CrOS EC (Embedded Controller)

Zephyr CrOS Board Information (CBI) Configuration

Overview

CrOS Board Info CBI is used to store static board information, such as BOARD_VERSION, SKU_ID and configuration information. This information allows a single firmware image to support multiple hardware variants.

Storage Backends

CBI data can be stored in several ways:

  • EC Flash (Recommended): On recent devices, CBI data is stored in a reserved region of the EC's internal flash memory. This approach simplifies the hardware design and reduces cost by eliminating the need for an external EEPROM chip. For detailed information, see the CBI In Flash documentation.
  • External EEPROM (Deprecated): Older devices stored CBI data in a dedicated external EEPROM chip on the I2C bus. While this provides more storage than GPIOs, it adds to the BOM cost and hardware complexity. See the EEPROM documentation for details.
  • GPIO Strapping (Deprecated): The simplest method, used on some older cost-sensitive devices, involves reading GPIO values (strapping resistors) to determine a limited set of information, typically just the BOARD_VERSION and SKU_ID. This method is highly constrained in the amount of data it can store.

Firmware-Relevant Configuration

The CBI contains information that the firmware uses at runtime to adapt its behavior to the specific hardware present on a device. Different mechanisms exist depending on the age of the device platform.

Legacy FW_CONFIG and SSFC

Legacy devices utilize two distinct mechanisms for firmware configuration:

  • Firmware Configuration (FW_CONFIG): A 32-bit field that stores non-probeable characteristics tied to a specific SKU, such as the presence of a backlit keyboard.
  • Second Source Factory Cache (SSFC): A 32-bit field used to handle probeable, second-source components (e.g., different codecs, sensors) that do not affect the device's SKU.

Unified Firmware and Second-source Configuration (UFSC)

Newer devices use a new unified system called Unified Firmware and Second-source Configuration (UFSC). This system replaces the separate FW_CONFIG and SSFC fields with a single, schema-driven 128-bit (4-DWORD) value.

Kconfig Options

Refer to Kconfig.cbi for all the Kconfig options that control CBI behavior. The appropriate configuration system (UFSC, FW_CONFIG, or SSFC) is typically enabled automatically based on the devicetree configuration.

Testing and Debugging

The ectool cbi command can be run from the kernel to get/set FW_CONFIG and SSFC values. The console has a “cbi” command that can be used to do the same thing from the EC console.