docs: update RESET_TO_BL31 documentation
The documentation has been updated to explicitly mention that with
RESET_TO_BL31, the platform can receive parameters based on their
actual boot sequence.
Change-Id: Ib482fb89e528ec836ff7ee175cac59dd2da2898b
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/design/reset-design.rst b/docs/design/reset-design.rst
index 7b10c95..f8c5a43 100644
--- a/docs/design/reset-design.rst
+++ b/docs/design/reset-design.rst
@@ -141,19 +141,27 @@
Platform initialization
~~~~~~~~~~~~~~~~~~~~~~~
-In this configuration, when the CPU resets to BL31 there are no parameters that
-can be passed in registers by previous boot stages. Instead, the platform code
-in BL31 needs to know, or be able to determine, the location of the BL32 (if
-required) and BL33 images and provide this information in response to the
-``bl31_plat_get_next_image_ep_info()`` function.
+In this configuration, since the CPU resets to BL31, no parameters are expected
+to be passed to BL31 (see notes below for clarification).
+Instead, the platform code in BL31 needs to know, or be able to determine, the
+location of the BL32 (if required) and BL33 images and provide this information
+in response to the ``bl31_plat_get_next_image_ep_info()`` function.
Additionally, platform software is responsible for carrying out any security
initialisation, for example programming a TrustZone address space controller.
This might be done by the Trusted Boot Firmware or by platform code in BL31.
+.. note::
+ Even though RESET_TO_BL31 is designed such that BL31 is the reset BL image,
+ some platforms may wish to pass some arguments to BL31 as per the defined
+ contract between BL31 and previous bootloaders. Previous bootloaders can
+ pass arguments through registers x0 through x3. BL31 will preserve them and
+ propagate them to platform code, which will handle these arguments in an
+ IMPDEF manner.
+
--------------
-*Copyright (c) 2015-2019, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2015-2023, Arm Limited and Contributors. All rights reserved.*
.. |Default reset code flow| image:: ../resources/diagrams/default_reset_code.png
.. |Reset code flow with programmable reset address| image:: ../resources/diagrams/reset_code_no_boot_type_check.png