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Change-Id: If20b695acedc6d1b49c8d9fb64efd6b6ba23f4a9
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>

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+ARM Trusted Firmware Reset Design
+=================================
+
+
+.. section-numbering::
+    :suffix: .
+
+.. contents::
+
+This document describes the high-level design of the framework to handle CPU
+resets in ARM Trusted Firmware. 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 `Firmware Design`_, which
+provides greater implementation details around the reset code, specifically
+for the cold boot path.
+
+General reset code flow
+-----------------------
+
+The ARM Trusted Firmware (TF) 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 Trusted Firmware 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, the TF 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 the TF with ``PROGRAMMABLE_RESET_ADDRESS=1``.
+This option only affects the TF 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, the TF 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 the TF with ``COLD_BOOT_SINGLE_CPU=1``. This
+option only affects the TF 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 the TF with both ``COLD_BOOT_SINGLE_CPU=1``
+and ``PROGRAMMABLE_RESET_ADDRESS=1``. These options only affect the TF 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 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 `User Guide`_ 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, 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.
+
+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.
+
+--------------
+
+*Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.*
+
+.. _Firmware Design: firmware-design.rst
+.. _User Guide: user-guide.rst
+
+.. |Default reset code flow| image:: diagrams/default_reset_code.png?raw=true
+.. |Reset code flow with programmable reset address| image:: diagrams/reset_code_no_boot_type_check.png?raw=true
+.. |Reset code flow with single CPU released out of reset| image:: diagrams/reset_code_no_cpu_check.png?raw=true
+.. |Reset code flow with programmable reset address and single CPU released out of reset| image:: diagrams/reset_code_no_checks.png?raw=true