Introduce the ARM TF reset design document

This patch introduces a new document presenting the ARM Trusted
Firmware Reset Design. It shows the reset code flow, lists the
different build options that affect it, in which case to use them
and what their exact effect is.

The section about using BL31 entrypoint as the reset address has
been moved from the general firmware design document to this one.
It's also been improved to explain why the FVP port supports the
RESET_TO_BL31 configuration, even though the reset vector address
can't be programmed dynamically.

This document includes some images, which have been generated using
Dia version 0.97.2. This tool can be obtained from:
https://wiki.gnome.org/Apps/Dia/Download
This patch provides:
 - the image files describing the different reset flow diagrams;
 - the source '.dia' file;
 - a script automating the generation of the images from the '.dia'
   file.
Note that the 2 latter files are not actually needed for the document
and are provided for convenience only, in case the reset images need
to be modified.

Change-Id: Ib6302e8209d418a5b31c4e85e55fd9e83caf2ca2

docs/diagrams/generate_reset_images.sh

@@ -0,0 +1,41 @@
+#! /bin/bash
+
+#
+# This script generates the image files used in the ARM Trusted Firmware Reset
+# Design document from the 'reset_code_flow.dia' file.
+#
+# The PNG files in the present directory have been generated using Dia version
+# 0.97.2, which can be obtained from https://wiki.gnome.org/Apps/Dia/Download
+#
+
+set -e
+
+# Usage: generate_image <layers> <image_filename>
+function generate_image
+{
+	dia				\
+		--show-layers=$1	\
+		--filter=png		\
+		--export=$2		\
+		reset_code_flow.dia
+
+}
+
+# The 'reset_code_flow.dia' file is organized in several layers.
+# Each image is generated by combining and exporting the appropriate set of
+# layers.
+generate_image								\
+	Frontground,Background,cpu_type_check,boot_type_check		\
+	default_reset_code.png
+
+generate_image								\
+	Frontground,Background,no_cpu_type_check,boot_type_check	\
+	reset_code_no_cpu_check.png
+
+generate_image								\
+	Frontground,Background,cpu_type_check,no_boot_type_check	\
+	reset_code_no_boot_type_check.png
+
+generate_image								\
+	Frontground,Background,no_cpu_type_check,no_boot_type_check	\
+	reset_code_no_checks.png

docs/firmware-design.md

@@ -43,13 +43,16 @@ Firmware Interrupt Management Design guide][INTRG] [4].
 2.  Cold boot
 -------------
 
-The cold boot path starts when the platform is physically turned on. One of
-the CPUs released from reset is chosen as the primary CPU, and the remaining
-CPUs are considered secondary CPUs. The primary CPU is chosen through
-platform-specific means. The cold boot path is mainly executed by the primary
-CPU, other than essential CPU initialization executed by all CPUs. The
-secondary CPUs are kept in a safe platform-specific state until the primary
-CPU has performed enough initialization to boot them.
+The cold boot path starts when the platform is physically turned on. If
+`COLD_BOOT_SINGLE_CPU=0`, one of the CPUs released from reset is chosen as the
+primary CPU, and the remaining CPUs are considered secondary CPUs. The primary
+CPU is chosen through platform-specific means. The cold boot path is mainly
+executed by the primary CPU, other than essential CPU initialization executed by
+all CPUs. The secondary CPUs are kept in a safe platform-specific state until
+the primary CPU has performed enough initialization to boot them.
+
+Refer to the [Reset Design] for more information on the effect of the
+`COLD_BOOT_SINGLE_CPU` platform build option.
 
 The cold boot path in this implementation of the ARM Trusted Firmware is divided
 into five steps (in order of execution):
@@ -78,8 +81,6 @@ The sections below provide the following details:
 *   initialization and execution of the first three stages during cold boot
 *   specification of the BL31 entrypoint requirements for use by alternative
     Trusted Boot Firmware in place of the provided BL1 and BL2
-*   changes in BL31 behavior when using the `RESET_TO_BL31` option which
-    allows BL31 to run without BL1 and BL2
 
 
 ### BL1
@@ -105,6 +106,10 @@ platform-specific state (see the `plat_secondary_cold_boot_setup()` function in
 the [Porting Guide]) while the primary CPU executes the remaining cold boot path
 as described in the following sections.
 
+This step only applies when `PROGRAMMABLE_RESET_ADDRESS=0`. Refer to the
+[Reset Design] for more information on the effect of the
+`PROGRAMMABLE_RESET_ADDRESS` platform build option.
+
 #### Architectural initialization
 
 BL1 performs minimal architectural initialization as follows.
@@ -510,53 +515,6 @@ platform power management code is then invoked as required to initialize all
 necessary system, cluster and CPU resources.
 
 
-### Using BL31 as the CPU reset vector
-
-On some platforms the runtime firmware (BL3x images) for the application
-processors are loaded by trusted 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.
-
-ARM Trusted Firmware provides a build-time option `RESET_TO_BL31` that includes
-some additional logic in the BL31 entrypoint 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 reset
-vector 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.
-
-The ARM FVP port supports 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.
-
-This 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. 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.
-
-As the first image to execute in this configuration BL31 must also ensure that
-any security initialisation, for example programming a TrustZone address space
-controller, is carried out during early platform initialisation.
-
-
 3.  EL3 runtime services framework
 ----------------------------------
 
@@ -1767,5 +1725,6 @@ _Copyright (c) 2013-2015, ARM Limited and Contributors. All rights reserved._
 [UUID]:             https://tools.ietf.org/rfc/rfc4122.txt "A Universally Unique IDentifier (UUID) URN Namespace"
 [User Guide]:       ./user-guide.md
 [Porting Guide]:    ./porting-guide.md
+[Reset Design]:     ./reset-design.md
 [INTRG]:            ./interrupt-framework-design.md
 [CPUBM]:            ./cpu-specific-build-macros.md.md

docs/reset-design.md

@@ -0,0 +1,172 @@
+ARM Trusted Firmware Reset Design
+=================================
+
+1.  [Introduction](#1--introduction)
+2.  [General reset code flow](#2--general-reset-code-flow)
+3.  [Programmable CPU reset address](#3--programmable-cpu-reset-address)
+4.  [Cold boot on a single CPU](#4--cold-boot-on-a-single-cpu)
+5.  [Programmable CPU reset address, Cold boot on a single CPU](#5--programmable-cpu-reset-address-cold-boot-on-a-single-cpu)
+6.  [Using BL31 entrypoint as the reset address](#6--using-bl31-entrypoint-as-the-reset-address)
+
+
+1.  Introduction
+----------------
+
+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.
+
+
+2.  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](diagrams/default_reset_code.png?raw=true)
+
+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.
+
+
+3.  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](
+diagrams/reset_code_no_boot_type_check.png?raw=true)
+
+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`.
+
+
+4.  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](
+diagrams/reset_code_no_cpu_check.png?raw=true)
+
+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`.
+
+
+5.  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](diagrams/reset_code_no_checks.png?raw=true)
+
+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`.
+
+
+6.  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._
+
+
+[User Guide]:       user-guide.md
+[Firmware Design]:  firmware-design.md