FWU: doc: Add documentation for the FWU feature

Add documentation for the FWU Multi Bank Update feature. The document
describes the steps needed for setting up the platform for the
feature, as well as steps for enabling the feature on the platform.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
index cd84706953..e0835beba4 100644
--- a/doc/develop/uefi/uefi.rst
+++ b/doc/develop/uefi/uefi.rst
@@ -277,6 +277,8 @@ Enable ``CONFIG_OPTEE``, ``CONFIG_CMD_OPTEE_RPMB`` and ``CONFIG_EFI_MM_COMM_TEE`
 
 [1] https://optee.readthedocs.io/en/latest/building/efi_vars/stmm.html
 
+.. _uefi_capsule_update_ref:
+
 Enabling UEFI Capsule Update feature
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -377,6 +379,16 @@ following command::
 
     dfu list
 
+When the FWU Multi Bank Update feature is enabled on the platform, the
+image index is used only to identify the image index with the image
+GUID. The image index would not correspond to the dfu alt number. This
+is because the FWU feature supports multiple partitions(banks) of
+updatable images, and the actual dfu alt number to which the image is
+to be written to is determined at runtime, based on the value of the
+update bank to which the image is to be written. For more information
+on the FWU Multi Bank Update feature, please refer `doc
+<doc/develop/uefi/fwu_updates.rst>`__.
+
 When using the FMP for FIT images, the image index value needs to be
 set to 1.