server/cros/servo/servo.py

# Copyright (c) 2012 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.
#
# Expects to be run in an environment with sudo and no interactive password
# prompt, such as within the Chromium OS development chroot.

import ast
import logging
import os
import re
import time
import xmlrpclib

from autotest_lib.client.common_lib import error
from autotest_lib.server import utils as server_utils
from autotest_lib.server.cros.servo import firmware_programmer

# Time to wait when probing for a usb device, it takes on avg 17 seconds
# to do a full probe.
_USB_PROBE_TIMEOUT = 40


def _extract_image_from_tarball(tarball, dest_dir, image_candidates):
    """Try extracting the image_candidates from the tarball.

    @param tarball: The path of the tarball.
    @param dest_path: The path of the destination.
    @param image_candidates: A tuple of the paths of image candidates.

    @return: The first path from the image candidates, which succeeds, or None
             if all the image candidates fail.
    """
    for image in image_candidates:
        status = server_utils.system(
                ('tar xf %s -C %s %s' % (tarball, dest_dir, image)),
                timeout=60, ignore_status=True)
        if status == 0:
            return image
    return None


class _PowerStateController(object):

    """Class to provide board-specific power operations.

    This class is responsible for "power on" and "power off"
    operations that can operate without making assumptions in
    advance about board state.  It offers an interface that
    abstracts out the different sequences required for different
    board types.

    """

    # Constants acceptable to be passed for the `rec_mode` parameter
    # to power_on().
    #
    # REC_ON:  Boot the DUT in recovery mode, i.e. boot from USB or
    #   SD card.
    # REC_OFF:  Boot in normal mode, i.e. boot from internal storage.

    REC_ON = 'rec'
    REC_OFF = 'on'
    REC_ON_FORCE_MRC = 'rec_force_mrc'

    # Delay in seconds needed between asserting and de-asserting
    # warm reset.
    _RESET_HOLD_TIME = 0.5

    def __init__(self, servo):
        """Initialize the power state control.

        @param servo Servo object providing the underlying `set` and `get`
                     methods for the target controls.

        """
        self._servo = servo

    def reset(self):
        """Force the DUT to reset.

        The DUT is guaranteed to be on at the end of this call,
        regardless of its previous state, provided that there is
        working OS software. This also guarantees that the EC has
        been restarted.

        """
        self._servo.set_nocheck('power_state', 'reset')

    def warm_reset(self):
        """Apply warm reset to the DUT.

        This asserts, then de-asserts the 'warm_reset' signal.
        Generally, this causes the board to restart.

        """
        self._servo.set_get_all(['warm_reset:on',
                                 'sleep:%.4f' % self._RESET_HOLD_TIME,
                                 'warm_reset:off'])

    def power_off(self):
        """Force the DUT to power off.

        The DUT is guaranteed to be off at the end of this call,
        regardless of its previous state, provided that there is
        working EC and boot firmware.  There is no requirement for
        working OS software.

        """
        self._servo.set_nocheck('power_state', 'off')

    def power_on(self, rec_mode=REC_OFF):
        """Force the DUT to power on.

        Prior to calling this function, the DUT must be powered off,
        e.g. with a call to `power_off()`.

        At power on, recovery mode is set as specified by the
        corresponding argument.  When booting with recovery mode on, it
        is the caller's responsibility to unplug/plug in a bootable
        external storage device.

        If the DUT requires a delay after powering on but before
        processing inputs such as USB stick insertion, the delay is
        handled by this method; the caller is not responsible for such
        delays.

        @param rec_mode Setting of recovery mode to be applied at
                        power on. default: REC_OFF aka 'off'

        """
        self._servo.set_nocheck('power_state', rec_mode)


class _Uart(object):
    """Class to capture UART streams of CPU, EC, Cr50, etc."""
    _UartToCapture = ('cpu', 'ec', 'cr50', 'servo_v4', 'servo_micro', 'usbpd')

    def __init__(self, servo):
        self._servo = servo
        self._streams = []
        self.logs_dir = None

    def _start_stop_capture(self, uart, start):
        """Helper function to start/stop capturing on specified UART.

        @param uart:  The UART name to start/stop capturing.
        @param start:  True to start capturing, otherwise stop.

        @returns True if the operation completes successfully. False if the UART
                 capturing is not supported or failed due to an error.
        """
        logging.debug('%s capturing %s UART.', 'Start' if start else 'Stop',
                      uart)
        try:
            self._servo.set('%s_uart_capture' % uart,
                            'on' if start else 'off')
        except error.TestFail as err:
            if 'No control named' in str(err):
                logging.debug("The servod doesn't support %s_uart_capture.",
                              uart)
            else:
                logging.debug("Can't caputre UART for %s: %s", uart, err)
            return False

        return True

    def start_capture(self):
        """Start capturing UART streams."""
        for uart in self._UartToCapture:
            if self._start_stop_capture(uart, True):
                self._streams.append(('%s_uart_stream' % uart, '%s_uart.log' %
                                      uart))

    def dump(self):
        """Dump UART streams to log files accordingly."""
        if not self.logs_dir:
            return

        for stream, logfile in self._streams:
            logfile_fullname = os.path.join(self.logs_dir, logfile)
            try:
                content = self._servo.get(stream)
            except Exception as err:
                logging.warn('Failed to get UART log for %s: %s', stream, err)
                continue

            # The UART stream may contain non-printable characters, and servo
            # returns it in string representation. We use `ast.leteral_eval`
            # to revert it back.
            with open(logfile_fullname, 'a') as fd:
                fd.write(ast.literal_eval(content))

    def stop_capture(self):
        """Stop capturing UART streams."""
        for uart in self._UartToCapture:
            try:
                self._start_stop_capture(uart, False)
            except Exception as err:
                logging.warn('Failed to stop UART logging for %s: %s', uart,
                             err)


class Servo(object):

    """Manages control of a Servo board.

    Servo is a board developed by hardware group to aide in the debug and
    control of various partner devices. Servo's features include the simulation
    of pressing the power button, closing the lid, and pressing Ctrl-d. This
    class manages setting up and communicating with a servo demon (servod)
    process. It provides both high-level functions for common servo tasks and
    low-level functions for directly setting and reading gpios.

    """

    # Power button press delays in seconds.
    #
    # The EC specification says that 8.0 seconds should be enough
    # for the long power press.  However, some platforms need a bit
    # more time.  Empirical testing has found these requirements:
    #   Alex: 8.2 seconds
    #   ZGB:  8.5 seconds
    # The actual value is set to the largest known necessary value.
    #
    # TODO(jrbarnette) Being generous is the right thing to do for
    # existing platforms, but if this code is to be used for
    # qualification of new hardware, we should be less generous.
    SHORT_DELAY = 0.1

    # Maximum number of times to re-read power button on release.
    GET_RETRY_MAX = 10

    # Delays to deal with DUT state transitions.
    SLEEP_DELAY = 6
    BOOT_DELAY = 10

    # Default minimum time interval between 'press' and 'release'
    # keyboard events.
    SERVO_KEY_PRESS_DELAY = 0.1

    # Time to toggle recovery switch on and off.
    REC_TOGGLE_DELAY = 0.1

    # Time to toggle development switch on and off.
    DEV_TOGGLE_DELAY = 0.1

    # Time between an usb disk plugged-in and detected in the system.
    USB_DETECTION_DELAY = 10
    # Time to keep USB power off before and after USB mux direction is changed
    USB_POWEROFF_DELAY = 2

    # Time to wait before timing out on servo initialization.
    INIT_TIMEOUT_SECS = 10


    def __init__(self, servo_host, servo_serial=None):
        """Sets up the servo communication infrastructure.

        @param servo_host: A ServoHost object representing
                           the host running servod.
        @param servo_serial: Serial number of the servo board.
        """
        # TODO(fdeng): crbug.com/298379
        # We should move servo_host object out of servo object
        # to minimize the dependencies on the rest of Autotest.
        self._servo_host = servo_host
        self._servo_serial = servo_serial
        self._server = servo_host.get_servod_server_proxy()
        self._power_state = _PowerStateController(self)
        self._uart = _Uart(self)
        self._usb_state = None
        self._programmer = None


    @property
    def servo_serial(self):
        """Returns the serial number of the servo board."""
        return self._servo_serial


    def get_power_state_controller(self):
        """Return the power state controller for this Servo.

        The power state controller provides board-independent
        interfaces for reset, power-on, power-off operations.

        """
        return self._power_state


    def initialize_dut(self, cold_reset=False):
        """Initializes a dut for testing purposes.

        This sets various servo signals back to default values
        appropriate for the target board.  By default, if the DUT
        is already on, it stays on.  If the DUT is powered off
        before initialization, its state afterward is unspecified.

        Rationale:  Basic initialization of servo sets the lid open,
        when there is a lid.  This operation won't affect powered on
        units; however, setting the lid open may power on a unit
        that's off, depending on the board type and previous state
        of the device.

        If `cold_reset` is a true value, the DUT and its EC will be
        reset, and the DUT rebooted in normal mode.

        @param cold_reset If True, cold reset the device after
                          initialization.
        """
        self._server.hwinit()
        self.set('usb_mux_oe1', 'on')
        self._usb_state = None
        self.switch_usbkey('off')
        self._uart.start_capture()
        if cold_reset:
            self._power_state.reset()
        logging.debug('Servo initialized, version is %s',
                      self._server.get_version())


    def is_localhost(self):
        """Is the servod hosted locally?

        Returns:
          True if local hosted; otherwise, False.
        """
        return self._servo_host.is_localhost()


    def power_long_press(self):
        """Simulate a long power button press."""
        # After a long power press, the EC may ignore the next power
        # button press (at least on Alex).  To guarantee that this
        # won't happen, we need to allow the EC one second to
        # collect itself.
        self._server.power_long_press()


    def power_normal_press(self):
        """Simulate a normal power button press."""
        self._server.power_normal_press()


    def power_short_press(self):
        """Simulate a short power button press."""
        self._server.power_short_press()


    def power_key(self, press_secs=''):
        """Simulate a power button press.

        @param press_secs : Str. Time to press key.
        """
        self._server.power_key(press_secs)


    def lid_open(self):
        """Simulate opening the lid and raise exception if all attempts fail"""
        self.set('lid_open', 'yes')


    def lid_close(self):
        """Simulate closing the lid and raise exception if all attempts fail

        Waits 6 seconds to ensure the device is fully asleep before returning.
        """
        self.set('lid_open', 'no')
        time.sleep(Servo.SLEEP_DELAY)

    def volume_up(self, timeout=300):
        """Simulate pushing the volume down button.

        @param timeout: Timeout for setting the volume.
        """
        self.set_get_all(['volume_up:yes',
                          'sleep:%.4f' % self.SERVO_KEY_PRESS_DELAY,
                          'volume_up:no'])
        # we need to wait for commands to take effect before moving on
        time_left = float(timeout)
        while time_left > 0.0:
            value = self.get('volume_up')
            if value == 'no':
                return
            time.sleep(self.SHORT_DELAY)
            time_left = time_left - self.SHORT_DELAY
        raise error.TestFail("Failed setting volume_up to no")

    def volume_down(self, timeout=300):
        """Simulate pushing the volume down button.

        @param timeout: Timeout for setting the volume.
        """
        self.set_get_all(['volume_down:yes',
                          'sleep:%.4f' % self.SERVO_KEY_PRESS_DELAY,
                          'volume_down:no'])
        # we need to wait for commands to take effect before moving on
        time_left = float(timeout)
        while time_left > 0.0:
            value = self.get('volume_down')
            if value == 'no':
                return
            time.sleep(self.SHORT_DELAY)
            time_left = time_left - self.SHORT_DELAY
        raise error.TestFail("Failed setting volume_down to no")

    def ctrl_d(self, press_secs=''):
        """Simulate Ctrl-d simultaneous button presses.

        @param press_secs : Str. Time to press key.
        """
        self._server.ctrl_d(press_secs)


    def ctrl_u(self):
        """Simulate Ctrl-u simultaneous button presses.

        @param press_secs : Str. Time to press key.
        """
        self._server.ctrl_u()


    def ctrl_enter(self, press_secs=''):
        """Simulate Ctrl-enter simultaneous button presses.

        @param press_secs : Str. Time to press key.
        """
        self._server.ctrl_enter(press_secs)


    def d_key(self, press_secs=''):
        """Simulate Enter key button press.

        @param press_secs : Str. Time to press key.
        """
        self._server.d_key(press_secs)


    def ctrl_key(self, press_secs=''):
        """Simulate Enter key button press.

        @param press_secs : Str. Time to press key.
        """
        self._server.ctrl_key(press_secs)


    def enter_key(self, press_secs=''):
        """Simulate Enter key button press.

        @param press_secs : Str. Time to press key.
        """
        self._server.enter_key(press_secs)


    def refresh_key(self, press_secs=''):
        """Simulate Refresh key (F3) button press.

        @param press_secs : Str. Time to press key.
        """
        self._server.refresh_key(press_secs)


    def ctrl_refresh_key(self, press_secs=''):
        """Simulate Ctrl and Refresh (F3) simultaneous press.

        This key combination is an alternative of Space key.

        @param press_secs : Str. Time to press key.
        """
        self._server.ctrl_refresh_key(press_secs)


    def imaginary_key(self, press_secs=''):
        """Simulate imaginary key button press.

        Maps to a key that doesn't physically exist.

        @param press_secs : Str. Time to press key.
        """
        self._server.imaginary_key(press_secs)


    def sysrq_x(self, press_secs=''):
        """Simulate Alt VolumeUp X simulataneous press.

        This key combination is the kernel system request (sysrq) X.

        @param press_secs : Str. Time to press key.
        """
        self._server.sysrq_x(press_secs)


    def toggle_recovery_switch(self):
        """Toggle recovery switch on and off."""
        self.enable_recovery_mode()
        time.sleep(self.REC_TOGGLE_DELAY)
        self.disable_recovery_mode()


    def enable_recovery_mode(self):
        """Enable recovery mode on device."""
        self.set('rec_mode', 'on')


    def disable_recovery_mode(self):
        """Disable recovery mode on device."""
        self.set('rec_mode', 'off')


    def toggle_development_switch(self):
        """Toggle development switch on and off."""
        self.enable_development_mode()
        time.sleep(self.DEV_TOGGLE_DELAY)
        self.disable_development_mode()


    def enable_development_mode(self):
        """Enable development mode on device."""
        self.set('dev_mode', 'on')


    def disable_development_mode(self):
        """Disable development mode on device."""
        self.set('dev_mode', 'off')

    def boot_devmode(self):
        """Boot a dev-mode device that is powered off."""
        self.power_short_press()
        self.pass_devmode()


    def pass_devmode(self):
        """Pass through boot screens in dev-mode."""
        time.sleep(Servo.BOOT_DELAY)
        self.ctrl_d()
        time.sleep(Servo.BOOT_DELAY)


    def get_board(self):
        """Get the board connected to servod."""
        return self._server.get_board()


    def get_base_board(self):
        """Get the board of the base connected to servod."""
        try:
            return self._server.get_base_board()
        except  xmlrpclib.Fault as e:
            # TODO(waihong): Remove the following compatibility check when
            # the new versions of hdctools are deployed.
            if 'not supported' in str(e):
                logging.warning('The servod is too old that get_base_board '
                        'not supported.')
                return ''
            raise


    def get_ec_active_copy(self):
        """Get the active copy of the EC image."""
        return self.get('ec_active_copy')


    def _get_xmlrpclib_exception(self, xmlexc):
        """Get meaningful exception string from xmlrpc.

        Args:
            xmlexc: xmlrpclib.Fault object

        xmlrpclib.Fault.faultString has the following format:

        <type 'exception type'>:'actual error message'

        Parse and return the real exception from the servod side instead of the
        less meaningful one like,
           <Fault 1: "<type 'exceptions.AttributeError'>:'tca6416' object has no
           attribute 'hw_driver'">

        Returns:
            string of underlying exception raised in servod.
        """
        return re.sub('^.*>:', '', xmlexc.faultString)


    def get(self, gpio_name):
        """Get the value of a gpio from Servod.

        @param gpio_name Name of the gpio.
        """
        assert gpio_name
        try:
            return self._server.get(gpio_name)
        except  xmlrpclib.Fault as e:
            err_msg = "Getting '%s' :: %s" % \
                (gpio_name, self._get_xmlrpclib_exception(e))
            raise error.TestFail(err_msg)


    def set(self, gpio_name, gpio_value):
        """Set and check the value of a gpio using Servod.

        @param gpio_name Name of the gpio.
        @param gpio_value New setting for the gpio.
        """
        self.set_nocheck(gpio_name, gpio_value)
        retry_count = Servo.GET_RETRY_MAX
        while gpio_value != self.get(gpio_name) and retry_count:
            logging.warning("%s != %s, retry %d", gpio_name, gpio_value,
                         retry_count)
            retry_count -= 1
            time.sleep(Servo.SHORT_DELAY)
        if not retry_count:
            assert gpio_value == self.get(gpio_name), \
                'Servo failed to set %s to %s' % (gpio_name, gpio_value)


    def set_nocheck(self, gpio_name, gpio_value):
        """Set the value of a gpio using Servod.

        @param gpio_name Name of the gpio.
        @param gpio_value New setting for the gpio.
        """
        assert gpio_name and gpio_value
        logging.info('Setting %s to %r', gpio_name, gpio_value)
        try:
            self._server.set(gpio_name, gpio_value)
        except  xmlrpclib.Fault as e:
            err_msg = "Setting '%s' to %r :: %s" % \
                (gpio_name, gpio_value, self._get_xmlrpclib_exception(e))
            raise error.TestFail(err_msg)


    def set_get_all(self, controls):
        """Set &| get one or more control values.

        @param controls: list of strings, controls to set &| get.

        @raise: error.TestError in case error occurs setting/getting values.
        """
        rv = []
        try:
            logging.info('Set/get all: %s', str(controls))
            rv = self._server.set_get_all(controls)
        except xmlrpclib.Fault as e:
            # TODO(waihong): Remove the following backward compatibility when
            # the new versions of hdctools are deployed.
            if 'not supported' in str(e):
                logging.warning('The servod is too old that set_get_all '
                        'not supported. Use set and get instead.')
                for control in controls:
                    if ':' in control:
                        (name, value) = control.split(':')
                        if name == 'sleep':
                            time.sleep(float(value))
                        else:
                            self.set_nocheck(name, value)
                        rv.append(True)
                    else:
                        rv.append(self.get(name))
            else:
                err_msg = "Problem with '%s' :: %s" % \
                    (controls, self._get_xmlrpclib_exception(e))
                raise error.TestFail(err_msg)
        return rv


    # TODO(waihong) It may fail if multiple servo's are connected to the same
    # host. Should look for a better way, like the USB serial name, to identify
    # the USB device.
    # TODO(sbasi) Remove this code from autoserv once firmware tests have been
    # updated.
    def probe_host_usb_dev(self, timeout=_USB_PROBE_TIMEOUT):
        """Probe the USB disk device plugged-in the servo from the host side.

        It uses servod to discover if there is a usb device attached to it.

        @param timeout The timeout period when probing for the usb host device.

        @return: String of USB disk path (e.g. '/dev/sdb') or None.
        """
        # Set up Servo's usb mux.
        self.switch_usbkey('host')
        return self._server.probe_host_usb_dev(timeout) or None


    def image_to_servo_usb(self, image_path=None,
                           make_image_noninteractive=False):
        """Install an image to the USB key plugged into the servo.

        This method may copy any image to the servo USB key including a
        recovery image or a test image.  These images are frequently used
        for test purposes such as restoring a corrupted image or conducting
        an upgrade of ec/fw/kernel as part of a test of a specific image part.

        @param image_path Path on the host to the recovery image.
        @param make_image_noninteractive Make the recovery image
                                   noninteractive, therefore the DUT
                                   will reboot automatically after
                                   installation.
        """
        # We're about to start plugging/unplugging the USB key.  We
        # don't know the state of the DUT, or what it might choose
        # to do to the device after hotplug.  To avoid surprises,
        # force the DUT to be off.
        self._server.hwinit()
        self._power_state.power_off()

        # Set up Servo's usb mux.
        self.switch_usbkey('host')
        if image_path:
            logging.info('Searching for usb device and copying image to it. '
                         'Please wait a few minutes...')
            if not self._server.download_image_to_usb(image_path):
                logging.error('Failed to transfer requested image to USB. '
                              'Please take a look at Servo Logs.')
                raise error.AutotestError('Download image to usb failed.')
            if make_image_noninteractive:
                logging.info('Making image noninteractive')
                if not self._server.make_image_noninteractive():
                    logging.error('Failed to make image noninteractive. '
                                  'Please take a look at Servo Logs.')

    def boot_in_recovery_mode(self):
        """Boot host DUT in recovery mode."""
        self._power_state.power_on(rec_mode=self._power_state.REC_ON)
        self.switch_usbkey('dut')


    def install_recovery_image(self, image_path=None,
                               make_image_noninteractive=False):
        """Install the recovery image specified by the path onto the DUT.

        This method uses google recovery mode to install a recovery image
        onto a DUT through the use of a USB stick that is mounted on a servo
        board specified by the usb_dev.  If no image path is specified
        we use the recovery image already on the usb image.

        @param image_path: Path on the host to the recovery image.
        @param make_image_noninteractive: Make the recovery image
                noninteractive, therefore the DUT will reboot automatically
                after installation.
        """
        self.image_to_servo_usb(image_path, make_image_noninteractive)
        self.boot_in_recovery_mode()


    def _scp_image(self, image_path):
        """Copy image to the servo host.

        When programming a firmware image on the DUT, the image must be
        located on the host to which the servo device is connected. Sometimes
        servo is controlled by a remote host, in this case the image needs to
        be transferred to the remote host.

        @param image_path: a string, name of the firmware image file to be
               transferred.
        @return: a string, full path name of the copied file on the remote.
        """

        dest_path = os.path.join('/tmp', os.path.basename(image_path))
        self._servo_host.send_file(image_path, dest_path)
        return dest_path


    def system(self, command, timeout=3600):
        """Execute the passed in command on the servod host.

        @param command Command to be executed.
        @param timeout Maximum number of seconds of runtime allowed. Default to
                       1 hour.
        """
        logging.info('Will execute on servo host: %s', command)
        self._servo_host.run(command, timeout=timeout)


    def system_output(self, command, timeout=3600,
                      ignore_status=False, args=()):
        """Execute the passed in command on the servod host, return stdout.

        @param command a string, the command to execute
        @param timeout an int, max number of seconds to wait til command
               execution completes. Default to 1 hour.
        @param ignore_status a Boolean, if true - ignore command's nonzero exit
               status, otherwise an exception will be thrown
        @param args a tuple of strings, each becoming a separate command line
               parameter for the command
        @return command's stdout as a string.
        """
        return self._servo_host.run(command, timeout=timeout,
                                    ignore_status=ignore_status,
                                    args=args).stdout.strip()


    def get_servo_version(self):
        """Get the version of the servo, e.g., servo_v2 or servo_v3.

        @return: The version of the servo.

        """
        return self._server.get_version()


    def _initialize_programmer(self, rw_only=False):
        """Initialize the firmware programmer.

        @param rw_only: True to initialize a programmer which only
                        programs the RW portions.
        """
        if self._programmer:
            return
        # Initialize firmware programmer
        servo_version = self.get_servo_version()
        if servo_version.startswith('servo_v2'):
            self._programmer = firmware_programmer.ProgrammerV2(self)
            self._programmer_rw = firmware_programmer.ProgrammerV2RwOnly(self)
        # Both servo v3 and v4 use the same programming methods so just leverage
        # ProgrammerV3 for servo v4 as well.
        elif (servo_version.startswith('servo_v3') or
              servo_version.startswith('servo_v4')):
            self._programmer = firmware_programmer.ProgrammerV3(self)
            self._programmer_rw = firmware_programmer.ProgrammerV3RwOnly(self)
        else:
            raise error.TestError(
                    'No firmware programmer for servo version: %s' %
                    servo_version)


    def program_bios(self, image, rw_only=False):
        """Program bios on DUT with given image.

        @param image: a string, file name of the BIOS image to program
                      on the DUT.
        @param rw_only: True to only program the RW portion of BIOS.

        """
        self._initialize_programmer()
        if not self.is_localhost():
            image = self._scp_image(image)
        if rw_only:
            self._programmer_rw.program_bios(image)
        else:
            self._programmer.program_bios(image)


    def program_ec(self, image, rw_only=False):
        """Program ec on DUT with given image.

        @param image: a string, file name of the EC image to program
                      on the DUT.
        @param rw_only: True to only program the RW portion of EC.

        """
        self._initialize_programmer()
        if not self.is_localhost():
            image = self._scp_image(image)
        if rw_only:
            self._programmer_rw.program_ec(image)
        else:
            self._programmer.program_ec(image)


    def _reprogram(self, tarball_path, firmware_name, image_candidates,
                   rw_only):
        """Helper function to reprogram firmware for EC or BIOS.

        @param tarball_path: The path of the downloaded build tarball.
        @param: firmware_name: either 'EC' or 'BIOS'.
        @param image_candidates: A tuple of the paths of image candidates.
        @param rw_only: True to only install firmware to its RW portions. Keep
                the RO portions unchanged.

        @raise: TestError if cannot extract firmware from the tarball.
        """
        dest_dir = os.path.dirname(tarball_path)
        image = _extract_image_from_tarball(tarball_path, dest_dir,
                                            image_candidates)
        if not image:
            if firmware_name == 'EC':
                logging.info('Not a Chrome EC, ignore re-programming it')
                return
            else:
                raise error.TestError('Failed to extract the %s image from '
                                      'tarball' % firmware_name)

        logging.info('Will re-program %s %snow', firmware_name,
                     'RW ' if rw_only else '')

        if firmware_name == 'EC':
            self.program_ec(os.path.join(dest_dir, image), rw_only)
        else:
            self.program_bios(os.path.join(dest_dir, image), rw_only)


    def program_firmware(self, model, tarball_path, rw_only=False):
        """Program firmware (EC, if applied, and BIOS) of the DUT.

        @param model: The DUT model name.
        @param tarball_path: The path of the downloaded build tarball.
        @param rw_only: True to only install firmware to its RW portions. Keep
                the RO portions unchanged.
        """
        ap_image_candidates = ('image.bin', 'image-%s.bin' % model)
        ec_image_candidates = ('ec.bin', '%s/ec.bin' % model)

        self._reprogram(tarball_path, 'EC', ec_image_candidates, rw_only)
        self._reprogram(tarball_path, 'BIOS', ap_image_candidates, rw_only)

        self.get_power_state_controller().reset()
        time.sleep(Servo.BOOT_DELAY)


    def _switch_usbkey_power(self, power_state, detection_delay=False):
        """Switch usbkey power.

        This function switches usbkey power by setting the value of
        'prtctl4_pwren'. If the power is already in the
        requested state, this function simply returns.

        @param power_state: A string, 'on' or 'off'.
        @param detection_delay: A boolean value, if True, sleep
                                for |USB_DETECTION_DELAY| after switching
                                the power on.
        """
        # TODO(kevcheng): Forgive me for this terrible hack. This is just to
        # handle beaglebones that haven't yet updated and have the
        # safe_switch_usbkey_power RPC.  I'll remove this once all beaglebones
        # have been updated and also think about a better way to handle
        # situations like this.
        try:
            self._server.safe_switch_usbkey_power(power_state)
        except Exception:
            self.set('prtctl4_pwren', power_state)
        if power_state == 'off':
            time.sleep(self.USB_POWEROFF_DELAY)
        elif detection_delay:
            time.sleep(self.USB_DETECTION_DELAY)


    def switch_usbkey(self, usb_state):
        """Connect USB flash stick to either host or DUT, or turn USB port off.

        This function switches the servo multiplexer to provide electrical
        connection between the USB port J3 and either host or DUT side. It
        can also be used to turn the USB port off.

        Switching to 'dut' or 'host' is accompanied by powercycling
        of the USB stick, because it sometimes gets wedged if the mux
        is switched while the stick power is on.

        @param usb_state: A string, one of 'dut', 'host', or 'off'.
                          'dut' and 'host' indicate which side the
                          USB flash device is required to be connected to.
                          'off' indicates turning the USB port off.

        @raise: error.TestError in case the parameter is not 'dut'
                'host', or 'off'.
        """
        if self.get_usbkey_direction() == usb_state:
            return

        if usb_state == 'off':
            self._switch_usbkey_power('off')
            self._usb_state = usb_state
            return
        elif usb_state == 'host':
            mux_direction = 'servo_sees_usbkey'
        elif usb_state == 'dut':
            mux_direction = 'dut_sees_usbkey'
        else:
            raise error.TestError('Unknown USB state request: %s' % usb_state)

        self._switch_usbkey_power('off')
        # TODO(kevcheng): Forgive me for this terrible hack. This is just to
        # handle beaglebones that haven't yet updated and have the
        # safe_switch_usbkey RPC.  I'll remove this once all beaglebones have
        # been updated and also think about a better way to handle situations
        # like this.
        try:
            self._server.safe_switch_usbkey(mux_direction)
        except Exception:
            self.set('usb_mux_sel1', mux_direction)
        time.sleep(self.USB_POWEROFF_DELAY)
        self._switch_usbkey_power('on', usb_state == 'host')
        self._usb_state = usb_state


    def get_usbkey_direction(self):
        """Get which side USB is connected to or 'off' if usb power is off.

        @return: A string, one of 'dut', 'host', or 'off'.
        """
        if not self._usb_state:
            if self.get('prtctl4_pwren') == 'off':
                self._usb_state = 'off'
            elif self.get('usb_mux_sel1').startswith('dut'):
                self._usb_state = 'dut'
            else:
                self._usb_state = 'host'
        return self._usb_state


    def set_servo_v4_role(self, role):
        """Set the power role of servo v4, either 'src' or 'snk'.

        It does nothing if not a servo v4.

        @param role: Power role for DUT port on servo v4, either 'src' or 'snk'.
        """
        servo_version = self.get_servo_version()
        if servo_version.startswith('servo_v4'):
            value = self.get('servo_v4_role')
            if value != role:
                self.set_nocheck('servo_v4_role', role)
            else:
                logging.debug('Already in the role: %s.', role)
        else:
            logging.debug('Not a servo v4, unable to set role to %s.', role)


    @property
    def uart_logs_dir(self):
        """Return the directory to save UART logs."""
        return self._uart.logs_dir if self._uart else ""


    @uart_logs_dir.setter
    def uart_logs_dir(self, logs_dir):
        """Set directory to save UART logs.

        @param logs_dir  String of directory name."""
        if self._uart:
            self._uart.logs_dir = logs_dir


    def close(self):
        """Close the servo object."""
        if self._uart:
            self._uart.stop_capture()
            self._uart.dump()
            self._uart = None