Docs: Add firmware update partition design document
Change-Id: Ia7e46eadb4c1e2a50d6641af6ed520b0c5123eca
Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
diff --git a/docs/getting_started/tfm_secure_boot.rst b/docs/getting_started/tfm_secure_boot.rst
index 3b1d8b7..0764956 100644
--- a/docs/getting_started/tfm_secure_boot.rst
+++ b/docs/getting_started/tfm_secure_boot.rst
@@ -791,4 +791,20 @@
--------------
+****************************************
+Integration with Firmware Update service
+****************************************
+The shim layer of the Firmware Update partition calls the APIs in
+bootutil_misc.c to control the image status.
+
+- Call ``boot_write_magic()`` to make the image as a candidate image for booting.
+- Call ``boot_set_confirmed()`` to make the image as a permanent image.
+
+.. Note::
+ Currently, in direct-xip mode and ram-load mode, TF-M cannot get the
+ information of which slot contains the running image from the bootloader.
+ So the Firmware Update partition cannot decide where to write the new
+ image. As a result, the firmware update service is not supported in
+ direct-xip mode and ram-load mode.
+
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