4.6. Firmware Configuration Framework
This document provides an overview of the FCONF framework.
4.6.1. Introduction
The Firmware CONfiguration Framework (FCONF) is an abstraction layer for platform specific data, allowing a “property” to be queried and a value retrieved without the requesting entity knowing what backing store is being used to hold the data.
It is used to bridge new and old ways of providing platform-specific data. Today, information like the Chain of Trust is held within several, nested platform-defined tables. In the future, it may be provided as part of a device blob, along with the rest of the information about images to load. Introducing this abstraction layer will make migration easier and will preserve functionality for platforms that cannot / don’t want to use device tree.
4.6.2. Accessing properties
Properties defined in the FCONF are grouped around namespaces and sub-namespaces: a.b.property. Examples namespace can be:
(TBBR) Chain of Trust data: tbbr.cot.trusted_boot_fw_cert
(TBBR) dynamic configuration info: tbbr.dyn_config.disable_auth
Arm io policies: arm.io_policies.bl2_image
GICv3 properties: hw_config.gicv3_config.gicr_base
Properties can be accessed with the FCONF_GET_PROPERTY(a,b,property) macro.
4.6.3. Defining properties
Properties composing the FCONF have to be stored in C structures. If
properties originate from a different backend source such as a device tree,
then the platform has to provide a populate() function which essentially
captures the property and stores them into a corresponding FCONF based C
structure.
Such a populate() function is usually platform specific and is associated
with a specific backend source. For example, a populator function which
captures the hardware topology of the platform from the HW_CONFIG device tree.
Hence each populate() function must be registered with a specific
config_type identifier. It broadly represents a logical grouping of
configuration properties which is usually a device tree file.
- Example:
FW_CONFIG: properties related to base address, maximum size and image id of other DTBs etc.
TB_FW: properties related to trusted firmware such as IO policies, mbedtls heap info etc.
HW_CONFIG: properties related to hardware configuration of the SoC such as topology, GIC controller, PSCI hooks, CPU ID etc.
Hence the populate() callback must be registered to the (FCONF) framework
with the FCONF_REGISTER_POPULATOR() macro. This ensures that the function
would be called inside the generic fconf_populate() function during
initialization.
int fconf_populate_topology(uintptr_t config)
{
/* read hw config dtb and fill soc_topology struct */
}
FCONF_REGISTER_POPULATOR(HW_CONFIG, topology, fconf_populate_topology);
Then, a wrapper has to be provided to match the FCONF_GET_PROPERTY() macro:
/* generic getter */
#define FCONF_GET_PROPERTY(a,b,property) a##__##b##_getter(property)
/* my specific getter */
#define hw_config__topology_getter(prop) soc_topology.prop
This second level wrapper can be used to remap the FCONF_GET_PROPERTY() to
anything appropriate: structure, array, function, etc..
To ensure a good interpretation of the properties, this documentation must explain how the properties are described for a specific backend. Refer to the Properties binding information section for more information and example.
4.6.4. Loading the property device tree
The fconf_load_config(image_id) must be called to load fw_config and
tb_fw_config devices tree containing the properties’ values. This must be done
after the io layer is initialized, as the DTB is stored on an external
device (FIP).
4.6.5. Populating the properties
Once a valid device tree is available, the fconf_populate(config) function
can be used to fill the C data structure with the data from the config DTB.
This function will call all the populate() callbacks which have been
registered with FCONF_REGISTER_POPULATOR() as described above.
4.6.6. Namespace guidance
As mentioned above, properties are logically grouped around namespaces and sub-namespaces. The following concepts should be considered when adding new properties/namespaces. The framework differentiates two types of properties:
Properties used inside common code.
Properties used inside platform specific code.
The first category applies to properties being part of the firmware and shared
across multiple platforms. They should be globally accessible and defined
inside the lib/fconf directory. The namespace must be chosen to reflect the
feature/data abstracted.
- Example:
TBBR related properties: tbbr.cot.bl2_id
Dynamic configuration information: dyn_cfg.dtb_info.hw_config_id
The second category should represent the majority of the properties defined within the framework: Platform specific properties. They must be accessed only within the platform API and are defined only inside the platform scope. The namespace must contain the platform name under which the properties defined belong.
- Example:
Arm io framework: arm.io_policies.bl31_id