CPU Reset
=========

This document describes the high-level design of the framework to handle CPU
resets in Trusted Firmware-A (TF-A). It also describes how the platform
integrator can tailor this code to the system configuration to some extent,
resulting in a simplified and more optimised boot flow.

This document should be used in conjunction with the :ref:`Firmware Design`
document which provides greater implementation details around the reset code,
specifically for the cold boot path.

General reset code flow
-----------------------

The TF-A reset code is implemented in BL1 by default. The following high-level
diagram illustrates this:

|Default reset code flow|

This diagram shows the default, unoptimised reset flow. Depending on the system
configuration, some of these steps might be unnecessary. The following sections
guide the platform integrator by indicating which build options exclude which
steps, depending on the capability of the platform.

.. note::
   If BL31 is used as the TF-A entry point instead of BL1, the diagram
   above is still relevant, as all these operations will occur in BL31 in
   this case. Please refer to section 6 "Using BL31 entrypoint as the reset
   address" for more information.

Programmable CPU reset address
------------------------------

By default, TF-A assumes that the CPU reset address is not programmable.
Therefore, all CPUs start at the same address (typically address 0) whenever
they reset. Further logic is then required to identify whether it is a cold or
warm boot to direct CPUs to the right execution path.

If the reset vector address (reflected in the reset vector base address register
``RVBAR_EL3``) is programmable then it is possible to make each CPU start directly
at the right address, both on a cold and warm reset. Therefore, the boot type
detection can be skipped, resulting in the following boot flow:

|Reset code flow with programmable reset address|

To enable this boot flow, compile TF-A with ``PROGRAMMABLE_RESET_ADDRESS=1``.
This option only affects the TF-A reset image, which is BL1 by default or BL31 if
``RESET_TO_BL31=1``.

On both the FVP and Juno platforms, the reset vector address is not programmable
so both ports use ``PROGRAMMABLE_RESET_ADDRESS=0``.

Cold boot on a single CPU
-------------------------

By default, TF-A assumes that several CPUs may be released out of reset.
Therefore, the cold boot code has to arbitrate access to hardware resources
shared amongst CPUs. This is done by nominating one of the CPUs as the primary,
which is responsible for initialising shared hardware and coordinating the boot
flow with the other CPUs.

If the platform guarantees that only a single CPU will ever be brought up then
no arbitration is required. The notion of primary/secondary CPU itself no longer
applies. This results in the following boot flow:

|Reset code flow with single CPU released out of reset|

To enable this boot flow, compile TF-A with ``COLD_BOOT_SINGLE_CPU=1``. This
option only affects the TF-A reset image, which is BL1 by default or BL31 if
``RESET_TO_BL31=1``.

On both the FVP and Juno platforms, although only one core is powered up by
default, there are platform-specific ways to release any number of cores out of
reset. Therefore, both platform ports use ``COLD_BOOT_SINGLE_CPU=0``.

Programmable CPU reset address, Cold boot on a single CPU
---------------------------------------------------------

It is obviously possible to combine both optimisations on platforms that have
a programmable CPU reset address and which release a single CPU out of reset.
This results in the following boot flow:


|Reset code flow with programmable reset address and single CPU released out of reset|

To enable this boot flow, compile TF-A with both ``COLD_BOOT_SINGLE_CPU=1``
and ``PROGRAMMABLE_RESET_ADDRESS=1``. These options only affect the TF-A reset
image, which is BL1 by default or BL31 if ``RESET_TO_BL31=1``.

Using BL31 entrypoint as the reset address
------------------------------------------

On some platforms the runtime firmware (BL3x images) for the application
processors are loaded by some firmware running on a secure system processor
on the SoC, rather than by BL1 and BL2 running on the primary application
processor. For this type of SoC it is desirable for the application processor
to always reset to BL31 which eliminates the need for BL1 and BL2.

TF-A provides a build-time option ``RESET_TO_BL31`` that includes some additional
logic in the BL31 entry point to support this use case.

In this configuration, the platform's Trusted Boot Firmware must ensure that
BL31 is loaded to its runtime address, which must match the CPU's ``RVBAR_EL3``
reset vector base address, before the application processor is powered on.
Additionally, platform software is responsible for loading the other BL3x images
required and providing entry point information for them to BL31. Loading these
images might be done by the Trusted Boot Firmware or by platform code in BL31.

Although the Arm FVP platform does not support programming the reset base
address dynamically at run-time, it is possible to set the initial value of the
``RVBAR_EL3`` register at start-up. This feature is provided on the Base FVP
only.

It allows the Arm FVP port to support the ``RESET_TO_BL31`` configuration, in
which case the ``bl31.bin`` image must be loaded to its run address in Trusted
SRAM and all CPU reset vectors be changed from the default ``0x0`` to this run
address. See the :ref:`Arm Fixed Virtual Platforms (FVP)` for details of running
the FVP models in this way.

Although technically it would be possible to program the reset base address with
the right support in the SCP firmware, this is currently not implemented so the
Juno port doesn't support the ``RESET_TO_BL31`` configuration.

The ``RESET_TO_BL31`` configuration requires some additions and changes in the
BL31 functionality:

Determination of boot path
~~~~~~~~~~~~~~~~~~~~~~~~~~

In this configuration, BL31 uses the same reset framework and code as the one
described for BL1 above. Therefore, it is affected by the
``PROGRAMMABLE_RESET_ADDRESS`` and ``COLD_BOOT_SINGLE_CPU`` build options in the
same way.

In the default, unoptimised BL31 reset flow, on a warm boot a CPU is directed
to the PSCI implementation via a platform defined mechanism. On a cold boot,
the platform must place any secondary CPUs into a safe state while the primary
CPU executes a modified BL31 initialization, as described below.

Platform initialization
~~~~~~~~~~~~~~~~~~~~~~~

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) 2015-2023, 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
.. |Reset code flow with single CPU released out of reset| image:: ../resources/diagrams/reset_code_no_cpu_check.png
.. |Reset code flow with programmable reset address and single CPU released out of reset| image:: ../resources/diagrams/reset_code_no_checks.png
