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.
    1. INFO -> the remaining 60 bytes are an informative message (providing progress or diagnostic messages). They should be displayed and then step #2 repeats
    2. 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.
    3. OKAY -> the requested command completed successfully. Go to #5
    4. 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:
    1. INFO -> display the remaining 60 bytes and return to #4
    2. FAIL -> display the remaining 60 bytes (if present) as a failure reason and consider the command failed. Stop.
    3. OKAY -> success. Go to #5
  5. Success. Stop.

Example Session

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.
"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.

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.