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docs: update RESET_TO_BL31 documentation

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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>
This commit is contained in:
Manish V Badarkhe 2023-02-21 12:35:09 +00:00
parent 3e14df6f63
commit d2baffbc03
1 changed files with 14 additions and 6 deletions
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20
docs/design/reset-design.rst
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@ -141,19 +141,27 @@ CPU executes a modified BL31 initialization, as described below.
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