Early Chromebook models have a special debug connector on their motherboard to interface with Servo Micro boards. This connector is not fitted on production devices, so a rework is necessary to add it. Newer models support the Closed Case Debugging (CCD) feature, using USB Type-C. It works with off-the-shelf devices without making any modifications, the case doesn’t need to be opened. It allows the same low-level controls as with a Servo Micro.
Interfaces that support CCD include the Type-C Servo v4 and SuzyQ. The control interface that actually has access to the hardware controls is the Cr50 which is reachable using CCD-capable USB Type-C interfaces.
For security reasons, CCD is not enabled in factory defaults. A manual procedure is necessary as described below, ensuring the user purposefully wants to enable it.
Connecting a CCD interface
The first step is to connect the host computer (laptop, NUC dispatcher…) to the Chromebook using a CCD-capable USB Type-C interface. Each type of interface has its own particularities as described below.
Connecting with SuzyQ
The SuzyQ cable contains a controller to enable CCD. It needs to be connected on a particular Type-C port of the device, in a particular orientation. This will vary depending on the device, so if there are 2x Type-C ports there will be 4 possibilities but only one that works.
Run the following command in terminal to watch for new devices:
sudo dmesg -w
Connect the Type-A USB end of the SuzyQ to the host (laptop, NUC
dispatcher…). A SuzyQable device should be detected as shown below:
[386753.698883] usb 1-2: new high-speed USB device number 49 using xhci_hcd
[386753.867487] usb 1-2: New USB device found, idVendor=18d1, idProduct=501f
[386753.867505] usb 1-2: New USB device strings: Mfr=10, Product=11, SerialNumber=12
[386753.867510] usb 1-2: Product: SuzyQable
[386753.867514] usb 1-2: Manufacturer: Google Inc.
[386753.867518] usb 1-2: SerialNumber: 0004002101BC
Then connect the Type-C end of the SuzyQ cable to the Chromebook. A Cr50
device should be detected by the host in the format of the list below:
[387078.927146] usb 1-2.2: new full-speed USB device number 51 using xhci_hcd
[387079.030988] usb 1-2.2: New USB device found, idVendor=18d1, idProduct=5014
[387079.030995] usb 1-2.2: New USB device strings: Mfr=1, Product=2, SerialNumber=11
[387079.031001] usb 1-2.2: Product: Cr50
[387079.031005] usb 1-2.2: Manufacturer: Google Inc.
[387079.031009] usb 1-2.2: SerialNumber: 07809050-93AA4889
If it doesn’t, try flipping the Type-C connector and also try other ports on the Chromebook.
Connect a Type-C USB power adapter to the other port of the Chromebook as the SuzyQ cable doesn’t provide power.
The serial number of the Cr50 device is the one to use in the
/etc/google-servo.conf file; in the example above 07809050-93AA4889.
See the section about running servod
below.
Connecting with Type-C Servo v4
To Do
Developer Mode and servod
Before using CCD, please enable Developer Mode as explained in the
Chromebooks and Servo boards documentation
and configure servod so a service gets started automatically when
connecting the Chromebook. The produre below requires the dut-control
command to work with the Chromebook, at least for the controls that are
active by default. Enabling CCD will unlock more controls.
Enabling Closed Case Debugging
See the CCD documentation from Google for more details.
Open a Cr50 console
Find the Cr50 serial console device by running this command:
dut-control -p XXXX cr50_uart_pty
where nnnn is the port number used in the file: ‘google-servo.conf’.
For example:
# dut-control -p 9932 cr50_uart_pty
cr50_uart_pty:/dev/pts/20
Then run minicom with the device path obtained from the previous command. For example:
minicom -D /dev/pts/20
You are now connected to an interactive console with the Cr50 interface. Run
the ccd command to check the CCD status, it should show “State: Locked” and
“TPM: dev_mode”, as in the example below:
2019-12-16 15:32:44 > ccd
2019-12-16 15:32:45 State: Locked
2019-12-16 15:32:45 Password: none
2019-12-16 15:32:45 Flags: 0x000000
2019-12-16 15:32:45 Capabilities: 0000000000000000
2019-12-16 15:32:45 UartGscRxAPTx Y 0=Default (Always)
2019-12-16 15:32:45 UartGscTxAPRx Y 0=Default (Always)
2019-12-16 15:32:45 UartGscRxECTx Y 0=Default (Always)
2019-12-16 15:32:45 UartGscTxECRx - 0=Default (IfOpened)
2019-12-16 15:32:45 FlashAP - 0=Default (IfOpened)
2019-12-16 15:32:45 FlashEC - 0=Default (IfOpened)
2019-12-16 15:32:45 OverrideWP - 0=Default (IfOpened)
2019-12-16 15:32:45 RebootECAP - 0=Default (IfOpened)
2019-12-16 15:32:45 GscFullConsole - 0=Default (IfOpened)
2019-12-16 15:32:45 UnlockNoReboot Y 0=Default (Always)
2019-12-16 15:32:45 UnlockNoShortPP Y 0=Default (Always)
2019-12-16 15:32:45 OpenNoTPMWipe - 0=Default (IfOpened)
2019-12-16 15:32:45 OpenNoLongPP - 0=Default (IfOpened)
2019-12-16 15:32:45 BatteryBypassPP Y 0=Default (Always)
2019-12-16 15:32:45 UpdateNoTPMWipe Y 0=Default (Always)
2019-12-16 15:32:45 I2C - 0=Default (IfOpened)
2019-12-16 15:32:45 FlashRead Y 0=Default (Always)
2019-12-16 15:32:45 OpenNoDevMode - 0=Default (IfOpened)
2019-12-16 15:32:45 OpenFromUSB - 0=Default (IfOpened)
2019-12-16 15:32:45 read_tpm_nvmem: object at 0x100a not found
2019-12-16 15:32:45 [431.145965 Console unlock allowed]
2019-12-16 15:32:45 TPM: dev_mode
2019-12-16 15:32:45 Capabilities are default.
2019-12-16 15:32:45 Use 'ccd help' to print subcommands
If it shows State: Opened then CCD has already been enabled on this
device in which case the step below can be skipped.
Start the CCD open process in the device terminal
In a Chromebook terminal, run the following command:
gsctool -a -o
Press the power button on the device every time it asks for it, keep pressing the power button as required. It will be waiting for a while before asking again a few times, this can take about 5 minutes to complete. It’s apparently a security measure to ensure a real person is interacting with the device.
The Chromebook will reboot automatically at the end of the procedure.
Then run ccd again in the Cr50 shell, it should show “State: Opened” this
time:
2019-12-16 15:43:59 State: Opened
2019-12-16 15:43:59 Password: none
2019-12-16 15:43:59 Flags: 0x000000
2019-12-16 15:43:59 Capabilities: 0000000000000000
2019-12-16 15:43:59 UartGscRxAPTx Y 0=Default (Always)
2019-12-16 15:43:59 UartGscTxAPRx Y 0=Default (Always)
2019-12-16 15:43:59 UartGscRxECTx Y 0=Default (Always)
2019-12-16 15:43:59 UartGscTxECRx Y 0=Default (IfOpened)
2019-12-16 15:43:59 FlashAP Y 0=Default (IfOpened)
2019-12-16 15:43:59 FlashEC Y 0=Default (IfOpened)
2019-12-16 15:43:59 OverrideWP Y 0=Default (IfOpened)
2019-12-16 15:43:59 RebootECAP Y 0=Default (IfOpened)
2019-12-16 15:43:59 GscFullConsole Y 0=Default (IfOpened)
2019-12-16 15:43:59 UnlockNoReboot Y 0=Default (Always)
2019-12-16 15:43:59 UnlockNoShortPP Y 0=Default (Always)
2019-12-16 15:43:59 OpenNoTPMWipe Y 0=Default (IfOpened)
2019-12-16 15:43:59 OpenNoLongPP Y 0=Default (IfOpened)
2019-12-16 15:43:59 BatteryBypassPP Y 0=Default (Always)
2019-12-16 15:43:59 UpdateNoTPMWipe Y 0=Default (Always)
2019-12-16 15:43:59 I2C Y 0=Default (IfOpened)
2019-12-16 15:43:59 FlashRead Y 0=Default (Always)
2019-12-16 15:43:59 OpenNoDevMode Y 0=Default (IfOpened)
2019-12-16 15:43:59 OpenFromUSB Y 0=Default (IfOpened)
2019-12-16 15:43:59 read_tpm_nvmem: object at 0x100a not found
2019-12-16 15:43:59 [1107.234474 Console unlock allowed]
2019-12-16 15:43:59 TPM:
2019-12-16 15:43:59 Capabilities are default.
2019-12-16 15:43:59 Use 'ccd help' to print subcommands
Enable these options in the Cr50 shell:
> ccd set OpenFromUSB Always
> ccd set OpenNoDevMode Always
Enable the “testlab” mode:
> ccd testlab enable
Press the power button several times as required.
Now CCD can be opened with this command:
> ccd testlab open
Disable write-protection:
> wp disable
If this worked as expected, congratulations! The CCD setup is now complete for the device.
However, if at any point the Chromebook device becomes unresponsive and needs the operating system to be recovered, follow the Recovery procedure and start again.