host/lib/bundle_firmware.py

# Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""This module builds a firmware image for a tegra-based board.

This modules uses a few rudimentary other libraries for its activity.

Here are the names we give to the various files we deal with. It is important
to keep these consistent!

  uboot     u-boot.bin (with no device tree)
  fdt       the fdt blob
  bct       the BCT file
  bootstub  uboot + fdt
  signed    (uboot + fdt + bct) signed blob
"""

import glob
import os
import re

import cros_output
from fdt import Fdt
from pack_firmware import PackFirmware
import shutil
import struct
import tempfile
from tools import CmdError
from tools import Tools
from write_firmware import WriteFirmware

# This data is required by bmpblk_utility. Does it ever change?
# It was stored with the chromeos-bootimage ebuild, but we want
# this utility to work outside the chroot.
yaml_data = '''
bmpblock: 1.0

images:
    devmode:    DeveloperBmp/DeveloperBmp.bmp
    recovery:   RecoveryBmp/RecoveryBmp.bmp
    rec_yuck:   RecoveryNoOSBmp/RecoveryNoOSBmp.bmp
    rec_insert: RecoveryMissingOSBmp/RecoveryMissingOSBmp.bmp

screens:
  dev_en:
    - [0, 0, devmode]
  rec_en:
    - [0, 0, recovery]
  yuck_en:
    - [0, 0, rec_yuck]
  ins_en:
    - [0, 0, rec_insert]

localizations:
  - [ dev_en, rec_en, yuck_en, ins_en ]
'''

class Bundle:
  """This class encapsulates the entire bundle firmware logic.

  Sequence of events:
    bundle = Bundle(tools.Tools(), cros_output.Output())
    bundle.SetDirs(...)
    bundle.SetFiles(...)
    bundle.SetOptions(...)
    bundle.SelectFdt(fdt.Fdt('filename.dtb')
    .. can call bundle.AddConfigList(), AddEnableList() if required
    bundle.Start(...)

  Public properties:
    fdt: The fdt object that we use for building our image. This wil be the
        one specified by the user, except that we might add config options
        to it. This is set up by SelectFdt() which must be called before
        bundling starts.
    uboot_fname: Full filename of the U-Boot binary we use.
    bct_fname: Full filename of the BCT file we use.
    spl_source: Source device to load U-Boot from, in SPL:
        straps: Select device according to CPU strap pins
        spi: Boot from SPI
        emmc: Boot from eMMC
  """

  def __init__(self, tools, output):
    """Set up a new Bundle object.

    Args:
      tools: A tools.Tools object to use for external tools.
      output: A cros_output.Output object to use for program output.
    """
    self._tools = tools
    self._out = output

    # Set up the things we need to know in order to operate.
    self._board = None          # Board name, e.g. tegra2_seaboard.
    self._fdt_fname = None      # Filename of our FDT.
    self.uboot_fname = None     # Filename of our U-Boot binary.
    self.bct_fname = None       # Filename of our BCT file.
    self.fdt = None             # Our Fdt object.
    self.bmpblk_fname = None    # Filename of our Bitmap Block
    self.coreboot_fname = None  # Filename of our coreboot binary.
    self.seabios_fname = None   # Filename of our SeaBIOS payload.
    self.exynos_bl1 = None      # Filename of Exynos BL1 (pre-boot)
    self.exynos_bl2 = None      # Filename of Exynos BL2 (SPL)
    self.spl_source = 'straps'  # SPL boot according to board settings
    self.skeleton_fname = None  # Filename of Coreboot skeleton file
    self.ecbin_fname = None     # Filename of EC file

  def SetDirs(self, keydir):
    """Set up directories required for Bundle.

    Args:
      keydir: Directory containing keys to use for signing firmware.
    """
    self._keydir = keydir

  def SetFiles(self, board, bct, uboot=None, bmpblk=None, coreboot=None,
               postload=None, seabios=None, exynos_bl1=None, exynos_bl2=None,
               skeleton=None, ecbin=None, kernel=None):
    """Set up files required for Bundle.

    Args:
      board: The name of the board to target (e.g. tegra2_seaboard).
      uboot: The filename of the u-boot.bin image to use.
      bct: The filename of the binary BCT file to use.
      bmpblk: The filename of bitmap block file to use.
      coreboot: The filename of the coreboot image to use (on x86)
      postload: The filename of the u-boot-post.bin image to use.
      seabios: The filename of the SeaBIOS payload to use if any.
      exynos_bl1: The filename of the exynos BL1 file
      exynos_bl2: The filename of the exynos BL2 file (U-Boot spl)
      skeleton: The filename of the coreboot skeleton file.
      ecbin: The filename of the EC (Embedded Controller) file.
      kernel: The filename of the kernel file if any.
    """
    self._board = board
    self.uboot_fname = uboot
    self.bct_fname = bct
    self.bmpblk_fname = bmpblk
    self.coreboot_fname = coreboot
    self.postload_fname = postload
    self.seabios_fname = seabios
    self.exynos_bl1 = exynos_bl1
    self.exynos_bl2 = exynos_bl2
    self.skeleton_fname = skeleton
    self.ecbin_fname = ecbin
    self.kernel_fname = kernel

  def SetOptions(self, small):
    """Set up options supported by Bundle.

    Args:
      small: Only create a signed U-Boot - don't produce the full packed
          firmware image. This is useful for devs who want to replace just the
          U-Boot part while keeping the keys, gbb, etc. the same.
    """
    self._small = small

  def CheckOptions(self):
    """Check provided options and select defaults."""
    if not self._board:
      raise ValueError('No board defined - please define a board to use')
    build_root = os.path.join('##', 'build', self._board, 'firmware')
    dir_name = os.path.join(build_root, 'dts')
    if not self._fdt_fname:
      # Figure out where the file should be, and the name we expect.
      base_name = re.sub('_', '-', self._board)

      # In case the name exists with a prefix or suffix, find it.
      wildcard = os.path.join(dir_name, '*%s*.dts' % base_name)
      found_list = glob.glob(self._tools.Filename(wildcard))
      if len(found_list) == 1:
        self._fdt_fname = found_list[0]
      else:
        # We didn't find anything definite, so set up our expected name.
        self._fdt_fname = os.path.join(dir_name, '%s.dts' % base_name)

    # Convert things like 'exynos5250-daisy' into a full path.
    root, ext = os.path.splitext(self._fdt_fname)
    if not ext and not os.path.dirname(root):
      self._fdt_fname = os.path.join(dir_name, '%s.dts' % root)

    if not self.uboot_fname:
      self.uboot_fname = os.path.join(build_root, 'u-boot.bin')
    if not self.bct_fname:
      self.bct_fname = os.path.join(build_root, 'bct', 'board.bct')
    if not self.bmpblk_fname:
      self.bmpblk_fname = os.path.join(build_root, 'default.bmpblk')
    if not self.exynos_bl1:
      self.exynos_bl1 = os.path.join(build_root, 'E5250.nbl1.bin')
    if not self.exynos_bl2:
      self.exynos_bl2 = os.path.join(build_root, 'smdk5250-spl.bin')
    if not self.coreboot_fname:
      self.coreboot_fname = os.path.join(build_root, 'coreboot.rom')
    if not self.skeleton_fname:
      self.skeleton_fname = os.path.join(build_root, 'skeleton.bin')
    if not self.ecbin_fname:
      self.ecbin_fname = os.path.join(build_root, 'ec.RW.bin')

  def GetFiles(self):
    """Get a list of files that we know about.

    This is the opposite of SetFiles except that we may have put in some
    default names. It returns a dictionary containing the filename for
    each of a number of pre-defined files.

    Returns:
      Dictionary, with one entry for each file.
    """
    file_list = {
        'bct' : self.bct_fname,
        'exynos-bl1' : self.exynos_bl1,
        'exynos-bl2' : self.exynos_bl2,
        }
    return file_list

  def _CreateGoogleBinaryBlock(self, hardware_id):
    """Create a GBB for the image.

    Args:
      hardware_id: Hardware ID to use for this board. If None, then the
          default from the Fdt will be used

    Returns:
      Path of the created GBB file.

    Raises:
      CmdError if a command fails.
    """
    if not hardware_id:
      hardware_id = self.fdt.GetString('/config', 'hwid')
    gbb_size = self.fdt.GetFlashPartSize('ro', 'gbb')
    odir = self._tools.outdir

    chromeos_config = self.fdt.GetProps("/chromeos-config")
    # Build GBB flags.
    # (src/platform/vboot_reference/firmware/include/gbb_header.h)
    flag_properties = {
        'fast-developer-mode': 0x01,
        'gbb-flag-dev-screen-short-delay': 0x00000001,
        'gbb-flag-load-option-roms': 0x00000002,
        'gbb-flag-enable-alternate-os': 0x00000004,
        'gbb-flag-force-dev-switch-on': 0x00000008,
        'gbb-flag-force-dev-boot-usb': 0x00000010,
        'gbb-flag-disable-fw-rollback-check': 0x00000020,
    }
    gbb_flags = 0
    for flag_name, flag_value in flag_properties.items():
      if flag_name not in chromeos_config:
        continue
      gbb_flags |= flag_value
      self._out.Notice("Enabling %s." % flag_name)

    self._out.Progress('Creating GBB')
    sizes = [0x100, 0x1000, gbb_size - 0x2180, 0x1000]
    sizes = ['%#x' % size for size in sizes]
    gbb = 'gbb.bin'
    keydir = self._tools.Filename(self._keydir)
    self._tools.Run('gbb_utility', ['-c', ','.join(sizes), gbb], cwd=odir)
    self._tools.Run('gbb_utility', ['-s',
        '--hwid=%s' % hardware_id,
        '--rootkey=%s/root_key.vbpubk' % keydir,
        '--recoverykey=%s/recovery_key.vbpubk' % keydir,
        '--bmpfv=%s' % self._tools.Filename(self.bmpblk_fname),
        '--flags=%d' % gbb_flags,
        gbb],
        cwd=odir)
    return os.path.join(odir, gbb)

  def _SignBootstub(self, bct, bootstub, text_base):
    """Sign an image so that the Tegra SOC will boot it.

    Args:
      bct: BCT file to use.
      bootstub: Boot stub (U-Boot + fdt) file to sign.
      text_base: Address of text base for image.

    Returns:
      filename of signed image.

    Raises:
      CmdError if a command fails.
    """
    # First create a config file - this is how we instruct cbootimage
    signed = os.path.join(self._tools.outdir, 'signed.bin')
    self._out.Progress('Signing Bootstub')
    config = os.path.join(self._tools.outdir, 'boot.cfg')
    fd = open(config, 'w')
    fd.write('Version    = 1;\n')
    fd.write('Redundancy = 1;\n')
    fd.write('Bctfile    = %s;\n' % bct)

    # TODO(dianders): Right now, we don't have enough space in our flash map
    # for two copies of the BCT when we're using NAND, so hack it to 1.  Not
    # sure what this does for reliability, but at least things will fit...
    is_nand = "NvBootDevType_Nand" in self._tools.Run('bct_dump', [bct])
    if is_nand:
      fd.write('Bctcopy = 1;\n')

    fd.write('BootLoader = %s,%#x,%#x,Complete;\n' % (bootstub, text_base,
        text_base))

    fd.close()

    self._tools.Run('cbootimage', [config, signed])
    self._tools.OutputSize('BCT', bct)
    self._tools.OutputSize('Signed image', signed)
    return signed

  def SetBootcmd(self, bootcmd, bootsecure):
    """Set the boot command for U-Boot.

    Args:
      bootcmd: Boot command to use, as a string (if None this this is a nop).
      bootsecure: We'll set '/config/bootsecure' to 1 if True and 0 if False.
    """
    if bootcmd:
      self.fdt.PutString('/config', 'bootcmd', bootcmd)
      self.fdt.PutInteger('/config', 'bootsecure', int(bootsecure))
      self._out.Info('Boot command: %s' % bootcmd)

  def SetNodeEnabled(self, node_name, enabled):
    """Set whether an node is enabled or disabled.

    This simply sets the 'status' property of a node to "ok", or "disabled".

    The node should either be a full path to the node (like '/uart@10200000')
    or an alias property.

    Aliases are supported like this:

        aliases {
                console = "/uart@10200000";
        };

    pointing to a node:

        uart@10200000 {
                status = "okay";
        };

    In this case, this function takes the name of the alias ('console' in
    this case) and updates the status of the node that is pointed to, to
    either ok or disabled. If the alias does not exist, a warning is
    displayed.

    Args:
      node_name: Name of node (e.g. '/uart@10200000') or alias alias
          (e.g. 'console') to adjust
      enabled: True to enable, False to disable
    """
    # Look up the alias if this is an alias reference
    if not node_name.startswith('/'):
      lookup = self.fdt.GetString('/aliases', node_name, '')
      if not lookup:
        self._out.Warning("Cannot find alias '%s' - ignoring" % node_name)
        return
      node_name = lookup
    if enabled:
      status = 'okay'
    else:
      status = 'disabled'
    self.fdt.PutString(node_name, 'status', status)

  def AddEnableList(self, enable_list):
    """Process a list of nodes to enable/disable.

    Args:
      config_list: List of (node, value) tuples to add to the fdt. For each
          tuple:
              node: The fdt node to write to will be <node> or pointed to by
                  /aliases/<node>. We can tell which
              value: 0 to disable the node, 1 to enable it
    """
    if enable_list:
      for node_name, enabled in enable_list:
        try:
          enabled = int(enabled)
          if enabled not in (0, 1):
            raise ValueError
        except ValueError as str:
          raise CmdError("Invalid enable option value '%s' "
              "(should be 0 or 1)" % enabled)
        self.SetNodeEnabled(node_name, enabled)

  def AddConfigList(self, config_list, use_int=False):
    """Add a list of config items to the fdt.

    Normally these values are written to the fdt as strings, but integers
    are also supported, in which case the values will be converted to integers
    (if necessary) before being stored.

    Args:
      config_list: List of (config, value) tuples to add to the fdt. For each
          tuple:
              config: The fdt node to write to will be /config/<config>.
              value: An integer or string value to write.
      use_int: True to only write integer values.

    Raises:
      CmdError: if a value is required to be converted to integer but can't be.
    """
    if config_list:
      for config in config_list:
        value = config[1]
        if use_int:
          try:
            value = int(value)
          except ValueError as str:
            raise CmdError("Cannot convert config option '%s' to integer" %
                value)
        if type(value) == type(1):
          self.fdt.PutInteger('/config', '%s' % config[0], value)
        else:
          self.fdt.PutString('/config', '%s' % config[0], value)

  def DecodeTextBase(self, data):
    """Look at a U-Boot image and try to decode its TEXT_BASE.

    This works because U-Boot has a header with the value 0x12345678
    immediately followed by the TEXT_BASE value. We can therefore read this
    from the image with some certainty. We check only the first 40 words
    since the header should be within that region.

    Since upstream Tegra has moved to having a 16KB SPL region at the start,
    and currently this does holds the U-Boot text base (e.g. 0x10c000) instead
    of the SPL one (e.g. 0x108000), we search in the U-Boot part as well.

    Args:
      data: U-Boot binary data

    Returns:
      Text base (integer) or None if none was found
    """
    found = False
    for start in (0, 0x4000):
      for i in range(start, start + 160, 4):
        word = data[i:i + 4]

        # TODO(sjg): This does not cope with a big-endian target
        value = struct.unpack('<I', word)[0]
        if found:
          return value - start
        if value == 0x12345678:
          found = True

    return None

  def CalcTextBase(self, name, fdt, fname):
    """Calculate the TEXT_BASE to use for U-Boot.

    Normally this value is in the fdt, so we just read it from there. But as
    a second check we look at the image itself in case this is different, and
    switch to that if it is.

    This allows us to flash any U-Boot even if its TEXT_BASE is different.
    This is particularly useful with upstream U-Boot which uses a different
    value (which we will move to).
    """
    data = self._tools.ReadFile(fname)
    fdt_text_base = fdt.GetInt('/chromeos-config', 'textbase', 0)
    text_base = self.DecodeTextBase(data)
    text_base_str = '%#x' % text_base if text_base else 'None'
    self._out.Info('TEXT_BASE: fdt says %#x, %s says %s' % (fdt_text_base,
        fname, text_base_str))

    # If they are different, issue a warning and switch over.
    if text_base and text_base != fdt_text_base:
      self._out.Warning("TEXT_BASE %x in %sU-Boot doesn't match "
              "fdt value of %x. Using %x" % (text_base, name,
                  fdt_text_base, text_base))
      fdt_text_base = text_base
    return fdt_text_base

  def _CreateBootStub(self, uboot, base_fdt, postload):
    """Create a boot stub and a signed boot stub.

    For postload:
    We add a /config/postload-text-offset entry to the signed bootstub's
    fdt so that U-Boot can find the postload code.

    The raw (unsigned) bootstub will have a value of -1 for this since we will
    simply append the postload code to the bootstub and it can find it there.
    This will be used for RW A/B firmware.

    For the signed case this value will specify where in the flash to find
    the postload code. This will be used for RO firmware.

    Args:
      uboot: Path to u-boot.bin (may be chroot-relative)
      base_fdt: Fdt object containing the flat device tree.
      postload: Path to u-boot-post.bin, or None if none.

    Returns:
      Tuple containing:
        Full path to bootstub (uboot + fdt(-1) + postload).
        Full path to signed (uboot + fdt(flash pos) + bct) + postload.

    Raises:
      CmdError if a command fails.
    """
    bootstub = os.path.join(self._tools.outdir, 'u-boot-fdt.bin')
    text_base = self.CalcTextBase('', self.fdt, uboot)
    uboot_data = self._tools.ReadFile(uboot)

    # Make a copy of the fdt for the bootstub
    fdt = base_fdt.Copy(os.path.join(self._tools.outdir, 'bootstub.dtb'))
    fdt.PutInteger('/config', 'postload-text-offset', 0xffffffff);
    fdt_data = self._tools.ReadFile(fdt.fname)

    self._tools.WriteFile(bootstub, uboot_data + fdt_data)
    self._tools.OutputSize('U-Boot binary', self.uboot_fname)
    self._tools.OutputSize('U-Boot fdt', self._fdt_fname)
    self._tools.OutputSize('Combined binary', bootstub)

    # Sign the bootstub; this is a combination of the board specific
    # bct and the stub u-boot image.
    signed = self._SignBootstub(self._tools.Filename(self.bct_fname),
        bootstub, text_base)

    signed_postload = os.path.join(self._tools.outdir, 'signed-postload.bin')
    data = self._tools.ReadFile(signed)

    if postload:
      # We must add postload to the bootstub since A and B will need to
      # be able to find it without the /config/postload-text-offset mechanism.
      bs_data = self._tools.ReadFile(bootstub)
      bs_data += self._tools.ReadFile(postload)
      bootstub = os.path.join(self._tools.outdir, 'u-boot-fdt-postload.bin')
      self._tools.WriteFile(bootstub, bs_data)
      self._tools.OutputSize('Combined binary with postload', bootstub)

      # Now that we know the file size, adjust the fdt and re-sign
      postload_bootstub = os.path.join(self._tools.outdir, 'postload.bin')
      fdt.PutInteger('/config', 'postload-text-offset', len(data))
      fdt_data = self._tools.ReadFile(fdt.fname)
      self._tools.WriteFile(postload_bootstub, uboot_data + fdt_data)
      signed = self._SignBootstub(self._tools.Filename(self.bct_fname),
          postload_bootstub, text_base)
      if len(data) != os.path.getsize(signed):
        raise CmdError('Signed file size changed from %d to %d after updating '
            'fdt' % (len(data), os.path.getsize(signed)))

      # Re-read the signed image, and add the post-load binary.
      data = self._tools.ReadFile(signed)
      data += self._tools.ReadFile(postload)
      self._tools.OutputSize('Post-load binary', postload)

    self._tools.WriteFile(signed_postload, data)
    self._tools.OutputSize('Final bootstub with postload', signed_postload)

    return bootstub, signed_postload

  def _CreateCorebootStub(self, uboot, coreboot, fdt, seabios):
    """Create a coreboot boot stub.

    Args:
      uboot: Path to u-boot.bin (may be chroot-relative)
      coreboot: Path to coreboot.rom
      fdt: Device Tree
      seabios: Path to SeaBIOS payload binary or None

    Returns:
      Full path to bootstub (coreboot + uboot + fdt).

    Raises:
      CmdError if a command fails.
    """
    bootstub = os.path.join(self._tools.outdir, 'coreboot-full.rom')

    # U-Boot itself does not put a .elf extension on the elf file.
    # The U-Boot ebuild does, but we shouldn't actually require it since
    # devs may want to just use what U-Boot creates.
    uboot_elf = uboot.replace('.bin', '')
    if not os.path.exists(self._tools.Filename(uboot_elf)):
      uboot_elf = uboot.replace('.bin', '.elf')
    shutil.copyfile(self._tools.Filename(coreboot), bootstub)
    if seabios:
        self._tools.Run('cbfstool', [bootstub, 'add-payload', seabios,
            'fallback/payload', 'lzma'])
        self._tools.Run('cbfstool', [bootstub, 'add-payload', uboot_elf,
            'img/U-Boot', 'lzma'])
    else:
        self._tools.Run('cbfstool', [bootstub, 'add-payload', uboot_elf,
            'fallback/payload', 'lzma'])
    self._tools.Run('cbfstool', [bootstub, 'add', fdt.fname, 'u-boot.dtb',
        '0xac'])
    return bootstub

  def _UpdateBl2Parameters(self, fdt, spl_load_size, data, pos):
    """Update the parameters in a BL2 blob.

    We look at the list in the parameter block, extract the value of each
    from the device tree, and write that value to the parameter block.

    Args:
      fdt: Device tree containing the parameter values.
      spl_load_size: Size of U-Boot image that SPL must load
      data: The BL2 data.
      pos: The position of the start of the parameter block.

    Returns:
      The new contents of the parameter block, after updating.
    """
    version, size = struct.unpack('<2L', data[pos + 4:pos + 12])
    if version != 1:
      raise CmdError("Cannot update machine parameter block version '%d'" %
          version)
    if size < 0 or pos + size > len(data):
      raise CmdError("Machine parameter block size %d is invalid: "
            "pos=%d, size=%d, space=%d, len=%d" %
            (size, pos, size, len(data) - pos, len(data)))

    # Move past the header and read the parameter list, which is terminated
    # with \0.
    pos += 12
    param_list = struct.unpack('<%ds' % (len(data) - pos), data[pos:])[0]
    param_len = param_list.find('\0')
    param_list = param_list[:param_len]
    pos += (param_len + 4) & ~3

    # Work through the parameters one at a time, adding each value
    new_data = ''
    upto = 0
    for param in param_list:
      value = struct.unpack('<1L', data[pos + upto:pos + upto + 4])[0]
      if param == 'm' :
        mem_type = fdt.GetString('/dmc', 'mem-type')
        mem_types = ['ddr2', 'ddr3', 'lpddr2', 'lpddr3']
        if not mem_type in mem_types:
          raise CmdError("Unknown memory type '%s'" % mem_type)
        value = mem_types.index(mem_type)
        self._out.Info('  Memory type: %s (%d)' % (mem_type, value))
      elif param == 'M' :
        mem_manuf = fdt.GetString('/dmc', 'mem-manuf')
        mem_manufs = ['autodetect', 'elpida', 'samsung']
        if not mem_manuf in mem_manufs:
          raise CmdError("Unknown memory manufacturer: '%s'" % mem_manuf)
        value = mem_manufs.index(mem_manuf)
        self._out.Info('  Memory manufacturer: %s (%d)' % (mem_manuf, value))
      elif param == 'v':
        value = 31
        self._out.Info('  Memory interleave: %#0x' % value)
      elif param == 'u':
        value = (spl_load_size + 0xfff) & ~0xfff
        self._out.Info('  U-Boot size: %#0x (rounded up from %#0x)' %
            (value, spl_load_size))
      elif param == 'b':
        # These values come from enum boot_mode in U-Boot's cpu.h
        if self.spl_source == 'straps':
          value = 32
        elif self.spl_source == 'emmc':
          value = 4
        elif self.spl_source == 'spi':
          value = 20
        elif self.spl_source == 'usb':
          value = 33
        else:
          raise CmdError("Invalid boot source '%s'" % self.spl_source)
        self._out.Info('  Boot source: %#0x' % value)
      else:
        self._out.Warning("Unknown machine parameter type '%s'" % param)
        self._out.Info('  Unknown value: %#0x' % value)
      new_data += struct.pack('<L', value)
      upto += 4

    # Put the data into our block.
    data = data[:pos] + new_data + data[pos + len(new_data):]
    self._out.Info('BL2 configuration complete')
    return data

  def _UpdateChecksum(self, data):
    """Update the BL2 checksum.

    The checksum is a 4 byte sum of all the bytes in the image before the
    last 4 bytes (which hold the checksum).

    Args:
      data: The BL2 data to update.

    Returns:
      The new contents of the BL2 data, after updating the checksum.
    """
    checksum = 0
    for ch in data[:-4]:
      checksum += ord(ch)
    return data[:-4] + struct.pack('<L', checksum & 0xffffffff)

  def ConfigureExynosBl2(self, fdt, spl_load_size, orig_bl2, name=''):
    """Configure an Exynos BL2 binary for our needs.

    We create a new modified BL2 and return its filename.

    Args:
      fdt: Device tree containing the parameter values.
      spl_load_size: Size of U-Boot image that SPL must load
      orig_bl2: Filename of original BL2 file to modify.
    """
    self._out.Info('Configuring BL2')
    bl2 = os.path.join(self._tools.outdir, 'updated-spl%s.bin' % name)
    data = self._tools.ReadFile(orig_bl2)
    self._tools.WriteFile(bl2, data)

    # Locate the parameter block
    data = self._tools.ReadFile(bl2)
    marker = struct.pack('<L', 0xdeadbeef)
    pos = data.rfind(marker)
    if not pos:
      raise CmdError("Could not find machine parameter block in '%s'" %
          orig_bl2)
    data = self._UpdateBl2Parameters(fdt, spl_load_size, data, pos)
    data = self._UpdateChecksum(data)
    self._tools.WriteFile(bl2, data)
    return bl2

  def _PackOutput(self, msg):
    """Helper function to write output from PackFirmware (verbose level 2).

    This is passed to PackFirmware for it to use to write output.

    Args:
      msg: Message to display.
    """
    self._out.Notice(msg)

  def _BuildBlob(self, pack, fdt, blob_type):
    """Build the blob data for a particular blob type.

    Args:
      blob_type: The type of blob to create data for. Supported types are:
          coreboot    A coreboot image (ROM plus U-boot and .dtb payloads).
          signed      Nvidia T20/T30 signed image (BCT, U-Boot, .dtb).
    """
    if blob_type == 'coreboot':
      coreboot = self._CreateCorebootStub(self.uboot_fname,
          self.coreboot_fname, fdt, self.seabios_fname)
      pack.AddProperty('coreboot', coreboot)
      pack.AddProperty('image', coreboot)
    elif blob_type == 'signed':
      bootstub, signed = self._CreateBootStub(self.uboot_fname, fdt,
                                              self.postload_fname)
      pack.AddProperty('bootstub', bootstub)
      pack.AddProperty('signed', signed)
      pack.AddProperty('image', signed)
    elif blob_type == 'exynos-bl1':
      pack.AddProperty(blob_type, self.exynos_bl1)
    elif blob_type == 'ecbin':
      pack.AddProperty(blob_type, self.ecbin_fname)
    elif blob_type == 'exynos-bl2':
      spl_payload = pack.GetBlobParams(blob_type)

      # TODO(sjg@chromium): Remove this later, when we remove boot+dtb
      # from all flash map files.
      if not spl_payload:
        spl_load_size = os.stat(pack.GetProperty('boot+dtb')).st_size
        prop_list = 'boot+dtb'

        # Do this later, when we remove boot+dtb.
        # raise CmdError("No parameters provided for blob type '%s'" %
        #     blob_type)
      else:
        prop_list = spl_payload[0].split(',')
        spl_load_size = len(pack.ConcatPropContents(prop_list)[0])
      self._out.Info("BL2/SPL contains '%s', size is %d / %#x" %
          (', '.join(prop_list), spl_load_size, spl_load_size))
      bl2 = self.ConfigureExynosBl2(fdt, spl_load_size, self.exynos_bl2)
      pack.AddProperty(blob_type, bl2)
    elif pack.GetProperty(blob_type):
      pass
    else:
      raise CmdError("Unknown blob type '%s' required in flash map" %
          blob_type)

  def _CreateImage(self, gbb, fdt):
    """Create a full firmware image, along with various by-products.

    This uses the provided u-boot.bin, fdt and bct to create a firmware
    image containing all the required parts. If the GBB is not supplied
    then this will just return a signed U-Boot as the image.

    Args:
      gbb:      Full path to the GBB file, or empty if a GBB is not required.
      fdt:      Fdt object containing required information.

    Returns:
      Path to image file

    Raises:
      CmdError if a command fails.
    """
    self._out.Notice("Model: %s" % fdt.GetString('/', 'model'))

    # Get the flashmap so we know what to build
    pack = PackFirmware(self._tools, self._out)
    pack.SelectFdt(fdt)

    # Get all our blobs ready
    pack.AddProperty('boot', self.uboot_fname)
    pack.AddProperty('skeleton', self.skeleton_fname)
    pack.AddProperty('dtb', fdt.fname)

    # If we are writing a kernel, add its offset from TEXT_BASE to the fdt.
    if self.kernel_fname:
      fdt.PutInteger('/config', 'kernel-offset', pack.image_size)

    # Make a copy of the fdt for the bootstub
    fdt_data = self._tools.ReadFile(fdt.fname)
    uboot_data = self._tools.ReadFile(self.uboot_fname)
    uboot_copy = os.path.join(self._tools.outdir, 'u-boot.bin')
    self._tools.WriteFile(uboot_copy, uboot_data)

    bootstub = os.path.join(self._tools.outdir, 'u-boot-dtb.bin')
    self._tools.WriteFile(bootstub, uboot_data + fdt_data)

    # TODO(sjg@chromium.org): Deprecate this property when all boards
    # use a comma-separated list for section contents. We will then
    # use only 'boot,dtb' instead of 'boot+dtb'
    pack.AddProperty('boot+dtb', bootstub)

    pack.AddProperty('gbb', self.uboot_fname)
    blob_list = pack.GetBlobList()
    self._out.Info('Building blobs %s\n' % blob_list)
    for blob_type in pack.GetBlobList():
      self._BuildBlob(pack, fdt, blob_type)

    if gbb:
      pack.RequireAllEntries()
      fwid = '.'.join([
          re.sub('[ ,]+', '_', fdt.GetString('/', 'model')),
          self._tools.GetChromeosVersion()])
      self._out.Notice('Firmware ID: %s' % fwid)
      pack.AddProperty('fwid', fwid)
      pack.AddProperty('gbb', gbb)
      pack.AddProperty('keydir', self._keydir)

    pack.CheckProperties()

    # Record position and size of all blob members in the FDT
    pack.UpdateBlobPositions(fdt)

    image = os.path.join(self._tools.outdir, 'image.bin')
    pack.PackImage(self._tools.outdir, image)
    pack.AddProperty('image', image)

    image = pack.GetProperty('image')
    self._tools.OutputSize('Final image', image)
    return image, pack

  def SelectFdt(self, fdt_fname):
    """Select an FDT to control the firmware bundling

    Args:
      fdt_fname: The filename of the fdt to use.

    Returns:
      The Fdt object of the original fdt file, which we will not modify.

    We make a copy of this which will include any on-the-fly changes we want
    to make.
    """
    self._fdt_fname = fdt_fname
    self.CheckOptions()
    fdt = Fdt(self._tools, self._fdt_fname)
    fdt.Compile()
    self.fdt = fdt.Copy(os.path.join(self._tools.outdir, 'updated.dtb'))
    return fdt

  def Start(self, hardware_id, output_fname, show_map):
    """This creates a firmware bundle according to settings provided.

      - Checks options, tools, output directory, fdt.
      - Creates GBB and image.

    Args:
      hardware_id: Hardware ID to use for this board. If None, then the
          default from the Fdt will be used
      output_fname: Output filename for the image. If this is not None, then
          the final image will be copied here.
      show_map: Show a flash map, with each area's name and position

    Returns:
      Filename of the resulting image (not the output_fname copy).
    """
    gbb = ''
    if not self._small:
      gbb = self._CreateGoogleBinaryBlock(hardware_id)

    # This creates the actual image.
    image, pack = self._CreateImage(gbb, self.fdt)
    if show_map:
      pack.ShowMap()
    if output_fname:
      shutil.copyfile(image, output_fname)
      self._out.Notice("Output image '%s'" % output_fname)
    return image, pack.props