doc: Add binding document for fconf.
Complete the documentation with information on how to write a DTS for
fconf. This patch adds the bindings information for dynamic
configuration properties.
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>
Change-Id: Ic6d9f927df53bb87315c23ec5a8943d0c3258d45
diff --git a/docs/components/fconf/index.rst b/docs/components/fconf/index.rst
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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
+- GICv3 properties: hw_config.gicv3_config.gicr_base
+
+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
+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:
+ - TB_FW: properties related to trusted firmware such as IO policies,
+ base address of other DTBs, 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
+:ref:`binding-document` section for more information and example.
+
+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).
+
+.. 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()`` as described above.
+
+.. uml:: ../../resources/diagrams/plantuml/fconf_bl2_populate.puml
+
+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
+
+.. _binding-document:
+
+Properties binding information
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. toctree::
+ :maxdepth: 1
+
+ fconf_properties