ARM Trusted Firmware Design
===========================

Contents :

1.  [Introduction](#1--introduction)
2.  [Cold boot](#2--cold-boot)
3.  [EL3 runtime services framework](#3--el3-runtime-services-framework)
4.  [Power State Coordination Interface](#4--power-state-coordination-interface)
5.  [Secure-EL1 Payloads and Dispatchers](#5--secure-el1-payloads-and-dispatchers)
6.  [Crash Reporting in BL31](#6--crash-reporting-in-bl31)
7.  [Guidelines for Reset Handlers](#7--guidelines-for-reset-handlers)
8.  [CPU specific operations framework](#8--cpu-specific-operations-framework)
9.  [Memory layout of BL images](#9-memory-layout-of-bl-images)
10. [Firmware Image Package (FIP)](#10--firmware-image-package-fip)
11. [Use of coherent memory in Trusted Firmware](#11--use-of-coherent-memory-in-trusted-firmware)
12. [Isolating code and read-only data on separate memory pages](#12--isolating-code-and-read-only-data-on-separate-memory-pages)
13. [Code Structure](#13--code-structure)
14. [References](#14--references)


1.  Introduction
----------------

The ARM Trusted Firmware implements a subset of the Trusted Board Boot
Requirements (TBBR) Platform Design Document (PDD) [1] for ARM reference
platforms. The TBB sequence starts when the platform is powered on and runs up
to the stage where it hands-off control to firmware running in the normal
world in DRAM. This is the cold boot path.

The ARM Trusted Firmware also implements the Power State Coordination Interface
([PSCI]) PDD [2] as a runtime service. PSCI is the interface from normal world
software to firmware implementing power management use-cases (for example,
secondary CPU boot, hotplug and idle). Normal world software can access ARM
Trusted Firmware runtime services via the ARM SMC (Secure Monitor Call)
instruction. The SMC instruction must be used as mandated by the [SMC Calling
Convention PDD][SMCCC] [3].

The ARM Trusted Firmware implements a framework for configuring and managing
interrupts generated in either security state. The details of the interrupt
management framework and its design can be found in [ARM Trusted
Firmware Interrupt Management Design guide][INTRG] [4].

2.  Cold boot
-------------

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):

*   Boot Loader stage 1 (BL1) _AP Trusted ROM_
*   Boot Loader stage 2 (BL2) _Trusted Boot Firmware_
*   Boot Loader stage 3-1 (BL31) _EL3 Runtime Firmware_
*   Boot Loader stage 3-2 (BL32) _Secure-EL1 Payload_ (optional)
*   Boot Loader stage 3-3 (BL33) _Non-trusted Firmware_

ARM development platforms (Fixed Virtual Platforms (FVPs) and Juno) implement a
combination of the following types of memory regions. Each bootloader stage uses
one or more of these memory regions.

*   Regions accessible from both non-secure and secure states. For example,
    non-trusted SRAM, ROM and DRAM.
*   Regions accessible from only the secure state. For example, trusted SRAM and
    ROM. The FVPs also implement the trusted DRAM which is statically
    configured. Additionally, the Base FVPs and Juno development platform
    configure the TrustZone Controller (TZC) to create a region in the DRAM
    which is accessible only from the secure state.


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


### BL1

This stage begins execution from the platform's reset vector at EL3. The reset
address is platform dependent but it is usually located in a Trusted ROM area.
The BL1 data section is copied to trusted SRAM at runtime.

On the ARM development platforms, BL1 code starts execution from the reset
vector defined by the constant `BL1_RO_BASE`. The BL1 data section is copied
to the top of trusted SRAM as defined by the constant `BL1_RW_BASE`.

The functionality implemented by this stage is as follows.

#### Determination of boot path

Whenever a CPU is released from reset, BL1 needs to distinguish between a warm
boot and a cold boot. This is done using platform-specific mechanisms (see the
`plat_get_my_entrypoint()` function in the [Porting Guide]). In the case of a
warm boot, a CPU is expected to continue execution from a separate
entrypoint. In the case of a cold boot, the secondary CPUs are placed in a safe
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.

*   Exception vectors

    BL1 sets up simple exception vectors for both synchronous and asynchronous
    exceptions. The default behavior upon receiving an exception is to populate
    a status code in the general purpose register `X0` and call the
    `plat_report_exception()` function (see the [Porting Guide]). The status
    code is one of:

        0x0 : Synchronous exception from Current EL with SP_EL0
        0x1 : IRQ exception from Current EL with SP_EL0
        0x2 : FIQ exception from Current EL with SP_EL0
        0x3 : System Error exception from Current EL with SP_EL0
        0x4 : Synchronous exception from Current EL with SP_ELx
        0x5 : IRQ exception from Current EL with SP_ELx
        0x6 : FIQ exception from Current EL with SP_ELx
        0x7 : System Error exception from Current EL with SP_ELx
        0x8 : Synchronous exception from Lower EL using aarch64
        0x9 : IRQ exception from Lower EL using aarch64
        0xa : FIQ exception from Lower EL using aarch64
        0xb : System Error exception from Lower EL using aarch64
        0xc : Synchronous exception from Lower EL using aarch32
        0xd : IRQ exception from Lower EL using aarch32
        0xe : FIQ exception from Lower EL using aarch32
        0xf : System Error exception from Lower EL using aarch32

    The `plat_report_exception()` implementation on the ARM FVP port programs
    the Versatile Express System LED register in the following format to
    indicate the occurence of an unexpected exception:

        SYS_LED[0]   - Security state (Secure=0/Non-Secure=1)
        SYS_LED[2:1] - Exception Level (EL3=0x3, EL2=0x2, EL1=0x1, EL0=0x0)
        SYS_LED[7:3] - Exception Class (Sync/Async & origin). This is the value
                       of the status code

    A write to the LED register reflects in the System LEDs (S6LED0..7) in the
    CLCD window of the FVP.

    BL1 does not expect to receive any exceptions other than the SMC exception.
    For the latter, BL1 installs a simple stub. The stub expects to receive a
    limited set of SMC types (determined by their function IDs in the general
    purpose register `X0`):
    -   `BL1_SMC_RUN_IMAGE`: This SMC is raised by BL2 to make BL1 pass control
        to BL31 (loaded by BL2) at EL3.
    -   All SMCs listed in section "BL1 SMC Interface" in the [Firmware Update]
        Design Guide.

    Any other SMC leads to an assertion failure.

*   CPU initialization

    BL1 calls the `reset_handler()` function which in turn calls the CPU
    specific reset handler function (see the section: "CPU specific operations
    framework").

*   Control register setup
    -   `SCTLR_EL3`. Instruction cache is enabled by setting the `SCTLR_EL3.I`
        bit. Alignment and stack alignment checking is enabled by setting the
        `SCTLR_EL3.A` and `SCTLR_EL3.SA` bits. Exception endianness is set to
        little-endian by clearing the `SCTLR_EL3.EE` bit.

    -  `SCR_EL3`. The register width of the next lower exception level is set
        to AArch64 by setting the `SCR.RW` bit. The `SCR.EA` bit is set to trap
        both External Aborts and SError Interrupts in EL3. The `SCR.SIF` bit is
        also set to disable instruction fetches from Non-secure memory when in
        secure state.

    -   `CPTR_EL3`. Accesses to the `CPACR_EL1` register from EL1 or EL2, or the
        `CPTR_EL2` register from EL2 are configured to not trap to EL3 by
        clearing the `CPTR_EL3.TCPAC` bit. Access to the trace functionality is
        configured not to trap to EL3 by clearing the `CPTR_EL3.TTA` bit.
        Instructions that access the registers associated with Floating Point
        and Advanced SIMD execution are configured to not trap to EL3 by
        clearing the `CPTR_EL3.TFP` bit.

    -   `DAIF`. The SError interrupt is enabled by clearing the SError interrupt
        mask bit.

#### Platform initialization

On ARM platforms, BL1 performs the following platform initializations:

*   Enable the Trusted Watchdog.
*   Initialize the console.
*   Configure the Interconnect to enable hardware coherency.
*   Enable the MMU and map the memory it needs to access.
*   Configure any required platform storage to load the next bootloader image
    (BL2).

#### Firmware Update detection and execution

After performing platform setup, BL1 common code calls
`bl1_plat_get_next_image_id()` to determine if [Firmware Update] is required or
to proceed with the normal boot process. If the platform code returns
`BL2_IMAGE_ID` then the normal boot sequence is executed as described in the
next section, else BL1 assumes that [Firmware Update] is required and execution
passes to the first image in the [Firmware Update] process. In either case, BL1
retrieves a descriptor of the next image by calling `bl1_plat_get_image_desc()`.
The image descriptor contains an `entry_point_info_t` structure, which BL1
uses to initialize the execution state of the next image.

#### BL2 image load and execution

In the normal boot flow, BL1 execution continues as follows:

1.  BL1 prints the following string from the primary CPU to indicate successful
    execution of the BL1 stage:

        "Booting Trusted Firmware"

2.  BL1 determines the amount of free trusted SRAM memory available by
    calculating the extent of its own data section, which also resides in
    trusted SRAM. BL1 loads a BL2 raw binary image from platform storage, at a
    platform-specific base address. If the BL2 image file is not present or if
    there is not enough free trusted SRAM the following error message is
    printed:

        "Failed to load BL2 firmware."

    If the load is successful, BL1 updates the limits of the remaining free
    trusted SRAM. It also populates information about the amount of trusted
    SRAM used by the BL2 image. The exact load location of the image is
    provided as a base address in the platform header. Further description of
    the memory layout can be found later in this document.

3.  BL1 passes control to the BL2 image at Secure EL1, starting from its load
    address.

4.  BL1 also passes information about the amount of trusted SRAM used and
    available for use. This information is populated at a platform-specific
    memory address.


### BL2

BL1 loads and passes control to BL2 at Secure-EL1. BL2 is linked against and
loaded at a platform-specific base address (more information can be found later
in this document). The functionality implemented by BL2 is as follows.

#### Architectural initialization

BL2 performs minimal architectural initialization required for subsequent
stages of the ARM Trusted Firmware and normal world software. EL1 and EL0 are
given access to Floating Point & Advanced SIMD registers by clearing the
`CPACR.FPEN` bits.

#### Platform initialization

On ARM platforms, BL2 performs the following platform initializations:

*   Initialize the console.
*   Configure any required platform storage to allow loading further bootloader
    images.
*   Enable the MMU and map the memory it needs to access.
*   Perform platform security setup to allow access to controlled components.
*   Reserve some memory for passing information to the next bootloader image
    (BL31) and populate it.
*   Define the extents of memory available for loading each subsequent
    bootloader image.

#### SCP_BL2 (System Control Processor Firmware) image load

Some systems have a separate System Control Processor (SCP) for power, clock,
reset and system control. BL2 loads the optional SCP_BL2 image from platform
storage into a platform-specific region of secure memory. The subsequent
handling of SCP_BL2 is platform specific. For example, on the Juno ARM
development platform port the image is transferred into SCP's internal memory
using the Boot Over MHU (BOM) protocol after being loaded in the trusted SRAM
memory. The SCP executes SCP_BL2 and signals to the Application Processor (AP)
for BL2 execution to continue.

#### BL31 (EL3 Runtime Firmware) image load

BL2 loads the BL31 image from platform storage into a platform-specific address
in trusted SRAM. If there is not enough memory to load the image or image is
missing it leads to an assertion failure. If the BL31 image loads successfully,
BL2 updates the amount of trusted SRAM used and available for use by BL31.
This information is populated at a platform-specific memory address.

#### BL32 (Secure-EL1 Payload) image load

BL2 loads the optional BL32 image from platform storage into a platform-
specific region of secure memory. The image executes in the secure world. BL2
relies on BL31 to pass control to the BL32 image, if present. Hence, BL2
populates a platform-specific area of memory with the entrypoint/load-address
of the BL32 image. The value of the Saved Processor Status Register (`SPSR`)
for entry into BL32 is not determined by BL2, it is initialized by the
Secure-EL1 Payload Dispatcher (see later) within BL31, which is responsible for
managing interaction with BL32. This information is passed to BL31.

#### BL33 (Non-trusted Firmware) image load

BL2 loads the BL33 image (e.g. UEFI or other test or boot software) from
platform storage into non-secure memory as defined by the platform.

BL2 relies on BL31 to pass control to BL33 once secure state initialization is
complete. Hence, BL2 populates a platform-specific area of memory with the
entrypoint and Saved Program Status Register (`SPSR`) of the normal world
software image. The entrypoint is the load address of the BL33 image. The
`SPSR` is determined as specified in Section 5.13 of the [PSCI PDD] [PSCI]. This
information is passed to BL31.

#### BL31 (EL3 Runtime Firmware) execution

BL2 execution continues as follows:

1.  BL2 passes control back to BL1 by raising an SMC, providing BL1 with the
    BL31 entrypoint. The exception is handled by the SMC exception handler
    installed by BL1.

2.  BL1 turns off the MMU and flushes the caches. It clears the
    `SCTLR_EL3.M/I/C` bits, flushes the data cache to the point of coherency
    and invalidates the TLBs.

3.  BL1 passes control to BL31 at the specified entrypoint at EL3.


### BL31

The image for this stage is loaded by BL2 and BL1 passes control to BL31 at
EL3. BL31 executes solely in trusted SRAM. BL31 is linked against and
loaded at a platform-specific base address (more information can be found later
in this document). The functionality implemented by BL31 is as follows.

#### Architectural initialization

Currently, BL31 performs a similar architectural initialization to BL1 as
far as system register settings are concerned. Since BL1 code resides in ROM,
architectural initialization in BL31 allows override of any previous
initialization done by BL1.

BL31 initializes the per-CPU data framework, which provides a cache of
frequently accessed per-CPU data optimised for fast, concurrent manipulation
on different CPUs. This buffer includes pointers to per-CPU contexts, crash
buffer, CPU reset and power down operations, PSCI data, platform data and so on.

It then replaces the exception vectors populated by BL1 with its own. BL31
exception vectors implement more elaborate support for handling SMCs since this
is the only mechanism to access the runtime services implemented by BL31 (PSCI
for example). BL31 checks each SMC for validity as specified by the
[SMC calling convention PDD][SMCCC] before passing control to the required SMC
handler routine.

BL31 programs the `CNTFRQ_EL0` register with the clock frequency of the system
counter, which is provided by the platform.

#### Platform initialization

BL31 performs detailed platform initialization, which enables normal world
software to function correctly.

On ARM platforms, this consists of the following:

*   Initialize the console.
*   Configure the Interconnect to enable hardware coherency.
*   Enable the MMU and map the memory it needs to access.
*   Initialize the generic interrupt controller.
*   Initialize the power controller device.
*   Detect the system topology.

#### Runtime services initialization

BL31 is responsible for initializing the runtime services. One of them is PSCI.

As part of the PSCI initializations, BL31 detects the system topology. It also
initializes the data structures that implement the state machine used to track
the state of power domain nodes. The state can be one of `OFF`, `RUN` or
`RETENTION`. All secondary CPUs are initially in the `OFF` state. The cluster
that the primary CPU belongs to is `ON`; any other cluster is `OFF`. It also
initializes the locks that protect them. BL31 accesses the state of a CPU or
cluster immediately after reset and before the data cache is enabled in the
warm boot path. It is not currently possible to use 'exclusive' based spinlocks,
therefore BL31 uses locks based on Lamport's Bakery algorithm instead.

The runtime service framework and its initialization is described in more
detail in the "EL3 runtime services framework" section below.

Details about the status of the PSCI implementation are provided in the
"Power State Coordination Interface" section below.

#### BL32 (Secure-EL1 Payload) image initialization

If a BL32 image is present then there must be a matching Secure-EL1 Payload
Dispatcher (SPD) service (see later for details). During initialization
that service must register a function to carry out initialization of BL32
once the runtime services are fully initialized. BL31 invokes such a
registered function to initialize BL32 before running BL33.

Details on BL32 initialization and the SPD's role are described in the
"Secure-EL1 Payloads and Dispatchers" section below.

#### BL33 (Non-trusted Firmware) execution

BL31 initializes the EL2 or EL1 processor context for normal-world cold
boot, ensuring that no secure state information finds its way into the
non-secure execution state. BL31 uses the entrypoint information provided
by BL2 to jump to the Non-trusted firmware image (BL33) at the highest
available Exception Level (EL2 if available, otherwise EL1).

### Using alternative Trusted Boot Firmware in place of BL1 and BL2

Some platforms have existing implementations of Trusted Boot Firmware that
would like to use ARM Trusted Firmware BL31 for the EL3 Runtime Firmware. To
enable this firmware architecture it is important to provide a fully documented
and stable interface between the Trusted Boot Firmware and BL31.

Future changes to the BL31 interface will be done in a backwards compatible
way, and this enables these firmware components to be independently enhanced/
updated to develop and exploit new functionality.

#### Required CPU state when calling `bl31_entrypoint()` during cold boot

This function must only be called by the primary CPU.

On entry to this function the calling primary CPU must be executing in AArch64
EL3, little-endian data access, and all interrupt sources masked:

    PSTATE.EL = 3
    PSTATE.RW = 1
    PSTATE.DAIF = 0xf
    SCTLR_EL3.EE = 0

X0 and X1 can be used to pass information from the Trusted Boot Firmware to the
platform code in BL31:

    X0 : Reserved for common Trusted Firmware information
    X1 : Platform specific information

BL31 zero-init sections (e.g. `.bss`) should not contain valid data on entry,
these will be zero filled prior to invoking platform setup code.

##### Use of the X0 and X1 parameters

The parameters are platform specific and passed from `bl31_entrypoint()` to
`bl31_early_platform_setup()`. The value of these parameters is never directly
used by the common BL31 code.

The convention is that `X0` conveys information regarding the BL31, BL32 and
BL33 images from the Trusted Boot firmware and `X1` can be used for other
platform specific purpose. This convention allows platforms which use ARM
Trusted Firmware's BL1 and BL2 images to transfer additional platform specific
information from Secure Boot without conflicting with future evolution of the
Trusted Firmware using `X0` to pass a `bl31_params` structure.

BL31 common and SPD initialization code depends on image and entrypoint
information about BL33 and BL32, which is provided via BL31 platform APIs.
This information is required until the start of execution of BL33. This
information can be provided in a platform defined manner, e.g. compiled into
the platform code in BL31, or provided in a platform defined memory location
by the Trusted Boot firmware, or passed from the Trusted Boot Firmware via the
Cold boot Initialization parameters. This data may need to be cleaned out of
the CPU caches if it is provided by an earlier boot stage and then accessed by
BL31 platform code before the caches are enabled.

ARM Trusted Firmware's BL2 implementation passes a `bl31_params` structure in
`X0` and the ARM development platforms interpret this in the BL31 platform
code.

##### MMU, Data caches & Coherency

BL31 does not depend on the enabled state of the MMU, data caches or
interconnect coherency on entry to `bl31_entrypoint()`. If these are disabled
on entry, these should be enabled during `bl31_plat_arch_setup()`.

##### Data structures used in the BL31 cold boot interface

These structures are designed to support compatibility and independent
evolution of the structures and the firmware images. For example, a version of
BL31 that can interpret the BL3x image information from different versions of
BL2, a platform that uses an extended entry_point_info structure to convey
additional register information to BL31, or a ELF image loader that can convey
more details about the firmware images.

To support these scenarios the structures are versioned and sized, which enables
BL31 to detect which information is present and respond appropriately. The
`param_header` is defined to capture this information:

    typedef struct param_header {
        uint8_t type;       /* type of the structure */
        uint8_t version;    /* version of this structure */
        uint16_t size;      /* size of this structure in bytes */
        uint32_t attr;      /* attributes: unused bits SBZ */
    } param_header_t;

The structures using this format are `entry_point_info`, `image_info` and
`bl31_params`. The code that allocates and populates these structures must set
the header fields appropriately, and the `SET_PARAM_HEAD()` a macro is defined
to simplify this action.

#### Required CPU state for BL31 Warm boot initialization

When requesting a CPU power-on, or suspending a running CPU, ARM Trusted
Firmware provides the platform power management code with a Warm boot
initialization entry-point, to be invoked by the CPU immediately after the
reset handler. On entry to the Warm boot initialization function the calling
CPU must be in AArch64 EL3, little-endian data access and all interrupt sources
masked:

    PSTATE.EL = 3
    PSTATE.RW = 1
    PSTATE.DAIF = 0xf
    SCTLR_EL3.EE = 0

The PSCI implementation will initialize the processor state and ensure that the
platform power management code is then invoked as required to initialize all
necessary system, cluster and CPU resources.


3.  EL3 runtime services framework
----------------------------------

Software executing in the non-secure state and in the secure state at exception
levels lower than EL3 will request runtime services using the Secure Monitor
Call (SMC) instruction. These requests will follow the convention described in
the SMC Calling Convention PDD ([SMCCC]). The [SMCCC] assigns function
identifiers to each SMC request and describes how arguments are passed and
returned.

The EL3 runtime services framework enables the development of services by
different providers that can be easily integrated into final product firmware.
The following sections describe the framework which facilitates the
registration, initialization and use of runtime services in EL3 Runtime
Firmware (BL31).

The design of the runtime services depends heavily on the concepts and
definitions described in the [SMCCC], in particular SMC Function IDs, Owning
Entity Numbers (OEN), Fast and Standard calls, and the SMC32 and SMC64 calling
conventions. Please refer to that document for more detailed explanation of
these terms.

The following runtime services are expected to be implemented first. They have
not all been instantiated in the current implementation.

1.  Standard service calls

    This service is for management of the entire system. The Power State
    Coordination Interface ([PSCI]) is the first set of standard service calls
    defined by ARM (see PSCI section later).

2.  Secure-EL1 Payload Dispatcher service

    If a system runs a Trusted OS or other Secure-EL1 Payload (SP) then
    it also requires a _Secure Monitor_ at EL3 to switch the EL1 processor
    context between the normal world (EL1/EL2) and trusted world (Secure-EL1).
    The Secure Monitor will make these world switches in response to SMCs. The
    [SMCCC] provides for such SMCs with the Trusted OS Call and Trusted
    Application Call OEN ranges.

    The interface between the EL3 Runtime Firmware and the Secure-EL1 Payload is
    not defined by the [SMCCC] or any other standard. As a result, each
    Secure-EL1 Payload requires a specific Secure Monitor that runs as a runtime
    service - within ARM Trusted Firmware this service is referred to as the
    Secure-EL1 Payload Dispatcher (SPD).

    ARM Trusted Firmware provides a Test Secure-EL1 Payload (TSP) and its
    associated Dispatcher (TSPD). Details of SPD design and TSP/TSPD operation
    are described in the "Secure-EL1 Payloads and Dispatchers" section below.

3.  CPU implementation service

    This service will provide an interface to CPU implementation specific
    services for a given platform e.g. access to processor errata workarounds.
    This service is currently unimplemented.

Additional services for ARM Architecture, SiP and OEM calls can be implemented.
Each implemented service handles a range of SMC function identifiers as
described in the [SMCCC].


### Registration

A runtime service is registered using the `DECLARE_RT_SVC()` macro, specifying
the name of the service, the range of OENs covered, the type of service and
initialization and call handler functions. This macro instantiates a `const
struct rt_svc_desc` for the service with these details (see `runtime_svc.h`).
This structure is allocated in a special ELF section `rt_svc_descs`, enabling
the framework to find all service descriptors included into BL31.

The specific service for a SMC Function is selected based on the OEN and call
type of the Function ID, and the framework uses that information in the service
descriptor to identify the handler for the SMC Call.

The service descriptors do not include information to identify the precise set
of SMC function identifiers supported by this service implementation, the
security state from which such calls are valid nor the capability to support
64-bit and/or 32-bit callers (using SMC32 or SMC64). Responding appropriately
to these aspects of a SMC call is the responsibility of the service
implementation, the framework is focused on integration of services from
different providers and minimizing the time taken by the framework before the
service handler is invoked.

Details of the parameters, requirements and behavior of the initialization and
call handling functions are provided in the following sections.


### Initialization

`runtime_svc_init()` in `runtime_svc.c` initializes the runtime services
framework running on the primary CPU during cold boot as part of the BL31
initialization. This happens prior to initializing a Trusted OS and running
Normal world boot firmware that might in turn use these services.
Initialization involves validating each of the declared runtime service
descriptors, calling the service initialization function and populating the
index used for runtime lookup of the service.

The BL31 linker script collects all of the declared service descriptors into a
single array and defines symbols that allow the framework to locate and traverse
the array, and determine its size.

The framework does basic validation of each descriptor to halt firmware
initialization if service declaration errors are detected. The framework does
not check descriptors for the following error conditions, and may behave in an
unpredictable manner under such scenarios:

1.  Overlapping OEN ranges
2.  Multiple descriptors for the same range of OENs and `call_type`
3.  Incorrect range of owning entity numbers for a given `call_type`

Once validated, the service `init()` callback is invoked. This function carries
out any essential EL3 initialization before servicing requests. The `init()`
function is only invoked on the primary CPU during cold boot. If the service
uses per-CPU data this must either be initialized for all CPUs during this call,
or be done lazily when a CPU first issues an SMC call to that service. If
`init()` returns anything other than `0`, this is treated as an initialization
error and the service is ignored: this does not cause the firmware to halt.

The OEN and call type fields present in the SMC Function ID cover a total of
128 distinct services, but in practice a single descriptor can cover a range of
OENs, e.g. SMCs to call a Trusted OS function. To optimize the lookup of a
service handler, the framework uses an array of 128 indices that map every
distinct OEN/call-type combination either to one of the declared services or to
indicate the service is not handled. This `rt_svc_descs_indices[]` array is
populated for all of the OENs covered by a service after the service `init()`
function has reported success. So a service that fails to initialize will never
have it's `handle()` function invoked.

The following figure shows how the `rt_svc_descs_indices[]` index maps the SMC
Function ID call type and OEN onto a specific service handler in the
`rt_svc_descs[]` array.

![Image 1](diagrams/rt-svc-descs-layout.png?raw=true)


### Handling an SMC

When the EL3 runtime services framework receives a Secure Monitor Call, the SMC
Function ID is passed in W0 from the lower exception level (as per the
[SMCCC]). If the calling register width is AArch32, it is invalid to invoke an
SMC Function which indicates the SMC64 calling convention: such calls are
ignored and return the Unknown SMC Function Identifier result code `0xFFFFFFFF`
in R0/X0.

Bit[31] (fast/standard call) and bits[29:24] (owning entity number) of the SMC
Function ID are combined to index into the `rt_svc_descs_indices[]` array. The
resulting value might indicate a service that has no handler, in this case the
framework will also report an Unknown SMC Function ID. Otherwise, the value is
used as a further index into the `rt_svc_descs[]` array to locate the required
service and handler.

The service's `handle()` callback is provided with five of the SMC parameters
directly, the others are saved into memory for retrieval (if needed) by the
handler. The handler is also provided with an opaque `handle` for use with the
supporting library for parameter retrieval, setting return values and context
manipulation; and with `flags` indicating the security state of the caller. The
framework finally sets up the execution stack for the handler, and invokes the
services `handle()` function.

On return from the handler the result registers are populated in X0-X3 before
restoring the stack and CPU state and returning from the original SMC.


4.  Power State Coordination Interface
--------------------------------------

TODO: Provide design walkthrough of PSCI implementation.

The PSCI v1.0 specification categorizes APIs as optional and mandatory. All the
mandatory APIs in PSCI v1.0 and all the APIs in PSCI v0.2 draft specification
[Power State Coordination Interface PDD] [PSCI] are implemented. The table lists
the PSCI v1.0 APIs and their support in generic code.

An API implementation might have a dependency on platform code e.g. CPU_SUSPEND
requires the platform to export a part of the implementation. Hence the level
of support of the mandatory APIs depends upon the support exported by the
platform port as well. The Juno and FVP (all variants) platforms export all the
required support.

| PSCI v1.0 API         |Supported| Comments                                  |
|:----------------------|:--------|:------------------------------------------|
|`PSCI_VERSION`         | Yes     | The version returned is 1.0               |
|`CPU_SUSPEND`          | Yes*    |                                           |
|`CPU_OFF`              | Yes*    |                                           |
|`CPU_ON`               | Yes*    |                                           |
|`AFFINITY_INFO`        | Yes     |                                           |
|`MIGRATE`              | Yes**   |                                           |
|`MIGRATE_INFO_TYPE`    | Yes**   |                                           |
|`MIGRATE_INFO_CPU`     | Yes**   |                                           |
|`SYSTEM_OFF`           | Yes*    |                                           |
|`SYSTEM_RESET`         | Yes*    |                                           |
|`PSCI_FEATURES`        | Yes     |                                           |
|`CPU_FREEZE`           | No      |                                           |
|`CPU_DEFAULT_SUSPEND`  | No      |                                           |
|`NODE_HW_STATE`        | Yes*    |                                           |
|`SYSTEM_SUSPEND`       | Yes*    |                                           |
|`PSCI_SET_SUSPEND_MODE`| No      |                                           |
|`PSCI_STAT_RESIDENCY`  | Yes*    |                                           |
|`PSCI_STAT_COUNT`      | Yes*    |                                           |

*Note : These PSCI APIs require platform power management hooks to be
registered with the generic PSCI code to be supported.

**Note : These PSCI APIs require appropriate Secure Payload Dispatcher
hooks to be registered with the generic PSCI code to be supported.


5.  Secure-EL1 Payloads and Dispatchers
---------------------------------------

On a production system that includes a Trusted OS running in Secure-EL1/EL0,
the Trusted OS is coupled with a companion runtime service in the BL31
firmware. This service is responsible for the initialisation of the Trusted
OS and all communications with it. The Trusted OS is the BL32 stage of the
boot flow in ARM Trusted Firmware. The firmware will attempt to locate, load
and execute a BL32 image.

ARM Trusted Firmware uses a more general term for the BL32 software that runs
at Secure-EL1 - the _Secure-EL1 Payload_ - as it is not always a Trusted OS.

The ARM Trusted Firmware provides a Test Secure-EL1 Payload (TSP) and a Test
Secure-EL1 Payload Dispatcher (TSPD) service as an example of how a Trusted OS
is supported on a production system using the Runtime Services Framework. On
such a system, the Test BL32 image and service are replaced by the Trusted OS
and its dispatcher service. The ARM Trusted Firmware build system expects that
the dispatcher will define the build flag `NEED_BL32` to enable it to include
the BL32 in the build either as a binary or to compile from source depending
on whether the `BL32` build option is specified or not.

The TSP runs in Secure-EL1. It is designed to demonstrate synchronous
communication with the normal-world software running in EL1/EL2. Communication
is initiated by the normal-world software

*   either directly through a Fast SMC (as defined in the [SMCCC])

*   or indirectly through a [PSCI] SMC. The [PSCI] implementation in turn
    informs the TSPD about the requested power management operation. This allows
    the TSP to prepare for or respond to the power state change

The TSPD service is responsible for.

*   Initializing the TSP

*   Routing requests and responses between the secure and the non-secure
    states during the two types of communications just described

### Initializing a BL32 Image

The Secure-EL1 Payload Dispatcher (SPD) service is responsible for initializing
the BL32 image. It needs access to the information passed by BL2 to BL31 to do
so. This is provided by:

    entry_point_info_t *bl31_plat_get_next_image_ep_info(uint32_t);

which returns a reference to the `entry_point_info` structure corresponding to
the image which will be run in the specified security state. The SPD uses this
API to get entry point information for the SECURE image, BL32.

In the absence of a BL32 image, BL31 passes control to the normal world
bootloader image (BL33). When the BL32 image is present, it is typical
that the SPD wants control to be passed to BL32 first and then later to BL33.

To do this the SPD has to register a BL32 initialization function during
initialization of the SPD service. The BL32 initialization function has this
prototype:

    int32_t init(void);

and is registered using the `bl31_register_bl32_init()` function.

Trusted Firmware supports two approaches for the SPD to pass control to BL32
before returning through EL3 and running the non-trusted firmware (BL33):

1.  In the BL32 setup function, use `bl31_set_next_image_type()` to
    request that the exit from `bl31_main()` is to the BL32 entrypoint in
    Secure-EL1. BL31 will exit to BL32 using the asynchronous method by
    calling `bl31_prepare_next_image_entry()` and `el3_exit()`.

    When the BL32 has completed initialization at Secure-EL1, it returns to
    BL31 by issuing an SMC, using a Function ID allocated to the SPD. On
    receipt of this SMC, the SPD service handler should switch the CPU context
    from trusted to normal world and use the `bl31_set_next_image_type()` and
    `bl31_prepare_next_image_entry()` functions to set up the initial return to
    the normal world firmware BL33. On return from the handler the framework
    will exit to EL2 and run BL33.

2.  The BL32 setup function registers an initialization function using
    `bl31_register_bl32_init()` which provides a SPD-defined mechanism to
    invoke a 'world-switch synchronous call' to Secure-EL1 to run the BL32
    entrypoint.
    NOTE: The Test SPD service included with the Trusted Firmware provides one
    implementation of such a mechanism.

    On completion BL32 returns control to BL31 via a SMC, and on receipt the
    SPD service handler invokes the synchronous call return mechanism to return
    to the BL32 initialization function. On return from this function,
    `bl31_main()` will set up the return to the normal world firmware BL33 and
    continue the boot process in the normal world.


6.  Crash Reporting in BL31
----------------------------

BL31 implements a scheme for reporting the processor state when an unhandled
exception is encountered. The reporting mechanism attempts to preserve all the
register contents and report it via a dedicated UART (PL011 console). BL31
reports the general purpose, EL3, Secure EL1 and some EL2 state registers.

A dedicated per-CPU crash stack is maintained by BL31 and this is retrieved via
the per-CPU pointer cache. The implementation attempts to minimise the memory
required for this feature. The file `crash_reporting.S` contains the
implementation for crash reporting.

The sample crash output is shown below.

    x0	:0x000000004F00007C
    x1	:0x0000000007FFFFFF
    x2	:0x0000000004014D50
    x3	:0x0000000000000000
    x4	:0x0000000088007998
    x5	:0x00000000001343AC
    x6	:0x0000000000000016
    x7	:0x00000000000B8A38
    x8	:0x00000000001343AC
    x9	:0x00000000000101A8
    x10	:0x0000000000000002
    x11	:0x000000000000011C
    x12	:0x00000000FEFDC644
    x13	:0x00000000FED93FFC
    x14	:0x0000000000247950
    x15	:0x00000000000007A2
    x16	:0x00000000000007A4
    x17	:0x0000000000247950
    x18	:0x0000000000000000
    x19	:0x00000000FFFFFFFF
    x20	:0x0000000004014D50
    x21	:0x000000000400A38C
    x22	:0x0000000000247950
    x23	:0x0000000000000010
    x24	:0x0000000000000024
    x25	:0x00000000FEFDC868
    x26	:0x00000000FEFDC86A
    x27	:0x00000000019EDEDC
    x28	:0x000000000A7CFDAA
    x29	:0x0000000004010780
    x30	:0x000000000400F004
    scr_el3	:0x0000000000000D3D
    sctlr_el3	:0x0000000000C8181F
    cptr_el3	:0x0000000000000000
    tcr_el3	:0x0000000080803520
    daif	:0x00000000000003C0
    mair_el3	:0x00000000000004FF
    spsr_el3	:0x00000000800003CC
    elr_el3	:0x000000000400C0CC
    ttbr0_el3	:0x00000000040172A0
    esr_el3	:0x0000000096000210
    sp_el3	:0x0000000004014D50
    far_el3	:0x000000004F00007C
    spsr_el1	:0x0000000000000000
    elr_el1	:0x0000000000000000
    spsr_abt	:0x0000000000000000
    spsr_und	:0x0000000000000000
    spsr_irq	:0x0000000000000000
    spsr_fiq	:0x0000000000000000
    sctlr_el1	:0x0000000030C81807
    actlr_el1	:0x0000000000000000
    cpacr_el1	:0x0000000000300000
    csselr_el1	:0x0000000000000002
    sp_el1	:0x0000000004028800
    esr_el1	:0x0000000000000000
    ttbr0_el1	:0x000000000402C200
    ttbr1_el1	:0x0000000000000000
    mair_el1	:0x00000000000004FF
    amair_el1	:0x0000000000000000
    tcr_el1	:0x0000000000003520
    tpidr_el1	:0x0000000000000000
    tpidr_el0	:0x0000000000000000
    tpidrro_el0	:0x0000000000000000
    dacr32_el2	:0x0000000000000000
    ifsr32_el2	:0x0000000000000000
    par_el1	:0x0000000000000000
    far_el1	:0x0000000000000000
    afsr0_el1	:0x0000000000000000
    afsr1_el1	:0x0000000000000000
    contextidr_el1	:0x0000000000000000
    vbar_el1	:0x0000000004027000
    cntp_ctl_el0	:0x0000000000000000
    cntp_cval_el0	:0x0000000000000000
    cntv_ctl_el0	:0x0000000000000000
    cntv_cval_el0	:0x0000000000000000
    cntkctl_el1	:0x0000000000000000
    fpexc32_el2	:0x0000000004000700
    sp_el0	:0x0000000004010780

7.  Guidelines for Reset Handlers
---------------------------------

Trusted Firmware implements a framework that allows CPU and platform ports to
perform actions very early after a CPU is released from reset in both the cold
and warm boot paths. This is done by calling the `reset_handler()` function in
both the BL1 and BL31 images. It in turn calls the platform and CPU specific
reset handling functions.

Details for implementing a CPU specific reset handler can be found in
Section 8. Details for implementing a platform specific reset handler can be
found in the [Porting Guide](see the `plat_reset_handler()` function).

When adding functionality to a reset handler, keep in mind that if a different
reset handling behavior is required between the first and the subsequent
invocations of the reset handling code, this should be detected at runtime.
In other words, the reset handler should be able to detect whether an action has
already been performed and act as appropriate. Possible courses of actions are,
e.g. skip the action the second time, or undo/redo it.

8.  CPU specific operations framework
-----------------------------

Certain aspects of the ARMv8 architecture are implementation defined,
that is, certain behaviours are not architecturally defined, but must be defined
and documented by individual processor implementations. The ARM Trusted
Firmware implements a framework which categorises the common implementation
defined behaviours and allows a processor to export its implementation of that
behaviour. The categories are:

1.  Processor specific reset sequence.

2.  Processor specific power down sequences.

3.  Processor specific register dumping as a part of crash reporting.

Each of the above categories fulfils a different requirement.

1.  allows any processor specific initialization before the caches and MMU
    are turned on, like implementation of errata workarounds, entry into
    the intra-cluster coherency domain etc.

2.  allows each processor to implement the power down sequence mandated in
    its Technical Reference Manual (TRM).

3.  allows a processor to provide additional information to the developer
    in the event of a crash, for example Cortex-A53 has registers which
    can expose the data cache contents.

Please note that only 2. is mandated by the TRM.

The CPU specific operations framework scales to accommodate a large number of
different CPUs during power down and reset handling. The platform can specify
any CPU optimization it wants to enable for each CPU. It can also specify
the CPU errata workarounds to be applied for each CPU type during reset
handling by defining CPU errata compile time macros. Details on these macros
can be found in the [cpu-specific-build-macros.md][CPUBM] file.

The CPU specific operations framework depends on the `cpu_ops` structure which
needs to be exported for each type of CPU in the platform. It is defined in
`include/lib/cpus/aarch64/cpu_macros.S` and has the following fields : `midr`,
`reset_func()`, `core_pwr_dwn()`, `cluster_pwr_dwn()` and `cpu_reg_dump()`.

The CPU specific files in `lib/cpus` export a `cpu_ops` data structure with
suitable handlers for that CPU.  For example, `lib/cpus/aarch64/cortex_a53.S`
exports the `cpu_ops` for Cortex-A53 CPU. According to the platform
configuration, these CPU specific files must be included in the build by
the platform makefile. The generic CPU specific operations framework code exists
in `lib/cpus/aarch64/cpu_helpers.S`.

### CPU specific Reset Handling

After a reset, the state of the CPU when it calls generic reset handler is:
MMU turned off, both instruction and data caches turned off and not part
of any coherency domain.

The BL entrypoint code first invokes the `plat_reset_handler()` to allow
the platform to perform any system initialization required and any system
errata workarounds that needs to be applied. The `get_cpu_ops_ptr()` reads
the current CPU midr, finds the matching `cpu_ops` entry in the `cpu_ops`
array and returns it. Note that only the part number and implementer fields
in midr are used to find the matching `cpu_ops` entry. The `reset_func()` in
the returned `cpu_ops` is then invoked which executes the required reset
handling for that CPU and also any errata workarounds enabled by the platform.
This function must preserve the values of general purpose registers x20 to x29.

Refer to Section "Guidelines for Reset Handlers" for general guidelines
regarding placement of code in a reset handler.

### CPU specific power down sequence

During the BL31 initialization sequence, the pointer to the matching `cpu_ops`
entry is stored in per-CPU data by `init_cpu_ops()` so that it can be quickly
retrieved during power down sequences.

The PSCI service, upon receiving a power down request, determines the highest
power level at which to execute power down sequence for a particular CPU and
invokes the corresponding 'prepare' power down handler in the CPU specific
operations framework. For example, when a CPU executes a power down for power
level 0, the `prepare_core_pwr_dwn()` retrieves the `cpu_ops` pointer from the
per-CPU data and the corresponding `core_pwr_dwn()` is invoked. Similarly when
a CPU executes power down at power level 1, the `prepare_cluster_pwr_dwn()`
retrieves the `cpu_ops` pointer and the corresponding `cluster_pwr_dwn()` is
invoked.

At runtime the platform hooks for power down are invoked by the PSCI service to
perform platform specific operations during a power down sequence, for example
turning off CCI coherency during a cluster power down.

### CPU specific register reporting during crash

If the crash reporting is enabled in BL31, when a crash occurs, the crash
reporting framework calls `do_cpu_reg_dump` which retrieves the matching
`cpu_ops` using `get_cpu_ops_ptr()` function. The `cpu_reg_dump()` in
`cpu_ops` is invoked, which then returns the CPU specific register values to
be reported and a pointer to the ASCII list of register names in a format
expected by the crash reporting framework.


9. Memory layout of BL images
-----------------------------

Each bootloader image can be divided in 2 parts:

 *    the static contents of the image. These are data actually stored in the
      binary on the disk. In the ELF terminology, they are called `PROGBITS`
      sections;

 *    the run-time contents of the image. These are data that don't occupy any
      space in the binary on the disk. The ELF binary just contains some
      metadata indicating where these data will be stored at run-time and the
      corresponding sections need to be allocated and initialized at run-time.
      In the ELF terminology, they are called `NOBITS` sections.

All PROGBITS sections are grouped together at the beginning of the image,
followed by all NOBITS sections. This is true for all Trusted Firmware images
and it is governed by the linker scripts. This ensures that the raw binary
images are as small as possible. If a NOBITS section was inserted in between
PROGBITS sections then the resulting binary file would contain zero bytes in
place of this NOBITS section, making the image unnecessarily bigger. Smaller
images allow faster loading from the FIP to the main memory.

### Linker scripts and symbols

Each bootloader stage image layout is described by its own linker script. The
linker scripts export some symbols into the program symbol table. Their values
correspond to particular addresses. The trusted firmware code can refer to these
symbols to figure out the image memory layout.

Linker symbols follow the following naming convention in the trusted firmware.

*   `__<SECTION>_START__`

    Start address of a given section named `<SECTION>`.

*   `__<SECTION>_END__`

    End address of a given section named `<SECTION>`. If there is an alignment
    constraint on the section's end address then `__<SECTION>_END__` corresponds
    to the end address of the section's actual contents, rounded up to the right
    boundary. Refer to the value of `__<SECTION>_UNALIGNED_END__`  to know the
    actual end address of the section's contents.

*   `__<SECTION>_UNALIGNED_END__`

    End address of a given section named `<SECTION>` without any padding or
    rounding up due to some alignment constraint.

*   `__<SECTION>_SIZE__`

    Size (in bytes) of a given section named `<SECTION>`. If there is an
    alignment constraint on the section's end address then `__<SECTION>_SIZE__`
    corresponds to the size of the section's actual contents, rounded up to the
    right boundary. In other words, `__<SECTION>_SIZE__ = __<SECTION>_END__ -
    _<SECTION>_START__`. Refer to the value of `__<SECTION>_UNALIGNED_SIZE__`
    to know the actual size of the section's contents.

*   `__<SECTION>_UNALIGNED_SIZE__`

    Size (in bytes) of a given section named `<SECTION>` without any padding or
    rounding up due to some alignment constraint. In other words,
    `__<SECTION>_UNALIGNED_SIZE__ = __<SECTION>_UNALIGNED_END__ -
    __<SECTION>_START__`.

Some of the linker symbols are mandatory as the trusted firmware code relies on
them to be defined. They are listed in the following subsections. Some of them
must be provided for each bootloader stage and some are specific to a given
bootloader stage.

The linker scripts define some extra, optional symbols. They are not actually
used by any code but they help in understanding the bootloader images' memory
layout as they are easy to spot in the link map files.

#### Common linker symbols

All BL images share the following requirements:

*   The BSS section must be zero-initialised before executing any C code.
*   The coherent memory section (if enabled) must be zero-initialised as well.
*   The MMU setup code needs to know the extents of the coherent and read-only
    memory regions to set the right memory attributes. When
    `SEPARATE_CODE_AND_RODATA=1`, it needs to know more specifically how the
    read-only memory region is divided between code and data.

The following linker symbols are defined for this purpose:

*   `__BSS_START__`          Must be aligned on a 16-byte boundary.
*   `__BSS_SIZE__`
*   `__COHERENT_RAM_START__` Must be aligned on a page-size boundary.
*   `__COHERENT_RAM_END__`   Must be aligned on a page-size boundary.
*   `__COHERENT_RAM_UNALIGNED_SIZE__`
*   `__RO_START__`
*   `__RO_END__`
*   `__TEXT_START__`
*   `__TEXT_END__`
*   `__RODATA_START__`
*   `__RODATA_END__`

#### BL1's linker symbols

BL1 being the ROM image, it has additional requirements. BL1 resides in ROM and
it is entirely executed in place but it needs some read-write memory for its
mutable data. Its `.data` section (i.e. its allocated read-write data) must be
relocated from ROM to RAM before executing any C code.

The following additional linker symbols are defined for BL1:

*   `__BL1_ROM_END__`    End address of BL1's ROM contents, covering its code
                         and `.data` section in ROM.
*   `__DATA_ROM_START__` Start address of the `.data` section in ROM. Must be
                         aligned on a 16-byte boundary.
*   `__DATA_RAM_START__` Address in RAM where the `.data` section should be
                         copied over. Must be aligned on a 16-byte boundary.
*   `__DATA_SIZE__`      Size of the `.data` section (in ROM or RAM).
*   `__BL1_RAM_START__`  Start address of BL1 read-write data.
*   `__BL1_RAM_END__`    End address of BL1 read-write data.


### How to choose the right base addresses for each bootloader stage image

There is currently no support for dynamic image loading in the Trusted Firmware.
This means that all bootloader images need to be linked against their ultimate
runtime locations and the base addresses of each image must be chosen carefully
such that images don't overlap each other in an undesired way. As the code
grows, the base addresses might need adjustments to cope with the new memory
layout.

The memory layout is completely specific to the platform and so there is no
general recipe for choosing the right base addresses for each bootloader image.
However, there are tools to aid in understanding the memory layout. These are
the link map files: `build/<platform>/<build-type>/bl<x>/bl<x>.map`, with `<x>`
being the stage bootloader. They provide a detailed view of the memory usage of
each image. Among other useful information, they provide the end address of
each image.

* `bl1.map` link map file provides `__BL1_RAM_END__` address.
* `bl2.map` link map file provides `__BL2_END__` address.
* `bl31.map` link map file provides `__BL31_END__` address.
* `bl32.map` link map file provides `__BL32_END__` address.

For each bootloader image, the platform code must provide its start address
as well as a limit address that it must not overstep. The latter is used in the
linker scripts to check that the image doesn't grow past that address. If that
happens, the linker will issue a message similar to the following:

    aarch64-none-elf-ld: BLx has exceeded its limit.

Additionally, if the platform memory layout implies some image overlaying like
on FVP, BL31 and TSP need to know the limit address that their PROGBITS
sections must not overstep. The platform code must provide those.

Trusted Firmware provides a mechanism to verify at boot time that the memory
to load a new image is free to prevent overwriting a previously loaded image.
For this mechanism to work, the platform must specify the memory available in
the system as regions, where each region consists of base address, total size
and the free area within it (as defined in the `meminfo_t` structure). Trusted
Firmware retrieves these memory regions by calling the corresponding platform
API:

*   `meminfo_t *bl1_plat_sec_mem_layout(void)`
*   `meminfo_t *bl2_plat_sec_mem_layout(void)`
*   `void bl2_plat_get_scp_bl2_meminfo(meminfo_t *scp_bl2_meminfo)`
*   `void bl2_plat_get_bl32_meminfo(meminfo_t *bl32_meminfo)`
*   `void bl2_plat_get_bl33_meminfo(meminfo_t *bl33_meminfo)`

For example, in the case of BL1 loading BL2, `bl1_plat_sec_mem_layout()` will
return the region defined by the platform where BL1 intends to load BL2. The
`load_image()` function will check that the memory where BL2 will be loaded is
within the specified region and marked as free.

The actual number of regions and their base addresses and sizes is platform
specific. The platform may return the same region or define a different one for
each API. However, the overlap verification mechanism applies only to a single
region. Hence, it is the platform responsibility to guarantee that different
regions do not overlap, or that if they do, the overlapping images are not
accessed at the same time. This could be used, for example, to load temporary
images (e.g. certificates) or firmware images prior to being transfered to its
corresponding processor (e.g. the SCP BL2 image).

To reduce fragmentation and simplify the tracking of free memory, all the free
memory within a region is always located in one single buffer defined by its
base address and size. Trusted Firmware implements a top/bottom load approach:
after a new image is loaded, it checks how much memory remains free above and
below the image. The smallest area is marked as unavailable, while the larger
area becomes the new free memory buffer. Platforms should take this behaviour
into account when defining the base address for each of the images. For example,
if an image is loaded near the middle of the region, small changes in image size
could cause a flip between a top load and a bottom load, which may result in an
unexpected memory layout.

The following diagram is an example of an image loaded in the bottom part of
the memory region. The region is initially free (nothing has been loaded yet):

               Memory region
               +----------+
               |          |
               |          |  <<<<<<<<<<<<<  Free
               |          |
               |----------|                 +------------+
               |  image   |  <<<<<<<<<<<<<  |   image    |
               |----------|                 +------------+
               | xxxxxxxx |  <<<<<<<<<<<<<  Marked as unavailable
               +----------+

And the following diagram is an example of an image loaded in the top part:

               Memory region
               +----------+
               | xxxxxxxx |  <<<<<<<<<<<<<  Marked as unavailable
               |----------|                 +------------+
               |  image   |  <<<<<<<<<<<<<  |   image    |
               |----------|                 +------------+
               |          |
               |          |  <<<<<<<<<<<<<  Free
               |          |
               +----------+


####  Memory layout on ARM development platforms

The following list describes the memory layout on the ARM development platforms:

*   A 4KB page of shared memory is used for communication between Trusted
    Firmware and the platform's power controller. This is located at the base of
    Trusted SRAM. The amount of Trusted SRAM available to load the bootloader
    images is reduced by the size of the shared memory.

    The shared memory is used to store the CPUs' entrypoint mailbox. On Juno,
    this is also used for the MHU payload when passing messages to and from the
    SCP.

*   On FVP, BL1 is originally sitting in the Trusted ROM at address `0x0`. On
    Juno, BL1 resides in flash memory at address `0x0BEC0000`. BL1 read-write
    data are relocated to the top of Trusted SRAM at runtime.

*   BL31 is loaded at the top of the Trusted SRAM, such that its NOBITS
    sections will overwrite BL1 R/W data. This implies that BL1 global variables
    remain valid only until execution reaches the BL31 entry point during
    a cold boot.

*   BL2 is loaded below BL31.

*   On Juno, SCP_BL2 is loaded temporarily into the BL31 memory region and
    transfered to the SCP before being overwritten by BL31.

*   BL32 can be loaded in one of the following locations:

    *   Trusted SRAM
    *   Trusted DRAM (FVP only)
    *   Secure region of DRAM (top 16MB of DRAM configured by the TrustZone
        controller)

    When BL32 is loaded into Trusted SRAM, its NOBITS sections are allowed to
    overlay BL2. This memory layout is designed to give the BL32 image as much
    memory as possible when it is loaded into Trusted SRAM.

The memory regions for the overlap detection mechanism at boot time are
defined as follows (shown per API):

*   `meminfo_t *bl1_plat_sec_mem_layout(void)`

    This region corresponds to the whole Trusted SRAM except for the shared
    memory at the base. This region is initially free. At boot time, BL1 will
    mark the BL1(rw) section within this region as occupied. The BL1(rw) section
    is placed at the top of Trusted SRAM.

*   `meminfo_t *bl2_plat_sec_mem_layout(void)`

    This region corresponds to the whole Trusted SRAM as defined by
    `bl1_plat_sec_mem_layout()`, but with the BL1(rw) section marked as
    occupied. This memory region is used to check that BL2 and BL31 do not
    overlap with each other. BL2_BASE and BL1_RW_BASE are carefully chosen so
    that the memory for BL31 is top loaded above BL2.

*   `void bl2_plat_get_scp_bl2_meminfo(meminfo_t *scp_bl2_meminfo)`

    This region is an exact copy of the region defined by
    `bl2_plat_sec_mem_layout()`. Being a disconnected copy means that all the
    changes made to this region by the Trusted Firmware will not be propagated.
    This approach is valid because the SCP BL2 image is loaded temporarily
    while it is being transferred to the SCP, so this memory is reused
    afterwards.

*   `void bl2_plat_get_bl32_meminfo(meminfo_t *bl32_meminfo)`

    This region depends on the location of the BL32 image. Currently, ARM
    platforms support three different locations (detailed below): Trusted SRAM,
    Trusted DRAM and the TZC-Secured DRAM.

*   `void bl2_plat_get_bl33_meminfo(meminfo_t *bl33_meminfo)`

    This region corresponds to the Non-Secure DDR-DRAM, excluding the
    TZC-Secured area.

The location of the BL32 image will result in different memory maps. This is
illustrated for both FVP and Juno in the following diagrams, using the TSP as
an example.

Note: Loading the BL32 image in TZC secured DRAM doesn't change the memory
layout of the other images in Trusted SRAM.

**FVP with TSP in Trusted SRAM (default option):**

               Trusted SRAM
    0x04040000 +----------+  loaded by BL2  ------------------
               | BL1 (rw) |  <<<<<<<<<<<<<  |  BL31 NOBITS   |
               |----------|  <<<<<<<<<<<<<  |----------------|
               |          |  <<<<<<<<<<<<<  | BL31 PROGBITS  |
               |----------|                 ------------------
               |   BL2    |  <<<<<<<<<<<<<  |  BL32 NOBITS   |
               |----------|  <<<<<<<<<<<<<  |----------------|
               |          |  <<<<<<<<<<<<<  | BL32 PROGBITS  |
    0x04001000 +----------+                 ------------------
               |  Shared  |
    0x04000000 +----------+

               Trusted ROM
    0x04000000 +----------+
               | BL1 (ro) |
    0x00000000 +----------+


**FVP with TSP in Trusted DRAM:**

               Trusted DRAM
    0x08000000 +----------+
               |  BL32   |
    0x06000000 +----------+

               Trusted SRAM
    0x04040000 +----------+  loaded by BL2  ------------------
               | BL1 (rw) |  <<<<<<<<<<<<<  |  BL31 NOBITS   |
               |----------|  <<<<<<<<<<<<<  |----------------|
               |          |  <<<<<<<<<<<<<  | BL31 PROGBITS  |
               |----------|                 ------------------
               |   BL2    |
               |----------|
               |          |
    0x04001000 +----------+
               |  Shared  |
    0x04000000 +----------+

               Trusted ROM
    0x04000000 +----------+
               | BL1 (ro) |
    0x00000000 +----------+

**FVP with TSP in TZC-Secured DRAM:**

                   DRAM
    0xffffffff +----------+
               |  BL32   |  (secure)
    0xff000000 +----------+
               |          |
               :          :  (non-secure)
               |          |
    0x80000000 +----------+

               Trusted SRAM
    0x04040000 +----------+  loaded by BL2  ------------------
               | BL1 (rw) |  <<<<<<<<<<<<<  |  BL31 NOBITS   |
               |----------|  <<<<<<<<<<<<<  |----------------|
               |          |  <<<<<<<<<<<<<  | BL31 PROGBITS  |
               |----------|                 ------------------
               |   BL2    |
               |----------|
               |          |
    0x04001000 +----------+
               |  Shared  |
    0x04000000 +----------+

               Trusted ROM
    0x04000000 +----------+
               | BL1 (ro) |
    0x00000000 +----------+


**Juno with BL32 in Trusted SRAM (default option):**

                  Flash0
    0x0C000000 +----------+
               :          :
    0x0BED0000 |----------|
               | BL1 (ro) |
    0x0BEC0000 |----------|
               :          :
    0x08000000 +----------+                  BL31 is loaded
                                             after SCP_BL2 has
               Trusted SRAM                  been sent to SCP
    0x04040000 +----------+  loaded by BL2  ------------------
               | BL1 (rw) |  <<<<<<<<<<<<<  |  BL31 NOBITS   |
               |----------|  <<<<<<<<<<<<<  |----------------|
               | SCP_BL2  |  <<<<<<<<<<<<<  | BL31 PROGBITS  |
               |----------|                 ------------------
               |   BL2    |  <<<<<<<<<<<<<  |  BL32 NOBITS   |
               |----------|  <<<<<<<<<<<<<  |----------------|
               |          |  <<<<<<<<<<<<<  | BL32 PROGBITS  |
    0x04001000 +----------+                 ------------------
               |   MHU    |
    0x04000000 +----------+


**Juno with BL32 in TZC-secured DRAM:**

                   DRAM
    0xFFE00000 +----------+
               |  BL32   |  (secure)
    0xFF000000 |----------|
               |          |
               :          :  (non-secure)
               |          |
    0x80000000 +----------+

                  Flash0
    0x0C000000 +----------+
               :          :
    0x0BED0000 |----------|
               | BL1 (ro) |
    0x0BEC0000 |----------|
               :          :
    0x08000000 +----------+                  BL31 is loaded
                                             after SCP_BL2 has
               Trusted SRAM                  been sent to SCP
    0x04040000 +----------+  loaded by BL2  ------------------
               | BL1 (rw) |  <<<<<<<<<<<<<  |  BL31 NOBITS   |
               |----------|  <<<<<<<<<<<<<  |----------------|
               | SCP_BL2  |  <<<<<<<<<<<<<  | BL31 PROGBITS  |
               |----------|                 ------------------
               |   BL2    |
               |----------|
               |          |
    0x04001000 +----------+
               |   MHU    |
    0x04000000 +----------+


10.  Firmware Image Package (FIP)
---------------------------------

Using a Firmware Image Package (FIP) allows for packing bootloader images (and
potentially other payloads) into a single archive that can be loaded by the ARM
Trusted Firmware from non-volatile platform storage. A driver to load images
from a FIP has been added to the storage layer and allows a package to be read
from supported platform storage. A tool to create Firmware Image Packages is
also provided and described below.

### Firmware Image Package layout

The FIP layout consists of a table of contents (ToC) followed by payload data.
The ToC itself has a header followed by one or more table entries. The ToC is
terminated by an end marker entry. All ToC entries describe some payload data
that has been appended to the end of the binary package. With the information
provided in the ToC entry the corresponding payload data can be retrieved.

    ------------------
    | ToC Header     |
    |----------------|
    | ToC Entry 0    |
    |----------------|
    | ToC Entry 1    |
    |----------------|
    | ToC End Marker |
    |----------------|
    |                |
    |     Data 0     |
    |                |
    |----------------|
    |                |
    |     Data 1     |
    |                |
    ------------------

The ToC header and entry formats are described in the header file
`include/firmware_image_package.h`. This file is used by both the tool and the
ARM Trusted firmware.

The ToC header has the following fields:

    `name`: The name of the ToC. This is currently used to validate the header.
    `serial_number`: A non-zero number provided by the creation tool
    `flags`: Flags associated with this data.
        Bits 0-13: Reserved
        Bits 32-47: Platform defined
        Bits 48-63: Reserved

A ToC entry has the following fields:

    `uuid`: All files are referred to by a pre-defined Universally Unique
        IDentifier [UUID] . The UUIDs are defined in
        `include/firmware_image_package`. The platform translates the requested
        image name into the corresponding UUID when accessing the package.
    `offset_address`: The offset address at which the corresponding payload data
        can be found. The offset is calculated from the ToC base address.
    `size`: The size of the corresponding payload data in bytes.
    `flags`: Flags associated with this entry. Non are yet defined.

### Firmware Image Package creation tool

The FIP creation tool can be used to pack specified images into a binary package
that can be loaded by the ARM Trusted Firmware from platform storage. The tool
currently only supports packing bootloader images. Additional image definitions
can be added to the tool as required.

The tool can be found in `tools/fiptool`.

### Loading from a Firmware Image Package (FIP)

The Firmware Image Package (FIP) driver can load images from a binary package on
non-volatile platform storage. For the ARM development platforms, this is
currently NOR FLASH.

Bootloader images are loaded according to the platform policy as specified by
the function `plat_get_image_source()`. For the ARM development platforms, this
means the platform will attempt to load images from a Firmware Image Package
located at the start of NOR FLASH0.

The ARM development platforms' policy is to only allow loading of a known set of
images. The platform policy can be modified to allow additional images.


11.  Use of coherent memory in Trusted Firmware
-----------------------------------------------

There might be loss of coherency when physical memory with mismatched
shareability, cacheability and memory attributes is accessed by multiple CPUs
(refer to section B2.9 of [ARM ARM] for more details). This possibility occurs
in Trusted Firmware during power up/down sequences when coherency, MMU and
caches are turned on/off incrementally.

Trusted Firmware defines coherent memory as a region of memory with Device
nGnRE attributes in the translation tables. The translation granule size in
Trusted Firmware is 4KB. This is the smallest possible size of the coherent
memory region.

By default, all data structures which are susceptible to accesses with
mismatched attributes from various CPUs are allocated in a coherent memory
region (refer to section 2.1 of [Porting Guide]). The coherent memory region
accesses are Outer Shareable, non-cacheable and they can be accessed
with the Device nGnRE attributes when the MMU is turned on. Hence, at the
expense of at least an extra page of memory, Trusted Firmware is able to work
around coherency issues due to mismatched memory attributes.

The alternative to the above approach is to allocate the susceptible data
structures in Normal WriteBack WriteAllocate Inner shareable memory. This
approach requires the data structures to be designed so that it is possible to
work around the issue of mismatched memory attributes by performing software
cache maintenance on them.

### Disabling the use of coherent memory in Trusted Firmware

It might be desirable to avoid the cost of allocating coherent memory on
platforms which are memory constrained. Trusted Firmware enables inclusion of
coherent memory in firmware images through the build flag `USE_COHERENT_MEM`.
This flag is enabled by default. It can be disabled to choose the second
approach described above.

The below sections analyze the data structures allocated in the coherent memory
region and the changes required to allocate them in normal memory.

### Coherent memory usage in PSCI implementation

The `psci_non_cpu_pd_nodes` data structure stores the platform's power domain
tree information for state management of power domains. By default, this data
structure is allocated in the coherent memory region in the Trusted Firmware
because it can be accessed by multple CPUs, either with caches enabled or
disabled.

    typedef struct non_cpu_pwr_domain_node {
        /*
         * Index of the first CPU power domain node level 0 which has this node
         * as its parent.
         */
        unsigned int cpu_start_idx;

        /*
         * Number of CPU power domains which are siblings of the domain indexed
         * by 'cpu_start_idx' i.e. all the domains in the range 'cpu_start_idx
         * -> cpu_start_idx + ncpus' have this node as their parent.
         */
        unsigned int ncpus;

        /*
         * Index of the parent power domain node.
         * TODO: Figure out whether to whether using pointer is more efficient.
         */
        unsigned int parent_node;

        plat_local_state_t local_state;

        unsigned char level;

        /* For indexing the psci_lock array*/
        unsigned char lock_index;
    } non_cpu_pd_node_t;

In order to move this data structure to normal memory, the use of each of its
fields must be analyzed. Fields like `cpu_start_idx`, `ncpus`, `parent_node`
`level` and `lock_index` are only written once during cold boot. Hence removing
them from coherent memory involves only doing a clean and invalidate of the
cache lines after these fields are written.

The field `local_state` can be concurrently accessed by multiple CPUs in
different cache states. A Lamport's Bakery lock `psci_locks` is used to ensure
mutual exlusion to this field and a clean and invalidate is needed after it
is written.

### Bakery lock data

The bakery lock data structure `bakery_lock_t` is allocated in coherent memory
and is accessed by multiple CPUs with mismatched attributes. `bakery_lock_t` is
defined as follows:

    typedef struct bakery_lock {
        /*
         * The lock_data is a bit-field of 2 members:
         * Bit[0]       : choosing. This field is set when the CPU is
         *                choosing its bakery number.
         * Bits[1 - 15] : number. This is the bakery number allocated.
         */
        volatile uint16_t lock_data[BAKERY_LOCK_MAX_CPUS];
    } bakery_lock_t;

It is a characteristic of Lamport's Bakery algorithm that the volatile per-CPU
fields can be read by all CPUs but only written to by the owning CPU.

Depending upon the data cache line size, the per-CPU fields of the
`bakery_lock_t` structure for multiple CPUs may exist on a single cache line.
These per-CPU fields can be read and written during lock contention by multiple
CPUs with mismatched memory attributes. Since these fields are a part of the
lock implementation, they do not have access to any other locking primitive to
safeguard against the resulting coherency issues. As a result, simple software
cache maintenance is not enough to allocate them in coherent memory. Consider
the following example.

CPU0 updates its per-CPU field with data cache enabled. This write updates a
local cache line which contains a copy of the fields for other CPUs as well. Now
CPU1 updates its per-CPU field of the `bakery_lock_t` structure with data cache
disabled. CPU1 then issues a DCIVAC operation to invalidate any stale copies of
its field in any other cache line in the system. This operation will invalidate
the update made by CPU0 as well.

To use bakery locks when `USE_COHERENT_MEM` is disabled, the lock data structure
has been redesigned. The changes utilise the characteristic of Lamport's Bakery
algorithm mentioned earlier. The bakery_lock structure only allocates the memory
for a single CPU. The macro `DEFINE_BAKERY_LOCK` allocates all the bakery locks
needed for a CPU into a section `bakery_lock`. The linker allocates the memory
for other cores by using the total size allocated for the bakery_lock section
and multiplying it with (PLATFORM_CORE_COUNT - 1). This enables software to
perform software cache maintenance on the lock data structure without running
into coherency issues associated with mismatched attributes.

The bakery lock data structure `bakery_info_t` is defined for use when
`USE_COHERENT_MEM` is disabled as follows:

    typedef struct bakery_info {
        /*
         * The lock_data is a bit-field of 2 members:
         * Bit[0]       : choosing. This field is set when the CPU is
         *                choosing its bakery number.
         * Bits[1 - 15] : number. This is the bakery number allocated.
         */
         volatile uint16_t lock_data;
    } bakery_info_t;

The `bakery_info_t` represents a single per-CPU field of one lock and
the combination of corresponding `bakery_info_t` structures for all CPUs in the
system represents the complete bakery lock. The view in memory for a system
with n bakery locks are:

    bakery_lock section start
    |----------------|
    | `bakery_info_t`| <-- Lock_0 per-CPU field
    |    Lock_0      |     for CPU0
    |----------------|
    | `bakery_info_t`| <-- Lock_1 per-CPU field
    |    Lock_1      |     for CPU0
    |----------------|
    | ....           |
    |----------------|
    | `bakery_info_t`| <-- Lock_N per-CPU field
    |    Lock_N      |     for CPU0
    ------------------
    |    XXXXX       |
    | Padding to     |
    | next Cache WB  | <--- Calculate PERCPU_BAKERY_LOCK_SIZE, allocate
    |  Granule       |       continuous memory for remaining CPUs.
    ------------------
    | `bakery_info_t`| <-- Lock_0 per-CPU field
    |    Lock_0      |     for CPU1
    |----------------|
    | `bakery_info_t`| <-- Lock_1 per-CPU field
    |    Lock_1      |     for CPU1
    |----------------|
    | ....           |
    |----------------|
    | `bakery_info_t`| <-- Lock_N per-CPU field
    |    Lock_N      |     for CPU1
    ------------------
    |    XXXXX       |
    | Padding to     |
    | next Cache WB  |
    |  Granule       |
    ------------------

Consider a system of 2 CPUs with 'N' bakery locks as shown above. For an
operation on Lock_N, the corresponding `bakery_info_t` in both CPU0 and CPU1
`bakery_lock` section need to be fetched and appropriate cache operations need
to be performed for each access.

On ARM Platforms, bakery locks are used in psci (`psci_locks`) and power controller
driver (`arm_lock`).


### Non Functional Impact of removing coherent memory

Removal of the coherent memory region leads to the additional software overhead
of performing cache maintenance for the affected data structures. However, since
the memory where the data structures are allocated is cacheable, the overhead is
mostly mitigated by an increase in performance.

There is however a performance impact for bakery locks, due to:
*   Additional cache maintenance operations, and
*   Multiple cache line reads for each lock operation, since the bakery locks
    for each CPU are distributed across different cache lines.

The implementation has been optimized to minimize this additional overhead.
Measurements indicate that when bakery locks are allocated in Normal memory, the
minimum latency of acquiring a lock is on an average 3-4 micro seconds whereas
in Device memory the same is 2 micro seconds. The measurements were done on the
Juno ARM development platform.

As mentioned earlier, almost a page of memory can be saved by disabling
`USE_COHERENT_MEM`. Each platform needs to consider these trade-offs to decide
whether coherent memory should be used. If a platform disables
`USE_COHERENT_MEM` and needs to use bakery locks in the porting layer, it can
optionally define macro `PLAT_PERCPU_BAKERY_LOCK_SIZE`  (see the [Porting
Guide]). Refer to the reference platform code for examples.


12.  Isolating code and read-only data on separate memory pages
---------------------------------------------------------------

In the ARMv8 VMSA, translation table entries include fields that define the
properties of the target memory region, such as its access permissions. The
smallest unit of memory that can be addressed by a translation table entry is
a memory page. Therefore, if software needs to set different permissions on two
memory regions then it needs to map them using different memory pages.

The default memory layout for each BL image is as follows:

       |        ...        |
       +-------------------+
       |  Read-write data  |
       +-------------------+ Page boundary
       |     <Padding>     |
       +-------------------+
       | Exception vectors |
       +-------------------+ 2 KB boundary
       |     <Padding>     |
       +-------------------+
       |  Read-only data   |
       +-------------------+
       |       Code        |
       +-------------------+ BLx_BASE

Note: The 2KB alignment for the exception vectors is an architectural
requirement.

The read-write data start on a new memory page so that they can be mapped with
read-write permissions, whereas the code and read-only data below are configured
as read-only.

However, the read-only data are not aligned on a page boundary. They are
contiguous to the code. Therefore, the end of the code section and the beginning
of the read-only data one might share a memory page. This forces both to be
mapped with the same memory attributes. As the code needs to be executable, this
means that the read-only data stored on the same memory page as the code are
executable as well. This could potentially be exploited as part of a security
attack.

TF provides the build flag `SEPARATE_CODE_AND_RODATA` to isolate the code and
read-only data on separate memory pages. This in turn allows independent control
of the access permissions for the code and read-only data. In this case,
platform code gets a finer-grained view of the image layout and can
appropriately map the code region as executable and the read-only data as
execute-never.

This has an impact on memory footprint, as padding bytes need to be introduced
between the code and read-only data to ensure the segragation of the two. To
limit the memory cost, this flag also changes the memory layout such that the
code and exception vectors are now contiguous, like so:

       |        ...        |
       +-------------------+
       |  Read-write data  |
       +-------------------+ Page boundary
       |     <Padding>     |
       +-------------------+
       |  Read-only data   |
       +-------------------+ Page boundary
       |     <Padding>     |
       +-------------------+
       | Exception vectors |
       +-------------------+ 2 KB boundary
       |     <Padding>     |
       +-------------------+
       |       Code        |
       +-------------------+ BLx_BASE

With this more condensed memory layout, the separation of read-only data will
add zero or one page to the memory footprint of each BL image. Each platform
should consider the trade-off between memory footprint and security.

This build flag is disabled by default, minimising memory footprint. On ARM
platforms, it is enabled.


13.  Code Structure
-------------------

Trusted Firmware code is logically divided between the three boot loader
stages mentioned in the previous sections. The code is also divided into the
following categories (present as directories in the source code):

*   **Platform specific.** Choice of architecture specific code depends upon
    the platform.
*   **Common code.** This is platform and architecture agnostic code.
*   **Library code.** This code comprises of functionality commonly used by all
    other code. The PSCI implementation and other EL3 runtime frameworks reside
    as Library components.
*   **Stage specific.** Code specific to a boot stage.
*   **Drivers.**
*   **Services.** EL3 runtime services (eg: SPD). Specific SPD services
    reside in the `services/spd` directory (e.g. `services/spd/tspd`).

Each boot loader stage uses code from one or more of the above mentioned
categories. Based upon the above, the code layout looks like this:

    Directory    Used by BL1?    Used by BL2?    Used by BL31?
    bl1          Yes             No              No
    bl2          No              Yes             No
    bl31         No              No              Yes
    plat         Yes             Yes             Yes
    drivers      Yes             No              Yes
    common       Yes             Yes             Yes
    lib          Yes             Yes             Yes
    services     No              No              Yes

The build system provides a non configurable build option IMAGE_BLx for each
boot loader stage (where x = BL stage). e.g. for BL1 , IMAGE_BL1 will be
defined by the build system. This enables the Trusted Firmware to compile
certain code only for specific boot loader stages

All assembler files have the `.S` extension. The linker source files for each
boot stage have the extension `.ld.S`. These are processed by GCC to create the
linker scripts which have the extension `.ld`.

FDTs provide a description of the hardware platform and are used by the Linux
kernel at boot time. These can be found in the `fdts` directory.


14.  References
---------------

1.  Trusted Board Boot Requirements CLIENT PDD (ARM DEN 0006B-5). Available
    under NDA through your ARM account representative.

2.  [Power State Coordination Interface PDD (ARM DEN 0022B.b)][PSCI].

3.  [SMC Calling Convention PDD (ARM DEN 0028A)][SMCCC].

4.  [ARM Trusted Firmware Interrupt Management Design guide][INTRG].

- - - - - - - - - - - - - - - - - - - - - - - - - -

_Copyright (c) 2013-2016, ARM Limited and Contributors. All rights reserved._

[ARM ARM]:          http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0487a.e/index.html "ARMv8-A Reference Manual (ARM DDI0487A.E)"
[PSCI]:             http://infocenter.arm.com/help/topic/com.arm.doc.den0022c/DEN0022C_Power_State_Coordination_Interface.pdf "Power State Coordination Interface PDD (ARM DEN 0022C)"
[SMCCC]:            http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html "SMC Calling Convention PDD (ARM DEN 0028A)"
[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
[Firmware Update]:  ./firmware-update.md
