Firmware Configuration Interface in coreboot


The firmware configuration interface in coreboot is designed to support a wide variety of configuration options in that are dictated by the hardware at runtime. This allows a single BIOS image to be used across a wide variety of devices which may have key differences but are otherwise similar enough to use the same coreboot build target.

The initial implementation is designed to take advantage of a bitmask returned by the Embedded Controller on Google Chrome OS devices which allows the manufacturer to use the same firmware image across multiple devices by selecting various options at runtime. See the Chromium OS Firmware Config documentation for more information.

This firmware configuration interface differs from the CMOS option interface in that this bitmask value is not intended as a user-configurable setting as the configuration values must match the actual hardware. In the case where a user was to swap their hardware this value would need to be updated or overridden.

Device Presence

One common example of why a firmware configuration interface is important is determining if a device is present in the system. With some bus topologies and hardware mechanisms it is possible to probe and enumerate this at runtime:

  • PCI is a self-discoverable bus and is very easy to handle.
  • I2C devices can often be probed with a combination of bus and address.
  • The use of GPIOs with external strap to ground or different voltages can be used to detect presence of a device.

However there are several cases where this is insufficient:

  • I2C peripherals that require different drivers but have the same bus address cannot be uniquely identified at runtime.
  • A mainboard may be designed with multiple daughter board combinations which contain devices and configurations that cannot be detected.
  • While presence detect GPIOs are a convenient way for a single device presence, they are unable to distinguish between different devices so it can require a large number of GPIOs to support relatively few options.

This presence detection can impact different stages of boot:


Devices that are not present should not provide an ACPI device indicating that they are present or the operating system may not be able to handle it correctly.

The ACPI devices are largely driven by chips defined in the mainboard devicetree.cb and the variant overridetree.cb. This means it is important to be able to specify when a device is present or not directly in devicetree.cb itself. Otherwise each mainboard needs custom code to parse the tree and disable unused devices.


GPIOs with multiple functions may need to be configured correctly depending on the attached device. Given the wide variety of GPIO configuration possibilities it is not feasible to specify all combinations directly in devicetree.cb and it is best left to code provided by the mainboard.


Enabling and disabling devices may require altering FSP UPD values that are provided to the various stages of FSP. These options are also not easy to specify multiple times for different configurations in devicetree.cb and can be provided by the mainboard as code.

Firmware Configuration Interface

The firmware configuration interface can be enabled by selecting CONFIG_FW_CONFIG and also providing a source for the value by defining an additional Kconfig option defined below.

If the firmware configuration interface is disabled via Kconfig then all probe attempts will return true.

Firmware Configuration Value

The 64-bit value used as the firmware configuration bitmask is meant to be determined at runtime but could also be defined at compile time if needed.

There are two supported sources for providing this information to coreboot.


The value can be provided with a 64-bit raw value in CBFS that is read by coreboot. The value can be set at build time but also adjusted in an existing image with cbfstool.

To enable this select the CONFIG_FW_CONFIG_CBFS option in the build configuration and add a raw 64-bit value to CBFS with the name of the current prefix at CONFIG_FW_PREFIX/fw_config.

When fw_config_probe_device() or fw_config_probe() is called it will look for the specified file in CBFS use the value it contains when matching fields and options.

Embedded Controller

Google Chrome OS devices support an Embedded Controller interface for reading and writing the firmware configuration value, along with other board-specific information. It is possible for coreboot to read this value at boot on systems that support this feature.

This option is selected by default for the mainboards that use it with CONFIG_FW_CONFIG_CHROME_EC_CBI and it is not typically necessary to adjust the value. It is possible by enabling the CBFS source and coreboot will look in CBFS first for a valid value before asking the embedded controller.

It is also possible to adjust the value in the embedded controller (after disabling write protection) with the ectool command in a Chrome OS environment.

For more information on the firmware configuration field on Chrome OS devices see the Chromium documentation for Firmware Config and Board Info.

Firmware Configuration Table

The firmware configuration table itself is defined in the mainboard devicetree.cb with special tokens for defining fields and options.

The table itself is enclosed in a fw_config token and terminated with end and it contains a mix of field and option definitions.

Each field is defined by providing the field name and the start and end bit marking the exact location in the bitmask. Field names must be at least three characters long in order to satisfy the sconfig parser requirements and they must be unique with non-overlapping masks.

field <name> <start-bit> <end-bit> [option...] end

For single-bit fields only one number is needed:

field <name> <bit> [option...] end

A field definition can also contain multiple sets of bit masks, which can be dis-contiguous. They are treated as if they are contiguous when defining option values. This allows for extending fields even after the bits after its current masks are occupied.

field <name> <start-bit0> <end-bit0> | <start-bit1> <end-bit1> | ...

For example, if more audio options need to be supported:

field AUDIO 3 3
	option AUDIO_0  0
	option AUDIO_1  1
field OTHER 4 4

the following can be done:

field AUDIO 3 3 | 5 5
	option AUDIO_FOO   0
	option AUDIO_BLAH  1
	option AUDIO_BAR   2
	option AUDIO_BAZ   3
field OTHER 4 4

In that case, the AUDIO masks are extended like so:

#define FW_CONFIG_FIELD_AUDIO_MASK						0x28

Each field definition starts a new block that can be composed of zero or more field options, and it is terminated with end.

Inside the field block the options can be defined by providing the option name and the field value that this option represents when the bit offsets are used to apply a mask and shift. Option names must also be at least three characters for the sconfig parser.

option <name> <value>

It is possible for there to be multiple fw_config blocks and for subsequent field blocks to add additional option definitions to the existing field. These subsequent definitions should not provide the field bitmask as it has already been defined earlier in the file and this is just matching an existing field by name.

field <name> [option...] end

This allows a baseboard to define the major fields and options in devicetree.cb and a board variant to add specific options to fields in or define new fields in the unused bitmask in overridetree.cb.

It is not possible to redefine a field mask or override the value of an existing option this way, only to add new options to a field or new fields to the table.

Firmware Configuration Table Example

In this example a baseboard defines a simple boolean feature that is enabled or disabled depending on the value of bit 0, and a field at bits 1-2 that indicates which daughter board is attached.

The baseboard itself defines one daughter board and the variant adds two more possibilities. This way each variant can support multiple possible daughter boards in addition to the one that was defined by the baseboard.


    field FEATURE 0
        option DISABLED 0
        option ENABLED 1
    field DAUGHTER_BOARD 1 2
        option NONE 0
        option REFERENCE_DB 1


        option VARIANT_DB_ONE 2
        option VARIANT_DB_TWO 3

The result of this table defined in devicetree.cb is a list of constants that can be used to check if fields match the firmware configuration options determined at runtime with a simple check of the field mask and the option value.


/* field: FEATURE */

/* field: DAUGHTER_BOARD */

Device Probing

One use of the firmware configuration interface in devicetree is to allow device probing to be specified directly with the devices themselves. A new probe token is introduced to allow a device to be probed by field and option name. Multiple probe entries may be present for each device and any successful probe will consider the device to be present.

Probing Example

Continuing with the previous example this device would be considered present if the field DAUGHTER_BOARD was set to either VARIANT_DB_ONE or VARIANT_DB_TWO:


chip drivers/generic/example
    device generic 0 on

If the field were set to any other option, including NONE and REFERENCE_DB and any undefined value then the device would be disabled.

Probe Overrides

When a device is declared with a probe in the baseboard devicetree.cb and the same device is also present in the overridetree.cb then the probing information from the baseboard is discarded and the override device must provide all necessary probing information.

In this example a device is listed in the baseboard with DAUGHTER_BOARD field probing for REFERENCE_DB as a field option, It is also defined as an override device with the field probing for the VARIANT_DB_ONE option instead.

In this case only the probe listed in the override is checked and a field option of REFERENCE_DB will not mark this device present. If both options are desired then the override device must list both. This allows an override device to remove a probe entry that was defined in the baseboard.


chip drivers/generic/example
    device generic 0 on


chip drivers/generic/example
    device generic 0 on

Automatic Device Probing

At boot time the firmware configuration interface will walk the device tree and apply any probe entries that were defined in devicetree.cb. This probing takes effect before the BS_DEV_ENUMERATE step during the boot state machine in ramstage.

Devices that have a probe list but do do not find a match are disabled by setting dev->enabled = 0 but the chip enable_dev() and device enable() handlers will still be executed to allow any device disable code to execute.

The result of this probe definition is to provide an array of structures describing each field and option to check.


 * struct fw_config - Firmware configuration field and option.
 * @field_name: Name of the field that this option belongs to.
 * @option_name: Name of the option within this field.
 * @mask: Bitmask of the field.
 * @value: Value of the option within the mask.
struct fw_config {
	const char *field_name;
	const char *option_name;
	uint64_t mask;
	uint64_t value;


STORAGE struct fw_config __devN_probe_list[] = {
	{ }

Runtime Probing

The device driver probing allows for seamless integration with the mainboard but it is only effective in ramstage and for specific devices declared in devicetree.cb. There are other situations where code may need to probe or check the value of a field in romstage or at other points in ramstage. For this reason it is also possible to use the firmware configuration interface directly.

 * fw_config_probe() - Check if field and option matches.
 * @match: Structure containing field and option to probe.
 * Return %true if match is found, %false if match is not found.
bool fw_config_probe(const struct fw_config *match);

The argument provided to this function can be created from a macro for easy use:

FW_CONFIG(field, option)

This example has a mainboard check if a feature is disabled and set an FSP UPD before memory training. This example expects that the default value of this register is set to true in devicetree.cb and this code is disabling that feature before FSP is executed.

#include <fw_config.h>

void mainboard_memory_init_params(FSPM_UPD *mupd)
	if (fw_config_probe_one(FW_CONFIG(FEATURE, DISABLED))
		mupd->ExampleFeature = false;