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  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
  ---------------
  
  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.