docs(fwu): add firmware update documentation

Added firmware update documentation for:
1. PSA firmware update build flag
2. Porting guidelines to set the addresses of FWU metadata image
   and updated components in I/O policy

Change-Id: Iad3eb68b4be01a0b5850b69a067c60fcb464f54b
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

docs/getting_started/build-options.rst

@@ -873,6 +873,14 @@ Firmware update options
    This flag is used in defining the firmware update metadata structure. This
    flag is by default set to '1'.
 
+-  ``PSA_FWU_SUPPORT``: Enable the firmware update mechanism as per the
+   `PSA FW update specification`_. The default value is 0, and this is an
+   experimental feature.
+   PSA firmware update implementation has some limitations, such as BL2 is
+   not part of the protocol-updatable images, if BL2 needs to be updated, then
+   it should be done through another platform-defined mechanism, and it assumes
+   that the platform's hardware supports CRC32 instructions.
+
 --------------
 
 *Copyright (c) 2019-2021, Arm Limited. All rights reserved.*

docs/getting_started/porting-guide.rst

@@ -894,6 +894,54 @@ On success the function should return 0 and a negative error code otherwise.
 Note that this API depends on ``DECRYPTION_SUPPORT`` build flag which is
 marked as experimental.
 
+Function : plat_fwu_set_images_source() [when PSA_FWU_SUPPORT == 1]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : struct fwu_metadata *metadata
+    Return   : void
+
+This function is mandatory when PSA_FWU_SUPPORT is enabled.
+It provides a means to retrieve image specification (offset in
+non-volatile storage and length) of active/updated images using the passed
+FWU metadata, and update I/O policies of active/updated images using retrieved
+image specification information.
+Further I/O layer operations such as I/O open, I/O read, etc. on these
+images rely on this function call.
+
+In Arm platforms, this function is used to set an I/O policy of the FIP image,
+container of all active/updated secure and non-secure images.
+
+Function : plat_fwu_set_metadata_image_source() [when PSA_FWU_SUPPORT == 1]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : unsigned int image_id, uintptr_t *dev_handle,
+               uintptr_t *image_spec
+    Return   : int
+
+This function is mandatory when PSA_FWU_SUPPORT is enabled. It is
+responsible for setting up the platform I/O policy of the requested metadata
+image (either FWU_METADATA_IMAGE_ID or BKUP_FWU_METADATA_IMAGE_ID) that will
+be used to load this image from the platform's non-volatile storage.
+
+FWU metadata can not be always stored as a raw image in non-volatile storage
+to define its image specification (offset in non-volatile storage and length)
+statically in I/O policy.
+For example, the FWU metadata image is stored as a partition inside the GUID
+partition table image. Its specification is defined in the partition table
+that needs to be parsed dynamically.
+This function provides a means to retrieve such dynamic information to set
+the I/O policy of the FWU metadata image.
+Further I/O layer operations such as I/O open, I/O read, etc. on FWU metadata
+image relies on this function call.
+
+It returns '0' on success, otherwise a negative error value on error.
+Alongside, returns device handle and image specification from the I/O policy
+of the requested FWU metadata image.
+
 Common optional modifications
 -----------------------------
 

docs/index.rst

@@ -30,6 +30,7 @@ such as:
 -  `SMC Calling Convention`_
 -  `System Control and Management Interface (SCMI)`_
 -  `Software Delegated Exception Interface (SDEI)`_
+-  `PSA FW update specification`_
 
 Where possible, the code is designed for reuse or porting to other Armv7-A and
 Armv8-A model and hardware platforms.
@@ -92,3 +93,4 @@ have previously been raised against the software.
 .. _System Control and Management Interface (SCMI): http://infocenter.arm.com/help/topic/com.arm.doc.den0056a/DEN0056A_System_Control_and_Management_Interface.pdf
 .. _Software Delegated Exception Interface (SDEI): http://infocenter.arm.com/help/topic/com.arm.doc.den0054a/ARM_DEN0054A_Software_Delegated_Exception_Interface.pdf
 .. _SMC Calling Convention: https://developer.arm.com/docs/den0028/latest
+.. _PSA FW update specification: https://developer.arm.com/documentation/den0118/a/