAIL", or "INFO".
Similar to #2:
a. INFO -> display the remaining 60 bytes and return to #4
b. FAIL -> display the remaining 60 bytes (if present) as a failure
reason and consider the command failed. Stop.
c. OKAY -> success. Go to #5
5. Success. Stop.
Example Session
---------------
.. code-block:: none
Host: "getvar:version" request version variable
Client: "OKAY0.4" return version "0.4"
Host: "getvar:nonexistant" request some undefined variable
Client: "OKAY" return value ""
Host: "download:00001234" request to send 0x1234 bytes of data
Client: "DATA00001234" ready to accept data
Host: < 0x1234 bytes > send data
Client: "OKAY" success
Host: "flash:bootloader" request to flash the data to the bootloader
Client: "INFOerasing flash" indicate status / progress
"INFOwriting flash"
"OKAY" indicate success
Host: "powerdown" send a command
Client: "FAILunknown command" indicate failure
Command Reference
-----------------
* Command parameters are indicated by printf-style escape sequences.
* Commands are ascii strings and sent without the quotes (which are
for illustration only here) and without a trailing 0 byte.
* Commands that begin with a lowercase letter are reserved for this
specification. OEM-specific commands should not begin with a
lowercase letter, to prevent incompatibilities with future specs.
.. code-block:: none
"getvar:%s" Read a config/version variable from the bootloader.
The variable contents will be returned after the
OKAY response.
"download:%08x" Write data to memory which will be later used
by "boot", "ramdisk", "flash", etc. The client
will reply with "DATA%08x" if it has enough
space in RAM or "FAIL" if not. The size of
the download is remembered.
"verify:%08x" Send a digital signature to verify the downloaded
data. Required if the bootloader is "secure"
otherwise "flash" and "boot" will be ignored.
"flash:%s" Write the previously downloaded image to the
named partition (if possible).
"erase:%s" Erase the indicated partition (clear to 0xFFs)
"boot" The previously downloaded data is a boot.img
and should be booted according to the normal
procedure for a boot.img
"continue" Continue booting as normal (if possible)
"reboot" Reboot the device.
"reboot-bootloader" Reboot back into the bootloader.
Useful for upgrade processes that require upgrading
the bootloader and then upgrading other partitions
using the new bootloader.
"powerdown" Power off the device.
"ucmd" execute any bootloader command and wait until it
finishs.
"acmd" execute any bootloader command, do not wait.
Client Variables
----------------
The ``getvar:%s`` command is used to read client variables which
represent various information about the device and the software
on it.
The various currently defined names are::
version Version of FastBoot protocol supported.
It should be "0.3" for this document.
version-bootloader Version string for the Bootloader.
version-baseband Version string of the Baseband Software
product Name of the product
serialno Product serial number
secure If the value is "yes", this is a secure
bootloader requiring a signature before
it will install or boot images.
Names starting with a lowercase character are reserved by this
specification. OEM-specific names should not start with lowercase
characters.
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