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Firmware Configuration Framework
This document provides an overview of the |FCONF| framework.
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.
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
Properties can be accessed with the FCONF_GET_PROPERTY(a,b,property) macro.
Defining properties
Properties composing the |FCONF| have to be stored in C structures. If another backing store is wanted to be used, the platform has to provide a populate() function to fill the corresponding C structure.
The populate() function 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_tbbr_dyn_config(uintptr_t config) { /* read dtb and fill tbbr_dyn_config struct */ } FCONF_REGISTER_POPULATOR(fconf_populate_tbbr_dyn_config);
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 tbbr__dyn_config_getter(id) tbbr_dyn_config.id
This second level wrapper can be used to remap the FCONF_GET_PROPERTY() to anything appropriate: structure, array, function, etc..
Loading the property device tree
The fconf_load_config() must be called to load the device 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).
System Message: ERROR/3 (<stdin>, line 75)
Unknown directive type "uml".
.. uml:: ../resources/diagrams/plantuml/fconf_bl1_load_config.puml
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().
System Message: ERROR/3 (<stdin>, line 85)
Unknown directive type "uml".
.. uml:: ../resources/diagrams/plantuml/fconf_bl2_populate.puml
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