86 lines
3.0 KiB
ReStructuredText
86 lines
3.0 KiB
ReStructuredText
Firmware Configuration Framework
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================================
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This document provides an overview of the |FCONF| framework.
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Introduction
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~~~~~~~~~~~~
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The Firmware CONfiguration Framework (|FCONF|) is an abstraction layer for
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platform specific data, allowing a "property" to be queried and a value
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retrieved without the requesting entity knowing what backing store is being used
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to hold the data.
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It is used to bridge new and old ways of providing platform-specific data.
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Today, information like the Chain of Trust is held within several, nested
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platform-defined tables. In the future, it may be provided as part of a device
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blob, along with the rest of the information about images to load.
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Introducing this abstraction layer will make migration easier and will preserve
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functionality for platforms that cannot / don't want to use device tree.
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Accessing properties
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~~~~~~~~~~~~~~~~~~~~
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Properties defined in the |FCONF| are grouped around namespaces and
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sub-namespaces: a.b.property.
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Examples namespace can be:
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- (|TBBR|) Chain of Trust data: tbbr.cot.trusted_boot_fw_cert
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- (|TBBR|) dynamic configuration info: tbbr.dyn_config.disable_auth
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- Arm io policies: arm.io_policies.bl2_image
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Properties can be accessed with the ``FCONF_GET_PROPERTY(a,b,property)`` macro.
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Defining properties
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~~~~~~~~~~~~~~~~~~~
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Properties composing the |FCONF| have to be stored in C structures. If another
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backing store is wanted to be used, the platform has to provide a ``populate()``
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function to fill the corresponding C structure.
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The ``populate()`` function must be registered to the |FCONF| framework with
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the ``FCONF_REGISTER_POPULATOR()`` macro. This ensures that the function would
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be called inside the generic ``fconf_populate()`` function during
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initialization.
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::
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int fconf_populate_tbbr_dyn_config(uintptr_t config)
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{
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/* read dtb and fill tbbr_dyn_config struct */
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}
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FCONF_REGISTER_POPULATOR(fconf_populate_tbbr_dyn_config);
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Then, a wrapper has to be provided to match the ``FCONF_GET_PROPERTY()`` macro:
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::
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/* generic getter */
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#define FCONF_GET_PROPERTY(a,b,property) a##__##b##_getter(property)
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/* my specific getter */
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#define tbbr__dyn_config_getter(id) tbbr_dyn_config.id
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This second level wrapper can be used to remap the ``FCONF_GET_PROPERTY()`` to
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anything appropriate: structure, array, function, etc..
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Loading the property device tree
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The ``fconf_load_config()`` must be called to load the device tree containing
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the properties' values. This must be done after the io layer is initialized, as
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the |DTB| is stored on an external device (FIP).
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.. uml:: ../resources/diagrams/plantuml/fconf_bl1_load_config.puml
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Populating the properties
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Once a valid device tree is available, the ``fconf_populate(config)`` function
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can be used to fill the C data structure with the data from the config |DTB|.
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This function will call all the ``populate()`` callbacks which have been
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registered with ``FCONF_REGISTER_POPULATOR()``.
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.. uml:: ../resources/diagrams/plantuml/fconf_bl2_populate.puml
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