firmware-design.md

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

Contents :

1.  Introduction
2.  Cold boot
3.  EL3 runtime services framework
4.  Power State Coordination Interface
5.  Secure-EL1 Payloads and Dispatchers
6.  Crash Reporting in BL3-1
7.  Memory layout on FVP platforms
8.  Firmware Image Package (FIP)
9.  Code Structure
10.  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. 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 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 (BL3-1) _EL3 Runtime Firmware_
*   Boot Loader stage 3-2 (BL3-2) _Secure-EL1 Payload_ (optional)
*   Boot Loader stage 3-3 (BL3-3) _Non-trusted Firmware_

The ARM Fixed Virtual Platforms (FVPs) provide trusted ROM, trusted SRAM and
trusted DRAM regions. Each boot loader stage uses one or more of these
memories for its code and data.

The sections below provide the following details:

*   initialization and execution of the first three stages during cold boot
*   specification of the BL3-1 entrypoint requirements for use by alternative
    Trusted Boot Firmware in place of the provided BL1 and BL2
*   changes in BL3-1 behavior when using the `RESET_TO_BL31` option which
    allows BL3-1 to run without BL1 and BL2


### BL1

This stage begins execution from the platform's reset vector in trusted ROM at
EL3. BL1 code starts at `0x00000000` (trusted ROM) in the FVP memory map. The
BL1 data section is placed at the start of trusted SRAM, `0x04000000`. 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 a platform-specific mechanism. The
ARM FVPs implement a simple power controller at `0x1c100000`. The `PSYS`
register (`0x10`) is used to distinguish between a cold and warm boot. This
information is contained in the `PSYS.WK[25:24]` field. Additionally, a
per-CPU mailbox is maintained in trusted DRAM (`0x00600000`), to which BL1
writes an entrypoint. Each CPU jumps to this entrypoint upon warm boot. During
cold boot, BL1 places the secondary CPUs in a safe platform-specific state while
the primary CPU executes the remaining cold boot path as described in the
following sections.

#### 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 set a
    status code. In the case of the FVP this code is written to the Versatile
    Express System LED register in the following format:

        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). The values for
                       each exception class are:

        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

    A write to the LED register reflects in the System LEDs (S6LED0..7) in the
    CLCD window of the FVP. This behavior is because this boot loader stage
    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
    only a single type of SMC (determined by its function ID in the general
    purpose register `X0`). This SMC is raised by BL2 to make BL1 pass control
    to BL3-1 (loaded by BL2) at EL3. Any other SMC leads to an assertion
    failure.

*   MMU setup

    BL1 sets up EL3 memory translation by creating page tables to cover the
    first 4GB of physical address space. This covers all the memories and
    peripherals needed by BL1.

*   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.

    -   `CPUECTLR`. When the FVP includes a model of a specific ARM processor
        implementation (for example A57 or A53), then intra-cluster coherency is
        enabled by setting the `CPUECTLR.SMPEN` bit. The AEMv8 Base FVP is
        inherently coherent so does not implement `CPUECTLR`.

    -   `SCR`. Use of the HVC instruction from EL1 is enabled by setting the
        `SCR.HCE` bit. FIQ exceptions are configured to be taken in EL3 by
        setting the `SCR.FIQ` bit. The register width of the next lower
        exception level is set to AArch64 by setting the `SCR.RW` bit. External
        Aborts and SError Interrupts are configured to be taken in EL3 by
        setting the `SCR.EA` bit.

    -   `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.

#### Platform initialization

BL1 enables issuing of snoop and DVM (Distributed Virtual Memory) requests from
the CCI-400 slave interface corresponding to the cluster that includes the
primary CPU. BL1 also initializes UART0 (PL011 console), which enables access to
the `printf` family of functions in BL1.

#### BL2 image load and execution

BL1 execution continues as follows:

1.  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 boot loader stage 2 (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.

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

        "Booting trusted firmware boot loader stage 1"

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. It sets up
Secure EL1 memory translation by creating page tables to address the first 4GB
of the physical address space in a similar way to BL1. EL1 and EL0 are given
access to Floating Point & Advanced SIMD registers by clearing the `CPACR.FPEN`
bits.

#### Platform initialization

BL2 copies the information regarding the trusted SRAM populated by BL1 using a
platform-specific mechanism. It calculates the limits of DRAM (main memory)
to determine whether there is enough space to load the BL3-3 image. A platform
defined base address is used to specify the load address for the BL3-1 image.
It also defines the extents of memory available for use by the BL3-2 image.
BL2 also initializes UART0 (PL011 console), which enables  access to the
`printf` family of functions in BL2. Platform security is initialized to allow
access to access controlled components. On the Base FVP a TrustZone controller
(TZC-400) is configured to give full access to the platform DRAM. The storage
abstraction layer is initialized which is used to load further bootloader
images.

#### BL3-0 (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 BL3-0 image from platform
storage into a platform-specific region of secure memory. The subsequent
handling of BL3-0 is platform specific. Typically the image is transferred into
SCP memory using a platform-specific protocol. The SCP executes BL3-0 and
signals to the Application Processor (AP) for BL2 execution to continue.

#### BL3-1 (EL3 Runtime Firmware) image load

BL2 loads the BL3-1 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 BL3-1 image loads successfully,
BL2 updates the amount of trusted SRAM used and available for use by BL3-1.
This information is populated at a platform-specific memory address.

#### BL3-2 (Secure-EL1 Payload) image load

BL2 loads the optional BL3-2 image from platform storage into a platform-
specific region of secure memory. The image executes in the secure world. BL2
relies on BL3-1 to pass control to the BL3-2 image, if present. Hence, BL2
populates a platform-specific area of memory with the entrypoint/load-address
of the BL3-2 image. The value of the Saved Processor Status Register (`SPSR`)
for entry into BL3-2 is not determined by BL2, it is initialized by the
Secure-EL1 Payload Dispatcher (see later) within BL3-1, which is responsible for
managing interaction with BL3-2. This information is passed to BL3-1.

#### BL3-3 (Non-trusted Firmware) image load

BL2 loads the BL3-3 image (e.g. UEFI or other test or boot software) from
platform storage into non-secure memory as defined by the platform
(`0x88000000` for FVPs).

BL2 relies on BL3-1 to pass control to BL3-3 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 BL3-3 image. The
`SPSR` is determined as specified in Section 5.13 of the [PSCI PDD] [PSCI]. This
information is passed to BL3-1.

#### BL3-1 (EL3 Runtime Firmware) execution

BL2 execution continues as follows:

1.  BL2 passes control back to BL1 by raising an SMC, providing BL1 with the
    BL3-1 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 BL3-1 at the specified entrypoint at EL3.


### BL3-1

The image for this stage is loaded by BL2 and BL1 passes control to BL3-1 at
EL3. BL3-1 executes solely in trusted SRAM. BL3-1 is linked against and
loaded at a platform-specific base address (more information can be found later
in this document). The functionality implemented by BL3-1 is as follows.

#### Architectural initialization

Currently, BL3-1 performs a similar architectural initialization to BL1 as
far as system register settings are concerned. Since BL1 code resides in ROM,
architectural initialization in BL3-1 allows override of any previous
initialization done by BL1. BL3-1 creates page tables to address the first
4GB of physical address space and initializes the MMU accordingly. It initializes
a buffer of frequently used pointers, called per-cpu pointer cache, in memory for
faster access. Currently the per-cpu pointer cache contains only the pointer
to crash stack. It then replaces the exception vectors populated by BL1 with its
own. BL3-1 exception vectors implement more elaborate support for
handling SMCs since this is the only mechanism to access the runtime services
implemented by BL3-1 (PSCI for example). BL3-1 checks each SMC for validity as
specified by the [SMC calling convention PDD][SMCCC] before passing control to
the required SMC handler routine. BL3-1 programs the `CNTFRQ_EL0` register with
the clock frequency of the system counter, which is provided by the platform.

#### Platform initialization

BL3-1 performs detailed platform initialization, which enables normal world
software to function correctly. It also retrieves entrypoint information for
the BL3-3 image loaded by BL2 from the platform defined memory address populated
by BL2. BL3-1 also initializes UART0 (PL011 console), which enables
access to the `printf` family of functions in BL3-1.  It enables the system
level implementation of the generic timer through the memory mapped interface.

* GICv2 initialization:

    -   Enable group0 interrupts in the GIC CPU interface.
    -   Configure group0 interrupts to be asserted as FIQs.
    -   Disable the legacy interrupt bypass mechanism.
    -   Configure the priority mask register to allow interrupts of all
        priorities to be signaled to the CPU interface.
    -   Mark SGIs 8-15, the secure physical timer interrupt (#29) and the
        trusted watchdog interrupt (#56) as group0 (secure).
    -   Target the trusted watchdog interrupt to CPU0.
    -   Enable these group0 interrupts in the GIC distributor.
    -   Configure all other interrupts as group1 (non-secure).
    -   Enable signaling of group0 interrupts in the GIC distributor.

*   GICv3 initialization:

    If a GICv3 implementation is available in the platform, BL3-1 initializes
    the GICv3 in GICv2 emulation mode with settings as described for GICv2
    above.

*   Power management initialization:

    BL3-1 implements a state machine to track CPU and cluster state. The state
    can be one of `OFF`, `ON_PENDING`, `SUSPEND` or `ON`. All secondary CPUs are
    initially in the `OFF` state. The cluster that the primary CPU belongs to is
    `ON`; any other cluster is `OFF`. BL3-1 initializes the data structures that
    implement the state machine, including the locks that protect them. BL3-1
    accesses the state of a CPU or cluster immediately after reset and before
    the MMU is enabled in the warm boot path. It is not currently possible to
    use 'exclusive' based spinlocks, therefore BL3-1 uses locks based on
    Lamport's Bakery algorithm instead. BL3-1 allocates these locks in device
    memory. They are accessible irrespective of MMU state.

*   Runtime services initialization:

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

    Details about the PSCI service are provided in the "Power State Coordination
    Interface" section below.

*   BL3-2 (Secure-EL1 Payload) image initialization

    If a BL3-2 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 BL3-2
    once the runtime services are fully initialized. BL3-1 invokes such a
    registered function to initialize BL3-2 before running BL3-3.

    Details on BL3-2 initialization and the SPD's role are described in the
    "Secure-EL1 Payloads and Dispatchers" section below.

*   BL3-3 (Non-trusted Firmware) execution

    BL3-1 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. BL3-1 uses the entrypoint information provided
    by BL2 to jump to the Non-trusted firmware image (BL3-3) 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 BL3-1 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 BL3-1.

Future changes to the BL3-1 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, if this is called by any
other CPU the firmware will abort.

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
    CTLR_EL3.EE = 0

X0 and X1 can be used to pass information from the Trusted Boot Firmware to the
platform code in BL3-1:

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

BL3-1 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 BL3-1 code.

The convention is that `X0` conveys information regarding the BL3-1, BL3-2 and
BL3-3 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.

BL3-1 common and SPD initialization code depends on image and entrypoint
information about BL3-3 and BL3-2, which is provided via BL3-1 platform APIs.
This information is required until the start of execution of BL3-3. This
information can be provided in a platform defined manner, e.g. compiled into
the platform code in BL3-1, 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
BL3-1 platform code before the caches are enabled.

ARM Trusted Firmware's BL2 implementation passes a `bl31_params` structure in
`X0` and the FVP port interprets this in the BL3-1 platform code.

##### MMU, Data caches & Coherency

BL3-1 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 BL3-1 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
BL3-1 that can interpret the BL3-x image information from different versions of
BL2, a platform that uses an extended entry_point_info structure to convey
additional register information to BL3-1, 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
BL3-1 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_PARA_HEAD()` a macro is defined
to simplify this action.

#### Required CPU state for BL3-1 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.


### Using BL3-1 as the CPU reset vector

On some platforms the runtime firmware (BL3-x 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 BL3-1 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 BL3-1 entrypoint to support this use case.

In this configuration, the platform's Trusted Boot Firmware must ensure that
BL3-1 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 BL3-x images required and
providing entry point information for them to BL3-1. Loading these images might
be done by the Trusted Boot Firmware or by platform code in BL3-1.

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 BL3-1
functionality:

#### Determination of boot path

In this configuration, BL3-1 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 BL3-1 initialization, as described below.

#### Architectural initialization

As the first image to execute in this configuration BL3-1 must ensure that
interconnect coherency is enabled (if required) before enabling the MMU.

#### Platform initialization

In this configuration, when the CPU resets to BL3-1 there are no parameters
that can be passed in registers by previous boot stages. Instead, the platform
code in BL3-1 needs to know, or be able to determine, the location of the BL3-2
(if required) and BL3-3 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 BL3-1 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
----------------------------------

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 (BL3-1).

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).

    NOTE: Currently this service is called PSCI since there are no other
    defined standard service calls.

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 BL3-1.

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 BL3-1
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 BL3-1 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 complete PSCI API is not yet implemented. The following functions are
currently implemented:

-   `PSCI_VERSION`
-   `CPU_OFF`
-   `CPU_ON`
-   `CPU_SUSPEND`
-   `AFFINITY_INFO`

The `CPU_ON`, `CPU_OFF` and `CPU_SUSPEND` functions implement the warm boot
path in ARM Trusted Firmware. `CPU_ON` and `CPU_OFF` have undergone testing
on all the supported FVPs. `CPU_SUSPEND` & `AFFINITY_INFO` have undergone
testing only on the AEM v8 Base FVP. Support for `AFFINITY_INFO` is still
experimental. Support for `CPU_SUSPEND` is stable for entry into power down
states. Standby states are currently not supported. `PSCI_VERSION` is
present but completely untested in this version of the software.

Unsupported PSCI functions can be divided into ones that can return
execution to the caller and ones that cannot. The following functions
return with a error code as documented in the [Power State Coordination
Interface PDD] [PSCI].

-   `MIGRATE` : -1 (NOT_SUPPORTED)
-   `MIGRATE_INFO_TYPE` : 2 (Trusted OS is either not present or does not
     require migration)
-   `MIGRATE_INFO_UP_CPU` : 0 (Return value is UNDEFINED)

The following unsupported functions do not return and signal an assertion
failure if invoked.

-   `SYSTEM_OFF`
-   `SYSTEM_RESET`


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 BL3-1
firmware. This service is responsible for the initialisation of the Trusted
OS and all communications with it. The Trusted OS is the BL3-2 stage of the
boot flow in ARM Trusted Firmware. The firmware will attempt to locate, load
and execute a BL3-2 image.

ARM Trusted Firmware uses a more general term for the BL3-2 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 BL3-2 image and service are replaced by the Trusted OS
and its dispatcher service.

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 BL3-2 Image

The Secure-EL1 Payload Dispatcher (SPD) service is responsible for initializing
the BL3-2 image. It needs access to the information passed by BL2 to BL3-1 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, BL3-2.

In the absence of a BL3-2 image, BL3-1 passes control to the normal world
bootloader image (BL3-3). When the BL3-2 image is present, it is typical
that the SPD wants control to be passed to BL3-2 first and then later to BL3-3.

To do this the SPD has to register a BL3-2 initialization function during
initialization of the SPD service. The BL3-2 initialization function has this
prototype:

    int32_t init();

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

Trusted Firmware supports two approaches for the SPD to pass control to BL3-2
before returning through EL3 and running the non-trusted firmware (BL3-3):

1.  In the BL3-2 initialization function, set up a secure context (see below
    for more details of CPU context support) for this CPU and use
    `bl31_set_next_image_type()` to request that the exit from `bl31_main()` is
    to the BL3-2 entrypoint in Secure-EL1.

    When the BL3-2 has completed initialization at Secure-EL1, it returns to
    BL3-1 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 BL3-3. On return from the handler the framework
    will exit to EL2 and run BL3-3.

2.  In the BL3-2 initialization function, use an SPD-defined mechanism to
    invoke a 'world-switch synchronous call' to Secure-EL1 to run the BL3-2
    entrypoint.
    NOTE: The Test SPD service included with the Trusted Firmware provides one
    implementation of such a mechanism.

    On completion BL3-2 returns control to BL3-1 via a SMC, and on receipt the
    SPD service handler invokes the synchronous call return mechanism to return
    to the BL3-2 initialization function. On return from this function,
    `bl31_main()` will set up the return to the normal world firmware BL3-3 and
    continue the boot process in the normal world.

6.  Crash Reporting in BL3-1
----------------------------------

The BL3-1 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 the default serial output. The general purpose
registers, EL3, Secure EL1 and some EL2 state registers are reported.

A dedicated per-cpu crash stack is maintained by BL3-1 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.  Memory layout on FVP platforms
----------------------------------

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 would sneak in between
PROGBITS sections then the resulting binary file would contain a bunch of zero
bytes at the location of this NOBITS section, making the image unnecessarily
bigger. Smaller images allow faster loading from the FIP to the main memory.

On FVP platforms, we use the Trusted ROM and Trusted SRAM to store the trusted
firmware binaries.

 *    BL1 is originally sitting in the Trusted ROM at address `0x0`. Its
      read-write data are relocated at the top of the Trusted SRAM at runtime.

 *    BL3-1 is loaded at the top of the Trusted SRAM, such that its NOBITS
      sections will overwrite BL1 R/W data.

 *    BL2 is loaded below BL3-1.

 *    The TSP is loaded as the BL3-2 image at the base of the Trusted SRAM. Its
      NOBITS sections are allowed to overlay BL2.

This memory layout is designed to give the BL3-2 image as much memory as
possible. It is illustrated by the following diagram.

               Trusted SRAM
    0x04040000 +----------+  loaded by BL2  ------------------
               | BL1 (rw) |  <<<<<<<<<<<<<  |  BL3-1 NOBITS  |
               |----------|  <<<<<<<<<<<<<  |----------------|
               |          |  <<<<<<<<<<<<<  | BL3-1 PROGBITS |
               |----------|                 ------------------
               |   BL2    |  <<<<<<<<<<<<<  |  BL3-2 NOBITS  |
               |----------|  <<<<<<<<<<<<<  |----------------|
               |          |  <<<<<<<<<<<<<  | BL3-2 PROGBITS |
    0x04000000 +----------+                 ------------------

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

The TSP image may be loaded in Trusted DRAM instead. This doesn't change the
memory layout of the other boot loader images in Trusted SRAM.

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

Early setup code needs to know the extents of the BSS section to zero-initialise
it before executing any C code. The following linker symbols are defined for
this purpose:

* `__BSS_START__` This address must be aligned on a 16-byte boundary.
* `__BSS_SIZE__`

Similarly, the coherent memory section must be zero-initialised. Also, the MMU
setup code needs to know the extents of this section to set the right memory
attributes for it. The following linker symbols are defined for this purpose:

* `__COHERENT_RAM_START__` This address must be aligned on a page-size boundary.
* `__COHERENT_RAM_END__` This address must be aligned on a page-size boundary.
* `__COHERENT_RAM_UNALIGNED_SIZE__`

### BL1's linker symbols

BL1's early setup code needs to know the extents of the .data section to
relocate it from ROM to RAM before executing any C code. The following linker
symbols are defined for this purpose:

* `__DATA_ROM_START__` This address must be aligned on a 16-byte boundary.
* `__DATA_RAM_START__` This address must be aligned on a 16-byte boundary.
* `__DATA_SIZE__`

BL1's platform setup code needs to know the extents of its read-write data
region to figure out its memory layout. The following linker symbols are defined
for this purpose:

* `__BL1_RAM_START__` This is the start address of BL1 RW data.
* `__BL1_RAM_END__` This is the end address of BL1 RW data.

### BL2's, BL3-1's and TSP's linker symbols

BL2, BL3-1 and TSP need to know the extents of their read-only section to set
the right memory attributes for this memory region in their MMU setup code. The
following linker symbols are defined for this purpose:

* `__RO_START__`
* `__RO_END__`

### 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, BL3-1 and TSP need to know the limit address that their PROGBITS
sections must not overstep. The platform code must provide those.

8.  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. None are yet defined.

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/fip_create`.

### 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 FVPs this is currently NOR FLASH.

Bootloader images are loaded according to the platform policy as specified in
`plat/<platform>/plat_io_storage.c`. For the FVPs this means the platform will
attempt to load images from a Firmware Image Package located at the start of NOR
FLASH0.

Currently the FVP's policy only allows loading of a known set of images. The
platform policy can be modified to allow additional images.


9.  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):

*   **Architecture specific.** This could be AArch32 or AArch64.
*   **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.
*   **Stage specific.** Code specific to a boot stage.
*   **Drivers.**
*   **Services.** EL3 runtime services, e.g. PSCI or 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 BL3-1?
    bl1          Yes             No              No
    bl2          No              Yes             No
    bl31         No              No              Yes
    arch         Yes             Yes             Yes
    plat         Yes             Yes             Yes
    drivers      Yes             No              Yes
    common       Yes             Yes             Yes
    lib          Yes             Yes             Yes
    services     No              No              Yes

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.


10.  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) 2013-2014, ARM Limited and Contributors. All rights reserved._


[PSCI]:             http://infocenter.arm.com/help/topic/com.arm.doc.den0022b/index.html "Power State Coordination Interface PDD (ARM DEN 0022B.b)"
[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
[INTRG]:            ./interrupt-framework-design.md