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ARM Trusted Firmware Porting Guide

Contents

  1. Introduction
  2. Common Modifications
  3. Boot Loader stage specific modifications
  4. Build flags
  5. C Library
  6. Storage abstraction layer
  1. Introduction

Porting the ARM Trusted Firmware to a new platform involves making some mandatory and optional modifications for both the cold and warm boot paths. Modifications consist of:

  • Implementing a platform-specific function or variable,
  • Setting up the execution context in a certain way, or
  • Defining certain constants (for example #defines).

The platform-specific functions and variables are all declared in include/plat/common/platform.h. The firmware provides a default implementation of variables and functions to fulfill the optional requirements. These implementations are all weakly defined; they are provided to ease the porting effort. Each platform port can override them with its own implementation if the default implementation is inadequate.

Some modifications are common to all Boot Loader (BL) stages. Section 2 discusses these in detail. The subsequent sections discuss the remaining modifications for each BL stage in detail.

This document should be read in conjunction with the ARM Trusted Firmware User Guide.

  1. Common modifications

This section covers the modifications that should be made by the platform for each BL stage to correctly port the firmware stack. They are categorized as either mandatory or optional.

2.1 Common mandatory modifications

A platform port must enable the Memory Management Unit (MMU) with identity mapped page tables, and enable both the instruction and data caches for each BL stage. In the ARM FVP port, each BL stage configures the MMU in its platform- specific architecture setup function, for example blX_plat_arch_setup().

If the build option USE_COHERENT_MEM is enabled, each platform must allocate a block of identity mapped secure memory with Device-nGnRE attributes aligned to page boundary (4K) for each BL stage. This memory is identified by the section name tzfw_coherent_mem so that its possible for the firmware to place variables in it using the following C code directive:

__attribute__ ((section("tzfw_coherent_mem")))

Or alternatively the following assembler code directive:

.section tzfw_coherent_mem

The tzfw_coherent_mem section is used to allocate any data structures that are accessed both when a CPU is executing with its MMU and caches enabled, and when it's running with its MMU and caches disabled. Examples are given below.

The following variables, functions and constants must be defined by the platform for the firmware to work correctly.

File : platform_def.h [mandatory]

Each platform must ensure that a header file of this name is in the system include path with the following constants defined. This may require updating the list of PLAT_INCLUDES in the platform.mk file. In the ARM FVP port, this file is found in plat/fvp/include/platform_def.h.

  • #define : PLATFORM_LINKER_FORMAT

    Defines the linker format used by the platform, for example elf64-littleaarch64 used by the FVP.

  • #define : PLATFORM_LINKER_ARCH

    Defines the processor architecture for the linker by the platform, for example aarch64 used by the FVP.

  • #define : PLATFORM_STACK_SIZE

    Defines the normal stack memory available to each CPU. This constant is used by plat/common/aarch64/platform_mp_stack.S and plat/common/aarch64/platform_up_stack.S.

  • #define : FIRMWARE_WELCOME_STR

    Defines the character string printed by BL1 upon entry into the bl1_main() function.

  • #define : BL2_IMAGE_NAME

    Name of the BL2 binary image on the host file-system. This name is used by BL1 to load BL2 into secure memory from non-volatile storage.

  • #define : BL31_IMAGE_NAME

    Name of the BL3-1 binary image on the host file-system. This name is used by BL2 to load BL3-1 into secure memory from platform storage.

  • #define : BL33_IMAGE_NAME

    Name of the BL3-3 binary image on the host file-system. This name is used by BL2 to load BL3-3 into non-secure memory from platform storage.

  • #define : BL2_CERT_NAME

    Name of the BL2 content certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : TRUSTED_KEY_CERT_NAME

    Name of the Trusted Key certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : BL31_KEY_CERT_NAME

    Name of the BL3-1 Key certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : BL31_CERT_NAME

    Name of the BL3-1 Content certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : BL33_KEY_CERT_NAME

    Name of the BL3-3 Key certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : BL33_CERT_NAME

    Name of the BL3-3 Content certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : PLATFORM_CACHE_LINE_SIZE

    Defines the size (in bytes) of the largest cache line across all the cache levels in the platform.

  • #define : PLATFORM_CLUSTER_COUNT

    Defines the total number of clusters implemented by the platform in the system.

  • #define : PLATFORM_CORE_COUNT

    Defines the total number of CPUs implemented by the platform across all clusters in the system.

  • #define : PLATFORM_MAX_CPUS_PER_CLUSTER

    Defines the maximum number of CPUs that can be implemented within a cluster on the platform.

  • #define : PLATFORM_NUM_AFFS

    Defines the total number of nodes in the affinity heirarchy at all affinity levels used by the platform.

  • #define : BL1_RO_BASE

    Defines the base address in secure ROM where BL1 originally lives. Must be aligned on a page-size boundary.

  • #define : BL1_RO_LIMIT

    Defines the maximum address in secure ROM that BL1's actual content (i.e. excluding any data section allocated at runtime) can occupy.

  • #define : BL1_RW_BASE

    Defines the base address in secure RAM where BL1's read-write data will live at runtime. Must be aligned on a page-size boundary.

  • #define : BL1_RW_LIMIT

    Defines the maximum address in secure RAM that BL1's read-write data can occupy at runtime.

  • #define : BL2_BASE

    Defines the base address in secure RAM where BL1 loads the BL2 binary image. Must be aligned on a page-size boundary.

  • #define : BL2_LIMIT

    Defines the maximum address in secure RAM that the BL2 image can occupy.

  • #define : BL31_BASE

    Defines the base address in secure RAM where BL2 loads the BL3-1 binary image. Must be aligned on a page-size boundary.

  • #define : BL31_LIMIT

    Defines the maximum address in secure RAM that the BL3-1 image can occupy.

  • #define : NS_IMAGE_OFFSET

    Defines the base address in non-secure DRAM where BL2 loads the BL3-3 binary image. Must be aligned on a page-size boundary.

If a BL3-0 image is supported by the platform, the following constants must also be defined:

  • #define : BL30_IMAGE_NAME

    Name of the BL3-0 binary image on the host file-system. This name is used by BL2 to load BL3-0 into secure memory from platform storage before being transfered to the SCP.

  • #define : BL30_KEY_CERT_NAME

    Name of the BL3-0 Key certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : BL30_CERT_NAME

    Name of the BL3-0 Content certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

If a BL3-2 image is supported by the platform, the following constants must also be defined:

  • #define : BL32_IMAGE_NAME

    Name of the BL3-2 binary image on the host file-system. This name is used by BL2 to load BL3-2 into secure memory from platform storage.

  • #define : BL32_KEY_CERT_NAME

    Name of the BL3-2 Key certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : BL32_CERT_NAME

    Name of the BL3-2 Content certificate on the host file-system (mandatory when Trusted Board Boot is enabled).

  • #define : BL32_BASE

    Defines the base address in secure memory where BL2 loads the BL3-2 binary image. Must be aligned on a page-size boundary.

  • #define : BL32_LIMIT

    Defines the maximum address that the BL3-2 image can occupy.

If the Test Secure-EL1 Payload (TSP) instantiation of BL3-2 is supported by the platform, the following constants must also be defined:

  • #define : TSP_SEC_MEM_BASE

    Defines the base address of the secure memory used by the TSP image on the platform. This must be at the same address or below BL32_BASE.

  • #define : TSP_SEC_MEM_SIZE

    Defines the size of the secure memory used by the BL3-2 image on the platform. TSP_SEC_MEM_BASE and TSP_SEC_MEM_SIZE must fully accomodate the memory required by the BL3-2 image, defined by BL32_BASE and BL32_LIMIT.

  • #define : TSP_IRQ_SEC_PHY_TIMER

    Defines the ID of the secure physical generic timer interrupt used by the TSP's interrupt handling code.

If the platform port uses the IO storage framework, the following constants must also be defined:

  • #define : MAX_IO_DEVICES

    Defines the maximum number of registered IO devices. Attempting to register more devices than this value using io_register_device() will fail with IO_RESOURCES_EXHAUSTED.

  • #define : MAX_IO_HANDLES

    Defines the maximum number of open IO handles. Attempting to open more IO entities than this value using io_open() will fail with IO_RESOURCES_EXHAUSTED.

If the platform needs to allocate data within the per-cpu data framework in BL3-1, it should define the following macro. Currently this is only required if the platform decides not to use the coherent memory section by undefining the USE_COHERENT_MEM build flag. In this case, the framework allocates the required memory within the the per-cpu data to minimize wastage.

  • #define : PLAT_PCPU_DATA_SIZE

    Defines the memory (in bytes) to be reserved within the per-cpu data structure for use by the platform layer.

The following constants are optional. They should be defined when the platform memory layout implies some image overlaying like on FVP.

  • #define : BL31_PROGBITS_LIMIT

    Defines the maximum address in secure RAM that the BL3-1's progbits sections can occupy.

  • #define : TSP_PROGBITS_LIMIT

    Defines the maximum address that the TSP's progbits sections can occupy.

File : plat_macros.S [mandatory]

Each platform must ensure a file of this name is in the system include path with the following macro defined. In the ARM FVP port, this file is found in plat/fvp/include/plat_macros.S.

  • Macro : plat_print_gic_regs

    This macro allows the crash reporting routine to print GIC registers in case of an unhandled exception in BL3-1. This aids in debugging and this macro can be defined to be empty in case GIC register reporting is not desired.

  • Macro : plat_print_interconnect_regs

    This macro allows the crash reporting routine to print interconnect registers in case of an unhandled exception in BL3-1. This aids in debugging and this macro can be defined to be empty in case interconnect register reporting is not desired. In the ARM FVP port, the CCI snoop control registers are reported.

Other mandatory modifications

The following mandatory modifications may be implemented in any file the implementer chooses. In the ARM FVP port, they are implemented in plat/fvp/aarch64/plat_common.c.

  • Function : uint64_t plat_get_syscnt_freq(void)

    This function is used by the architecture setup code to retrieve the counter frequency for the CPU's generic timer. This value will be programmed into the CNTFRQ_EL0 register. In the ARM FVP port, it returns the base frequency of the system counter, which is retrieved from the first entry in the frequency modes table.

2.2 Handling Reset

BL1 by default implements the reset vector where execution starts from a cold or warm boot. BL3-1 can be optionally set as a reset vector using the RESET_TO_BL31 make variable.

For each CPU, the reset vector code is responsible for the following tasks:

  1. Distinguishing between a cold boot and a warm boot.

  2. In the case of a cold boot and the CPU being a secondary CPU, ensuring that the CPU is placed in a platform-specific state until the primary CPU performs the necessary steps to remove it from this state.

  3. In the case of a warm boot, ensuring that the CPU jumps to a platform- specific address in the BL3-1 image in the same processor mode as it was when released from reset.

The following functions need to be implemented by the platform port to enable reset vector code to perform the above tasks.

Function : platform_get_entrypoint() [mandatory]

Argument : unsigned long
Return   : unsigned int

This function is called with the SCTLR.M and SCTLR.C bits disabled. The CPU is identified by its MPIDR, which is passed as the argument. The function is responsible for distinguishing between a warm and cold reset using platform- specific means. If it's a warm reset then it returns the entrypoint into the BL3-1 image that the CPU must jump to. If it's a cold reset then this function must return zero.

This function is also responsible for implementing a platform-specific mechanism to handle the condition where the CPU has been warm reset but there is no entrypoint to jump to.

This function does not follow the Procedure Call Standard used by the Application Binary Interface for the ARM 64-bit architecture. The caller should not assume that callee saved registers are preserved across a call to this function.

This function fulfills requirement 1 and 3 listed above.

Function : plat_secondary_cold_boot_setup() [mandatory]

Argument : void
Return   : void

This function is called with the MMU and data caches disabled. It is responsible for placing the executing secondary CPU in a platform-specific state until the primary CPU performs the necessary actions to bring it out of that state and allow entry into the OS.

In the ARM FVP port, each secondary CPU powers itself off. The primary CPU is responsible for powering up the secondary CPU when normal world software requires them.

This function fulfills requirement 2 above.

Function : platform_is_primary_cpu() [mandatory]

Argument : unsigned long
Return   : unsigned int

This function identifies a CPU by its MPIDR, which is passed as the argument, to determine whether this CPU is the primary CPU or a secondary CPU. A return value of zero indicates that the CPU is not the primary CPU, while a non-zero return value indicates that the CPU is the primary CPU.

Function : platform_mem_init() [mandatory]

Argument : void
Return   : void

This function is called before any access to data is made by the firmware, in order to carry out any essential memory initialization.

The ARM FVP port uses this function to initialize the mailbox memory used for providing the warm-boot entry-point addresses.

Function: plat_match_rotpk()

Argument : const unsigned char *, unsigned int
Return   : int

This function is mandatory when Trusted Board Boot is enabled. It receives a pointer to a buffer containing a signing key and its size as parameters and returns 0 (success) if that key matches the ROT (Root Of Trust) key stored in the platform. Any other return value means a mismatch.

2.3 Common optional modifications

The following are helper functions implemented by the firmware that perform common platform-specific tasks. A platform may choose to override these definitions.

Function : platform_get_core_pos()

Argument : unsigned long
Return   : int

A platform may need to convert the MPIDR of a CPU to an absolute number, which can be used as a CPU-specific linear index into blocks of memory (for example while allocating per-CPU stacks). This routine contains a simple mechanism to perform this conversion, using the assumption that each cluster contains a maximum of 4 CPUs:

linear index = cpu_id + (cluster_id * 4)

cpu_id = 8-bit value in MPIDR at affinity level 0
cluster_id = 8-bit value in MPIDR at affinity level 1

Function : platform_set_stack()

Argument : unsigned long
Return   : void

This function sets the current stack pointer to the normal memory stack that has been allocated for the CPU specificed by MPIDR. For BL images that only require a stack for the primary CPU the parameter is ignored. The size of the stack allocated to each CPU is specified by the platform defined constant PLATFORM_STACK_SIZE.

Common implementations of this function for the UP and MP BL images are provided in plat/common/aarch64/platform_up_stack.S and plat/common/aarch64/platform_mp_stack.S

Function : platform_get_stack()

Argument : unsigned long
Return   : unsigned long

This function returns the base address of the normal memory stack that has been allocated for the CPU specificed by MPIDR. For BL images that only require a stack for the primary CPU the parameter is ignored. The size of the stack allocated to each CPU is specified by the platform defined constant PLATFORM_STACK_SIZE.

Common implementations of this function for the UP and MP BL images are provided in plat/common/aarch64/platform_up_stack.S and plat/common/aarch64/platform_mp_stack.S

Function : plat_report_exception()

Argument : unsigned int
Return   : void

A platform may need to report various information about its status when an exception is taken, for example the current exception level, the CPU security state (secure/non-secure), the exception type, and so on. This function is called in the following circumstances:

  • In BL1, whenever an exception is taken.
  • In BL2, whenever an exception is taken.

The default implementation doesn't do anything, to avoid making assumptions about the way the platform displays its status information.

This function receives the exception type as its argument. Possible values for exceptions types are listed in the include/runtime_svc.h header file. Note that these constants are not related to any architectural exception code; they are just an ARM Trusted Firmware convention.

Function : plat_reset_handler()

Argument : void
Return   : void

A platform may need to do additional initialization after reset. This function allows the platform to do the platform specific intializations. Platform specific errata workarounds could also be implemented here. The api should preserve the values of callee saved registers x19 to x29.

The default implementation doesn't do anything. If a platform needs to override the default implementation, refer to the Firmware Design Guide for general guidelines regarding placement of code in a reset handler.

Function : plat_disable_acp()

Argument : void
Return   : void

This api allows a platform to disable the Accelerator Coherency Port (if present) during a cluster power down sequence. The default weak implementation doesn't do anything. Since this api is called during the power down sequence, it has restrictions for stack usage and it can use the registers x0 - x17 as scratch registers. It should preserve the value in x18 register as it is used by the caller to store the return address.

  1. Modifications specific to a Boot Loader stage

3.1 Boot Loader Stage 1 (BL1)

BL1 implements the reset vector where execution starts from after a cold or warm boot. For each CPU, BL1 is responsible for the following tasks:

  1. Handling the reset as described in section 2.2

  2. In the case of a cold boot and the CPU being the primary CPU, ensuring that only this CPU executes the remaining BL1 code, including loading and passing control to the BL2 stage.

  3. Loading the BL2 image from non-volatile storage into secure memory at the address specified by the platform defined constant BL2_BASE.

  4. Populating a meminfo structure with the following information in memory, accessible by BL2 immediately upon entry.

    meminfo.total_base = Base address of secure RAM visible to BL2
meminfo.total_size = Size of secure RAM visible to BL2
meminfo.free_base  = Base address of secure RAM available for
                     allocation to BL2
meminfo.free_size  = Size of secure RAM available for allocation to BL2

    BL1 places this meminfo structure at the beginning of the free memory available for its use. Since BL1 cannot allocate memory dynamically at the moment, its free memory will be available for BL2's use as-is. However, this means that BL2 must read the meminfo structure before it starts using its free memory (this is discussed in Section 3.2).

    In future releases of the ARM Trusted Firmware it will be possible for the platform to decide where it wants to place the meminfo structure for BL2.

    BL1 implements the bl1_init_bl2_mem_layout() function to populate the BL2 meminfo structure. The platform may override this implementation, for example if the platform wants to restrict the amount of memory visible to BL2. Details of how to do this are given below.

The following functions need to be implemented by the platform port to enable BL1 to perform the above tasks.

Function : bl1_plat_arch_setup() [mandatory]

Argument : void
Return   : void

This function performs any platform-specific and architectural setup that the platform requires. Platform-specific setup might include configuration of memory controllers, configuration of the interconnect to allow the cluster to service cache snoop requests from another cluster, and so on.

In the ARM FVP port, this function enables CCI snoops into the cluster that the primary CPU is part of. It also enables the MMU.

This function helps fulfill requirement 2 above.

Function : bl1_platform_setup() [mandatory]

Argument : void
Return   : void

This function executes with the MMU and data caches enabled. It is responsible for performing any remaining platform-specific setup that can occur after the MMU and data cache have been enabled.

This function is also responsible for initializing the storage abstraction layer which is used to load further bootloader images.

This function helps fulfill requirement 3 above.

Function : bl1_plat_sec_mem_layout() [mandatory]

Argument : void
Return   : meminfo *

This function should only be called on the cold boot path. It executes with the MMU and data caches enabled. The pointer returned by this function must point to a meminfo structure containing the extents and availability of secure RAM for the BL1 stage.

meminfo.total_base = Base address of secure RAM visible to BL1
meminfo.total_size = Size of secure RAM visible to BL1
meminfo.free_base  = Base address of secure RAM available for allocation
                     to BL1
meminfo.free_size  = Size of secure RAM available for allocation to BL1

This information is used by BL1 to load the BL2 image in secure RAM. BL1 also populates a similar structure to tell BL2 the extents of memory available for its own use.

This function helps fulfill requirement 3 above.

Function : bl1_init_bl2_mem_layout() [optional]

Argument : meminfo *, meminfo *, unsigned int, unsigned long
Return   : void

BL1 needs to tell the next stage the amount of secure RAM available for it to use. This information is populated in a meminfo structure.

Depending upon where BL2 has been loaded in secure RAM (determined by BL2_BASE), BL1 calculates the amount of free memory available for BL2 to use. BL1 also ensures that its data sections resident in secure RAM are not visible to BL2. An illustration of how this is done in the ARM FVP port is given in the User Guide, in the Section "Memory layout on Base FVP".

Function : bl1_plat_set_bl2_ep_info() [mandatory]

Argument : image_info *, entry_point_info *
Return   : void

This function is called after loading BL2 image and it can be used to overwrite the entry point set by loader and also set the security state and SPSR which represents the entry point system state for BL2.

On FVP, we are setting the security state and the SPSR for the BL2 entrypoint

3.2 Boot Loader Stage 2 (BL2)

The BL2 stage is executed only by the primary CPU, which is determined in BL1 using the platform_is_primary_cpu() function. BL1 passed control to BL2 at BL2_BASE. BL2 executes in Secure EL1 and is responsible for:

  1. (Optional) Loading the BL3-0 binary image (if present) from platform provided non-volatile storage. To load the BL3-0 image, BL2 makes use of the meminfo returned by the bl2_plat_get_bl30_meminfo() function. The platform also defines the address in memory where BL3-0 is loaded through the optional constant BL30_BASE. BL2 uses this information to determine if there is enough memory to load the BL3-0 image. Subsequent handling of the BL3-0 image is platform-specific and is implemented in the bl2_plat_handle_bl30() function. If BL30_BASE is not defined then this step is not performed.

  2. Loading the BL3-1 binary image into secure RAM from non-volatile storage. To load the BL3-1 image, BL2 makes use of the meminfo structure passed to it by BL1. This structure allows BL2 to calculate how much secure RAM is available for its use. The platform also defines the address in secure RAM where BL3-1 is loaded through the constant BL31_BASE. BL2 uses this information to determine if there is enough memory to load the BL3-1 image.

  3. (Optional) Loading the BL3-2 binary image (if present) from platform provided non-volatile storage. To load the BL3-2 image, BL2 makes use of the meminfo returned by the bl2_plat_get_bl32_meminfo() function. The platform also defines the address in memory where BL3-2 is loaded through the optional constant BL32_BASE. BL2 uses this information to determine if there is enough memory to load the BL3-2 image. If BL32_BASE is not defined then this and the next step is not performed.

  4. (Optional) Arranging to pass control to the BL3-2 image (if present) that has been pre-loaded at BL32_BASE. BL2 populates an entry_point_info structure in memory provided by the platform with information about how BL3-1 should pass control to the BL3-2 image.

  5. Loading the normal world BL3-3 binary image into non-secure DRAM from platform storage and arranging for BL3-1 to pass control to this image. This address is determined using the plat_get_ns_image_entrypoint() function described below.

  6. BL2 populates an entry_point_info structure in memory provided by the platform with information about how BL3-1 should pass control to the other BL images.

The following functions must be implemented by the platform port to enable BL2 to perform the above tasks.

Function : bl2_early_platform_setup() [mandatory]

Argument : meminfo *
Return   : void

This function executes with the MMU and data caches disabled. It is only called by the primary CPU. The arguments to this function is the address of the meminfo structure populated by BL1.

The platform must copy the contents of the meminfo structure into a private variable as the original memory may be subsequently overwritten by BL2. The copied structure is made available to all BL2 code through the bl2_plat_sec_mem_layout() function.

Function : bl2_plat_arch_setup() [mandatory]

Argument : void
Return   : void

This function executes with the MMU and data caches disabled. It is only called by the primary CPU.

The purpose of this function is to perform any architectural initialization that varies across platforms, for example enabling the MMU (since the memory map differs across platforms).

Function : bl2_platform_setup() [mandatory]

Argument : void
Return   : void

This function may execute with the MMU and data caches enabled if the platform port does the necessary initialization in bl2_plat_arch_setup(). It is only called by the primary CPU.

The purpose of this function is to perform any platform initialization specific to BL2. Platform security components are configured if required. For the Base FVP the TZC-400 TrustZone controller is configured to only grant non-secure access to DRAM. This avoids aliasing between secure and non-secure accesses in the TLB and cache - secure execution states can use the NS attributes in the MMU translation tables to access the DRAM.

This function is also responsible for initializing the storage abstraction layer which is used to load further bootloader images.

Function : bl2_plat_sec_mem_layout() [mandatory]

Argument : void
Return   : meminfo *

This function should only be called on the cold boot path. It may execute with the MMU and data caches enabled if the platform port does the necessary initialization in bl2_plat_arch_setup(). It is only called by the primary CPU.

The purpose of this function is to return a pointer to a meminfo structure populated with the extents of secure RAM available for BL2 to use. See bl2_early_platform_setup() above.

Function : bl2_plat_get_bl30_meminfo() [mandatory]

Argument : meminfo *
Return   : void

This function is used to get the memory limits where BL2 can load the BL3-0 image. The meminfo provided by this is used by load_image() to validate whether the BL3-0 image can be loaded within the given memory from the given base.

Function : bl2_plat_handle_bl30() [mandatory]

Argument : image_info *
Return   : int

This function is called after loading BL3-0 image and it is used to perform any platform-specific actions required to handle the SCP firmware. Typically it transfers the image into SCP memory using a platform-specific protocol and waits until SCP executes it and signals to the Application Processor (AP) for BL2 execution to continue.

This function returns 0 on success, a negative error code otherwise.

Function : bl2_plat_get_bl31_params() [mandatory]

Argument : void
Return   : bl31_params *

BL2 platform code needs to return a pointer to a bl31_params structure it will use for passing information to BL3-1. The bl31_params structure carries the following information. - Header describing the version information for interpreting the bl31_param structure - Information about executing the BL3-3 image in the bl33_ep_info field - Information about executing the BL3-2 image in the bl32_ep_info field - Information about the type and extents of BL3-1 image in the bl31_image_info field - Information about the type and extents of BL3-2 image in the bl32_image_info field - Information about the type and extents of BL3-3 image in the bl33_image_info field

The memory pointed by this structure and its sub-structures should be accessible from BL3-1 initialisation code. BL3-1 might choose to copy the necessary content, or maintain the structures until BL3-3 is initialised.

Funtion : bl2_plat_get_bl31_ep_info() [mandatory]

Argument : void
Return   : entry_point_info *

BL2 platform code returns a pointer which is used to populate the entry point information for BL3-1 entry point. The location pointed by it should be accessible from BL1 while processing the synchronous exception to run to BL3-1.

On FVP this is allocated inside an bl2_to_bl31_params_mem structure which is allocated at an address pointed by PARAMS_BASE.

Function : bl2_plat_set_bl31_ep_info() [mandatory]

Argument : image_info *, entry_point_info *
Return   : void

This function is called after loading BL3-1 image and it can be used to overwrite the entry point set by loader and also set the security state and SPSR which represents the entry point system state for BL3-1.

On FVP, we are setting the security state and the SPSR for the BL3-1 entrypoint.

Function : bl2_plat_set_bl32_ep_info() [mandatory]

Argument : image_info *, entry_point_info *
Return   : void

This function is called after loading BL3-2 image and it can be used to overwrite the entry point set by loader and also set the security state and SPSR which represents the entry point system state for BL3-2.

On FVP, we are setting the security state and the SPSR for the BL3-2 entrypoint

Function : bl2_plat_set_bl33_ep_info() [mandatory]

Argument : image_info *, entry_point_info *
Return   : void

This function is called after loading BL3-3 image and it can be used to overwrite the entry point set by loader and also set the security state and SPSR which represents the entry point system state for BL3-3.

On FVP, we are setting the security state and the SPSR for the BL3-3 entrypoint

Function : bl2_plat_get_bl32_meminfo() [mandatory]

Argument : meminfo *
Return   : void

This function is used to get the memory limits where BL2 can load the BL3-2 image. The meminfo provided by this is used by load_image() to validate whether the BL3-2 image can be loaded with in the given memory from the given base.

Function : bl2_plat_get_bl33_meminfo() [mandatory]

Argument : meminfo *
Return   : void

This function is used to get the memory limits where BL2 can load the BL3-3 image. The meminfo provided by this is used by load_image() to validate whether the BL3-3 image can be loaded with in the given memory from the given base.

Function : bl2_plat_flush_bl31_params() [mandatory]

Argument : void
Return   : void

Once BL2 has populated all the structures that needs to be read by BL1 and BL3-1 including the bl31_params structures and its sub-structures, the bl31_ep_info structure and any platform specific data. It flushes all these data to the main memory so that it is available when we jump to later Bootloader stages with MMU off

Function : plat_get_ns_image_entrypoint() [mandatory]

Argument : void
Return   : unsigned long

As previously described, BL2 is responsible for arranging for control to be passed to a normal world BL image through BL3-1. This function returns the entrypoint of that image, which BL3-1 uses to jump to it.

BL2 is responsible for loading the normal world BL3-3 image (e.g. UEFI).

3.2 Boot Loader Stage 3-1 (BL3-1)

During cold boot, the BL3-1 stage is executed only by the primary CPU. This is determined in BL1 using the platform_is_primary_cpu() function. BL1 passes control to BL3-1 at BL31_BASE. During warm boot, BL3-1 is executed by all CPUs. BL3-1 executes at EL3 and is responsible for:

  1. Re-initializing all architectural and platform state. Although BL1 performs some of this initialization, BL3-1 remains resident in EL3 and must ensure that EL3 architectural and platform state is completely initialized. It should make no assumptions about the system state when it receives control.

  2. Passing control to a normal world BL image, pre-loaded at a platform- specific address by BL2. BL3-1 uses the entry_point_info structure that BL2 populated in memory to do this.

  3. Providing runtime firmware services. Currently, BL3-1 only implements a subset of the Power State Coordination Interface (PSCI) API as a runtime service. See Section 3.3 below for details of porting the PSCI implementation.

  4. Optionally passing control to the BL3-2 image, pre-loaded at a platform- specific address by BL2. BL3-1 exports a set of apis that allow runtime services to specify the security state in which the next image should be executed and run the corresponding image. BL3-1 uses the entry_point_info structure populated by BL2 to do this.

If BL3-1 is a reset vector, It also needs to handle the reset as specified in section 2.2 before the tasks described above.

The following functions must be implemented by the platform port to enable BL3-1 to perform the above tasks.

Function : bl31_early_platform_setup() [mandatory]

Argument : bl31_params *, void *
Return   : void

This function executes with the MMU and data caches disabled. It is only called by the primary CPU. The arguments to this function are:

  • The address of the bl31_params structure populated by BL2.
  • An opaque pointer that the platform may use as needed.

The platform can copy the contents of the bl31_params structure and its sub-structures into private variables if the original memory may be subsequently overwritten by BL3-1 and similarly the void * pointing to the platform data also needs to be saved.

On the ARM FVP port, BL2 passes a pointer to a bl31_params structure populated in the secure DRAM at address 0x6000000 in the bl31_params * argument and it does not use opaque pointer mentioned earlier. BL3-1 does not copy this information to internal data structures as it guarantees that the secure DRAM memory will not be overwritten. It maintains an internal reference to this information in the bl2_to_bl31_params variable.

Function : bl31_plat_arch_setup() [mandatory]

Argument : void
Return   : void

This function executes with the MMU and data caches disabled. It is only called by the primary CPU.

The purpose of this function is to perform any architectural initialization that varies across platforms, for example enabling the MMU (since the memory map differs across platforms).

Function : bl31_platform_setup() [mandatory]

Argument : void
Return   : void

This function may execute with the MMU and data caches enabled if the platform port does the necessary initialization in bl31_plat_arch_setup(). It is only called by the primary CPU.

The purpose of this function is to complete platform initialization so that both BL3-1 runtime services and normal world software can function correctly.

The ARM FVP port does the following:

  • Initializes the generic interrupt controller.
  • Configures the CLCD controller.
  • Enables system-level implementation of the generic timer counter.
  • Grants access to the system counter timer module
  • Initializes the FVP power controller device
  • Detects the system topology.

Function : bl31_get_next_image_info() [mandatory]

Argument : unsigned int
Return   : entry_point_info *

This function may execute with the MMU and data caches enabled if the platform port does the necessary initializations in bl31_plat_arch_setup().

This function is called by bl31_main() to retrieve information provided by BL2 for the next image in the security state specified by the argument. BL3-1 uses this information to pass control to that image in the specified security state. This function must return a pointer to the entry_point_info structure (that was copied during bl31_early_platform_setup()) if the image exists. It should return NULL otherwise.

3.3 Power State Coordination Interface (in BL3-1)

The ARM Trusted Firmware's implementation of the PSCI API is based around the concept of an affinity instance. Each affinity instance can be uniquely identified in a system by a CPU ID (the processor MPIDR is used in the PSCI interface) and an affinity level. A processing element (for example, a CPU) is at level 0. If the CPUs in the system are described in a tree where the node above a CPU is a logical grouping of CPUs that share some state, then affinity level 1 is that group of CPUs (for example, a cluster), and affinity level 2 is a group of clusters (for example, the system). The implementation assumes that the affinity level 1 ID can be computed from the affinity level 0 ID (for example, a unique cluster ID can be computed from the CPU ID). The current implementation computes this on the basis of the recommended use of MPIDR affinity fields in the ARM Architecture Reference Manual.

BL3-1's platform initialization code exports a pointer to the platform-specific power management operations required for the PSCI implementation to function correctly. This information is populated in the plat_pm_ops structure. The PSCI implementation calls members of the plat_pm_ops structure for performing power management operations for each affinity instance. For example, the target CPU is specified by its MPIDR in a PSCI CPU_ON call. The affinst_on() handler (if present) is called for each affinity instance as the PSCI implementation powers up each affinity level implemented in the MPIDR (for example, CPU, cluster and system).

The following functions must be implemented to initialize PSCI functionality in the ARM Trusted Firmware.

Function : plat_get_aff_count() [mandatory]

Argument : unsigned int, unsigned long
Return   : unsigned int

This function may execute with the MMU and data caches enabled if the platform port does the necessary initializations in bl31_plat_arch_setup(). It is only called by the primary CPU.

This function is called by the PSCI initialization code to detect the system topology. Its purpose is to return the number of affinity instances implemented at a given affinity level (specified by the first argument) and a given MPIDR (specified by the second argument). For example, on a dual-cluster system where first cluster implements 2 CPUs and the second cluster implements 4 CPUs, a call to this function with an MPIDR corresponding to the first cluster (0x0) and affinity level 0, would return 2. A call to this function with an MPIDR corresponding to the second cluster (0x100) and affinity level 0, would return 4.

Function : plat_get_aff_state() [mandatory]

Argument : unsigned int, unsigned long
Return   : unsigned int

This function may execute with the MMU and data caches enabled if the platform port does the necessary initializations in bl31_plat_arch_setup(). It is only called by the primary CPU.

This function is called by the PSCI initialization code. Its purpose is to return the state of an affinity instance. The affinity instance is determined by the affinity ID at a given affinity level (specified by the first argument) and an MPIDR (specified by the second argument). The state can be one of PSCI_AFF_PRESENT or PSCI_AFF_ABSENT. The latter state is used to cater for system topologies where certain affinity instances are unimplemented. For example, consider a platform that implements a single cluster with 4 CPUs and another CPU implemented directly on the interconnect with the cluster. The MPIDRs of the cluster would range from 0x0-0x3. The MPIDR of the single CPU would be 0x100 to indicate that it does not belong to cluster 0. Cluster 1 is missing but needs to be accounted for to reach this single CPU in the topology tree. Hence it is marked as PSCI_AFF_ABSENT.

Function : plat_get_max_afflvl() [mandatory]

Argument : void
Return   : int

This function may execute with the MMU and data caches enabled if the platform port does the necessary initializations in bl31_plat_arch_setup(). It is only called by the primary CPU.

This function is called by the PSCI implementation both during cold and warm boot, to determine the maximum affinity level that the power management operations should apply to. ARMv8-A has support for 4 affinity levels. It is likely that hardware will implement fewer affinity levels. This function allows the PSCI implementation to consider only those affinity levels in the system that the platform implements. For example, the Base AEM FVP implements two clusters with a configurable number of CPUs. It reports the maximum affinity level as 1, resulting in PSCI power control up to the cluster level.

Function : platform_setup_pm() [mandatory]

Argument : const plat_pm_ops **
Return   : int

This function may execute with the MMU and data caches enabled if the platform port does the necessary initializations in bl31_plat_arch_setup(). It is only called by the primary CPU.

This function is called by PSCI initialization code. Its purpose is to export handler routines for platform-specific power management actions by populating the passed pointer with a pointer to BL3-1's private plat_pm_ops structure.

A description of each member of this structure is given below. Please refer to the ARM FVP specific implementation of these handlers in [plat/fvp/fvp_pm.c] as an example. A platform port is expected to implement these handlers if the corresponding PSCI operation is to be supported and these handlers are expected to succeed if the return type is void.

plat_pm_ops.affinst_standby()

Perform the platform-specific setup to enter the standby state indicated by the passed argument. The generic code expects the handler to succeed.

plat_pm_ops.affinst_on()

Perform the platform specific setup to power on an affinity instance, specified by the MPIDR (first argument) and affinity level (third argument). The state (fourth argument) contains the current state of that affinity instance (ON or OFF). This is useful to determine whether any action must be taken. For example, while powering on a CPU, the cluster that contains this CPU might already be in the ON state. The platform decides what actions must be taken to transition from the current state to the target state (indicated by the power management operation). The generic code expects the platform to return E_SUCCESS on success or E_INTERN_FAIL for any failure.

plat_pm_ops.affinst_off()

Perform the platform specific setup to power off an affinity instance of the calling CPU. It is called by the PSCI CPU_OFF API implementation.

The affinity level (first argument) and state (second argument) have a similar meaning as described in the affinst_on() operation. They are used to identify the affinity instance on which the call is made and its current state. This gives the platform port an indication of the state transition it must make to perform the requested action. For example, if the calling CPU is the last powered on CPU in the cluster, after powering down affinity level 0 (CPU), the platform port should power down affinity level 1 (the cluster) as well. The generic code expects the handler to succeed.

plat_pm_ops.affinst_suspend()

Perform the platform specific setup to power off an affinity instance of the calling CPU. It is called by the PSCI CPU_SUSPEND API implementation.

The affinity level (second argument) and state (third argument) have a similar meaning as described in the affinst_on() operation. They are used to identify the affinity instance on which the call is made and its current state. This gives the platform port an indication of the state transition it must make to perform the requested action. For example, if the calling CPU is the last powered on CPU in the cluster, after powering down affinity level 0 (CPU), the platform port should power down affinity level 1 (the cluster) as well.

The difference between turning an affinity instance off versus suspending it is that in the former case, the affinity instance is expected to re-initialize its state when its next powered on (see affinst_on_finish()). In the latter case, the affinity instance is expected to save enough state so that it can resume execution by restoring this state when its powered on (see affinst_suspend_finish()).The generic code expects the handler to succeed.

plat_pm_ops.affinst_on_finish()

This function is called by the PSCI implementation after the calling CPU is powered on and released from reset in response to an earlier PSCI CPU_ON call. It performs the platform-specific setup required to initialize enough state for this CPU to enter the normal world and also provide secure runtime firmware services.

The affinity level (first argument) and state (second argument) have a similar meaning as described in the previous operations. The generic code expects the handler to succeed.

plat_pm_ops.affinst_on_suspend()

This function is called by the PSCI implementation after the calling CPU is powered on and released from reset in response to an asynchronous wakeup event, for example a timer interrupt that was programmed by the CPU during the CPU_SUSPEND call. It performs the platform-specific setup required to restore the saved state for this CPU to resume execution in the normal world and also provide secure runtime firmware services.

The affinity level (first argument) and state (second argument) have a similar meaning as described in the previous operations. The generic code expects the platform to succeed.

plat_pm_ops.validate_power_state()

This function is called by the PSCI implementation during the CPU_SUSPEND call to validate the power_state parameter of the PSCI API. If the power_state is known to be invalid, the platform must return PSCI_E_INVALID_PARAMS as error, which is propagated back to the normal world PSCI client.

plat_pm_ops.validate_ns_entrypoint()

This function is called by the PSCI implementation during the CPU_SUSPEND and CPU_ON calls to validate the non-secure entry_point parameter passed by the normal world. If the entry_point is known to be invalid, the platform must return PSCI_E_INVALID_PARAMS as error, which is propagated back to the normal world PSCI client.

BL3-1 platform initialization code must also detect the system topology and the state of each affinity instance in the topology. This information is critical for the PSCI runtime service to function correctly. More details are provided in the description of the plat_get_aff_count() and plat_get_aff_state() functions above.

3.4 Interrupt Management framework (in BL3-1)

BL3-1 implements an Interrupt Management Framework (IMF) to manage interrupts generated in either security state and targeted to EL1 or EL2 in the non-secure state or EL3/S-EL1 in the secure state. The design of this framework is described in the IMF Design Guide

A platform should export the following APIs to support the IMF. The following text briefly describes each api and its implementation on the FVP port. The API implementation depends upon the type of interrupt controller present in the platform. The FVP implements an ARM Generic Interrupt Controller (ARM GIC) as per the version 2.0 of the ARM GIC Architecture Specification

Function : plat_interrupt_type_to_line() [mandatory]

Argument : uint32_t, uint32_t
Return   : uint32_t

The ARM processor signals an interrupt exception either through the IRQ or FIQ interrupt line. The specific line that is signaled depends on how the interrupt controller (IC) reports different interrupt types from an execution context in either security state. The IMF uses this API to determine which interrupt line the platform IC uses to signal each type of interrupt supported by the framework from a given security state.

The first parameter will be one of the INTR_TYPE_* values (see IMF Design Guide) indicating the target type of the interrupt, the second parameter is the security state of the originating execution context. The return result is the bit position in the SCR_EL3 register of the respective interrupt trap: IRQ=1, FIQ=2.

The FVP port configures the ARM GIC to signal S-EL1 interrupts as FIQs and Non-secure interrupts as IRQs from either security state.

Function : plat_ic_get_pending_interrupt_type() [mandatory]

Argument : void
Return   : uint32_t

This API returns the type of the highest priority pending interrupt at the platform IC. The IMF uses the interrupt type to retrieve the corresponding handler function. INTR_TYPE_INVAL is returned when there is no interrupt pending. The valid interrupt types that can be returned are INTR_TYPE_EL3, INTR_TYPE_S_EL1 and INTR_TYPE_NS.

The FVP port reads the Highest Priority Pending Interrupt Register (GICC_HPPIR) to determine the id of the pending interrupt. The type of interrupt depends upon the id value as follows.

  1. id < 1022 is reported as a S-EL1 interrupt
  2. id = 1022 is reported as a Non-secure interrupt.
  3. id = 1023 is reported as an invalid interrupt type.

Function : plat_ic_get_pending_interrupt_id() [mandatory]

Argument : void
Return   : uint32_t

This API returns the id of the highest priority pending interrupt at the platform IC. The IMF passes the id returned by this API to the registered handler for the pending interrupt if the IMF_READ_INTERRUPT_ID build time flag is set. INTR_ID_UNAVAILABLE is returned when there is no interrupt pending.

The FVP port reads the Highest Priority Pending Interrupt Register (GICC_HPPIR) to determine the id of the pending interrupt. The id that is returned by API depends upon the value of the id read from the interrupt controller as follows.

  1. id < 1022. id is returned as is.
  2. id = 1022. The Aliased Highest Priority Pending Interrupt Register (GICC_AHPPIR) is read to determine the id of the non-secure interrupt. This id is returned by the API.
  3. id = 1023. INTR_ID_UNAVAILABLE is returned.

Function : plat_ic_acknowledge_interrupt() [mandatory]

Argument : void
Return   : uint32_t

This API is used by the CPU to indicate to the platform IC that processing of the highest pending interrupt has begun. It should return the id of the interrupt which is being processed.

The FVP port reads the Interrupt Acknowledge Register (GICC_IAR). This changes the state of the highest priority pending interrupt from pending to active in the interrupt controller. It returns the value read from the GICC_IAR. This value is the id of the interrupt whose state has been changed.

The TSP uses this API to start processing of the secure physical timer interrupt.

Function : plat_ic_end_of_interrupt() [mandatory]

Argument : uint32_t
Return   : void

This API is used by the CPU to indicate to the platform IC that processing of the interrupt corresponding to the id (passed as the parameter) has finished. The id should be the same as the id returned by the plat_ic_acknowledge_interrupt() API.

The FVP port writes the id to the End of Interrupt Register (GICC_EOIR). This deactivates the corresponding interrupt in the interrupt controller.

The TSP uses this API to finish processing of the secure physical timer interrupt.

Function : plat_ic_get_interrupt_type() [mandatory]

Argument : uint32_t
Return   : uint32_t

This API returns the type of the interrupt id passed as the parameter. INTR_TYPE_INVAL is returned if the id is invalid. If the id is valid, a valid interrupt type (one of INTR_TYPE_EL3, INTR_TYPE_S_EL1 and INTR_TYPE_NS) is returned depending upon how the interrupt has been configured by the platform IC.

The FVP port configures S-EL1 interrupts as Group0 interrupts and Non-secure interrupts as Group1 interrupts. It reads the group value corresponding to the interrupt id from the relevant Interrupt Group Register (GICD_IGROUPRn). It uses the group value to determine the type of interrupt.

3.5 Crash Reporting mechanism (in BL3-1)

BL3-1 implements a crash reporting mechanism which prints the various registers of the CPU to enable quick crash analysis and debugging. It requires that a console is designated as the crash console by the platform which will be used to print the register dump.

The following functions must be implemented by the platform if it wants crash reporting mechanism in BL3-1. The functions are implemented in assembly so that they can be invoked without a C Runtime stack.

Function : plat_crash_console_init

Argument : void
Return   : int

This API is used by the crash reporting mechanism to initialize the crash console. It should only use the general purpose registers x0 to x2 to do the initialization and returns 1 on success.

The FVP port designates the PL011_UART0 as the crash console and calls the console_core_init() to initialize the console.

Function : plat_crash_console_putc

Argument : int
Return   : int

This API is used by the crash reporting mechanism to print a character on the designated crash console. It should only use general purpose registers x1 and x2 to do its work. The parameter and the return value are in general purpose register x0.

The FVP port designates the PL011_UART0 as the crash console and calls the console_core_putc() to print the character on the console.

  1. Build flags

There are some build flags which can be defined by the platform to control inclusion or exclusion of certain BL stages from the FIP image. These flags need to be defined in the platform makefile which will get included by the build system.

  • NEED_BL30 This flag if defined by the platform mandates that a BL3-0 binary should be included in the FIP image. The path to the BL3-0 binary can be specified by the BL30 build option (see build options in the User Guide).

  • NEED_BL33 By default, this flag is defined yes by the build system and BL33 build option should be supplied as a build option. The platform has the option of excluding the BL3-3 image in the fip image by defining this flag to no.

  1. C Library

To avoid subtle toolchain behavioral dependencies, the header files provided by the compiler are not used. The software is built with the -nostdinc flag to ensure no headers are included from the toolchain inadvertently. Instead the required headers are included in the ARM Trusted Firmware source tree. The library only contains those C library definitions required by the local implementation. If more functionality is required, the needed library functions will need to be added to the local implementation.

Versions of FreeBSD headers can be found in include/stdlib. Some of these headers have been cut down in order to simplify the implementation. In order to minimize changes to the header files, the FreeBSD layout has been maintained. The generic C library definitions can be found in include/stdlib with more system and machine specific declarations in include/stdlib/sys and include/stdlib/machine.

The local C library implementations can be found in lib/stdlib. In order to extend the C library these files may need to be modified. It is recommended to use a release version of FreeBSD as a starting point.

The C library header files in the FreeBSD source tree are located in the include and sys/sys directories. FreeBSD machine specific definitions can be found in the sys/<machine-type> directories. These files define things like 'the size of a pointer' and 'the range of an integer'. Since an AArch64 port for FreeBSD does not yet exist, the machine specific definitions are based on existing machine types with similar properties (for example SPARC64).

Where possible, C library function implementations were taken from FreeBSD as found in the lib/libc directory.

A copy of the FreeBSD sources can be downloaded with git.

git clone git://github.com/freebsd/freebsd.git -b origin/release/9.2.0
  1. Storage abstraction layer

In order to improve platform independence and portability an storage abstraction layer is used to load data from non-volatile platform storage.

Each platform should register devices and their drivers via the Storage layer. These drivers then need to be initialized by bootloader phases as required in their respective blx_platform_setup() functions. Currently storage access is only required by BL1 and BL2 phases. The load_image() function uses the storage layer to access non-volatile platform storage.

It is mandatory to implement at least one storage driver. For the FVP the Firmware Image Package(FIP) driver is provided as the default means to load data from storage (see the "Firmware Image Package" section in the User Guide). The storage layer is described in the header file include/drivers/io/io_storage.h. The implementation of the common library is in drivers/io/io_storage.c and the driver files are located in drivers/io/.

Each IO driver must provide io_dev_* structures, as described in drivers/io/io_driver.h. These are returned via a mandatory registration function that is called on platform initialization. The semi-hosting driver implementation in io_semihosting.c can be used as an example.

The Storage layer provides mechanisms to initialize storage devices before IO operations are called. The basic operations supported by the layer include open(), close(), read(), write(), size() and seek(). Drivers do not have to implement all operations, but each platform must provide at least one driver for a device capable of supporting generic operations such as loading a bootloader image.

The current implementation only allows for known images to be loaded by the firmware. These images are specified by using their names, as defined in include/plat/common/platform.h. The platform layer (plat_get_image_source()) then returns a reference to a device and a driver-specific spec which will be understood by the driver to allow access to the image data.

The layer is designed in such a way that is it possible to chain drivers with other drivers. For example, file-system drivers may be implemented on top of physical block devices, both represented by IO devices with corresponding drivers. In such a case, the file-system "binding" with the block device may be deferred until the file-system device is initialised.

The abstraction currently depends on structures being statically allocated by the drivers and callers, as the system does not yet provide a means of dynamically allocating memory. This may also have the affect of limiting the amount of open resources per driver.

Copyright (c) 2013-2014, ARM Limited and Contributors. All rights reserved.