coreboot Console

coreboot supports multiple ways to access its console.

SMBus Console

SMBus is a two-wire interface which is based on the principles of operation of I2C. SMBus, was first was designed to allow a battery to communicate with the charger, the system host, and/or other power-related components in the system.

Enable the SMBus console with CONSOLE_I2C_SMBUS Kconfig. Set CONSOLE_I2C_SMBUS_SLAVE_ADDRESS and CONSOLE_I2C_SMBUS_SLAVE_DATA_REGISTER configuration values of the slave I2C device which you will use to capture I2C packets.

Modern computer Random Access Memory (RAM) slot has SMBus in it according to the JEDEC standards. We can use a breakout-board to expose those SMBus pins. Some mainboard have SMBus pins in the PCIe slot as well.

This feature has been tested on the following platforms:

A minimal DDR3 DIMM breakout board PCB design with only the SDA(Data line) and SCL(Clock line) pins of I2C/SMBus can be found here. See the PCB layout here

NOTE: To capture the I2C packets, an I2C slave device is required. The easiest way to capture the log message is to use a I2C to UART converter chip with a UART to USB converter chip. The setup would be as follows.

+---------+    +-------------+    +-------------+
+   PC    +----+ UART to USB +----+ I2C to UART |
+---------+    +-------------+    +-------------+
                                              | |
------------------------------------------------+--  System Management
----------------------------------------------+----  Bus

Watch this video to see how it is set up. A backup of the video is available here

If you are using any of the SC16IS740/750/760 I2C to UART converter chip, you can enable the SC16IS7XX_INIT option to initialize the chip.

If not we can use other I2C slave devices like an Arduino or a Beagleboard.

This feature was added as part of a GSoC 2022 project. Checkout the following blog posts for more details.