suit: Add description of SUIT manifest var

Add a brief description of manifest variables, including API
description.

Ref: NCSDK-NONE

Signed-off-by: Tomasz Chyrowicz <[email protected]>

Author: tomchy

doc/nrf/libraries/dfu/suit_mfst_var.rst

@@ -0,0 +1,72 @@
+.. _subsys_suit_mfst_var:
+
+SUIT Manifest-accessible variables
+##################################
+
+.. contents::
+   :local:
+   :depth: 2
+
+The SUIT Manifest-accessible variables are new type of components that allow to store integer values.
+They are readable by any SUIT manifest, the SDFW, and the IPC clients.
+The modifiability of these variables depends on the variable ID.
+
+Overview
+********
+
+The manifest-controlled variable can be defined as one of the components inside the SUIT manifest.
+The component ID of the variables is constructed as follows:
+
+.. code-block:: console
+
+   [ MFST_VAR, <id> ]
+
+There is a fixed number of such components for a given platform.
+Their functionality and access privileges are defined by using ``<id>`` values from the following predefined ranges:
+
+  * IDs from ``0`` to ``7`` are meant to hold persistently stored configuration data, which can be modified by OEM-controlled elements.
+    They can be utilized to tune the installation and boot process.
+    They are stored in two copies, protected by the digest, ensuring that only a valid value will be returned.
+    If the system detects that it is impossible to synchronize those two copies, further updates of all NVM variables are blocked.
+
+    .. warning::
+       Non-volatile variables are stored at the fixed location inside the memory.
+       There are no mechanisms managing the NVM wearing, so the frequency of the variable updates should be managed by manifest designer.
+
+    The non-volatile variables can be modified by Application, Radio Manifests, and by IPC clients running on Application and Radio Domains.
+
+  * IDs from ``128`` to ``129`` are meant to hold runtime, volatile information, provided by the SDFW.
+    Currently there are no functionalities assigned to those variables.
+
+    The platform-controlled variables cannot be modified by Manifests, and IPC clients.
+
+  * IDs from ``256`` to ``259`` are meant to hold runtime, volatile information, provided by OEM-controlled manifests.
+    These variables can be used to pass the information about executed boot scenario (selected set of images boot in the "A/B" method) as well as allow for a minimal manifest execution tracing.
+
+    The volatile variables can be modified by Application and Radio Manifests.
+    IPC clients are not allowed to modify them.
+
+Configuration
+*************
+
+All configuration option values are defined by the SDFW.
+They are also available in read-only mode for all types of applications.
+All the APIs are serialized using the SSF framework.
+
+The following configuration options are defined:
+
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_NVM_BASE_ID` - Defines the first ID of manifest variables, that are stored in the non-volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_NVM_COUNT` - Defines the number of available manifest variables, that are stored in the non-volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_NVM_ACCESS_MASK` - Defines the access mask of manifest variables, that are stored in the non-volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_PLAT_VOLATILE_BASE_ID` - Defines the first ID of platform-controlled manifest variables, that are stored in the volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_PLAT_VOLATILE_COUNT` - Defines the number of available platform-controlled manifest variables, that are stored in the volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_PLAT_VOLATILE_ACCESS_MASK` - Defines the access mask of platform-controlled manifest variables, that are stored in the volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_MFST_VOLATILE_BASE_ID` - Defines the first ID of manifest variables, that are stored in the volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_MFST_VOLATILE_COUNT` - Defines the number of available manifest variables, that are stored in the volatile memory.
+* :kconfig:option:`CONFIG_SUIT_MANIFEST_VARIABLES_MFST_VOLATILE_ACCESS_MASK` - Defines the access mask of manifest variables, that are stored in the volatile memory.
+
+API documentation
+*****************
+
+| Header file: :file:`subsys/suit/manifest_variables/include/suit_manifest_variables.h`
+| Source files: :file:`subsys/suit/manifest_variables/src/suit_manifest_variables.c`