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Trusted Firmware-A Porting Guide

Contents

1.   Introduction

Porting Trusted Firmware-A (TF-A) 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 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.

Platform ports that want to be aligned with standard Arm platforms (for example FVP and Juno) may also use include/plat/arm/common/plat_arm.h and the corresponding source files in plat/arm/common/. These provide standard implementations for some of the required platform porting functions. However, using these functions requires the platform port to implement additional Arm standard platform porting functions. These additional functions are not documented here.

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 TF-A User Guide.

Please refer to the Platform compatibility policy for the policy regarding compatibility and deprecation of these porting interfaces.

2.   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.

3.   Common mandatory modifications

A platform port must enable the Memory Management Unit (MMU) as well as the instruction and data caches for each BL stage. Setting up the translation tables is the responsibility of the platform port because memory maps differ across platforms. A memory translation library (see lib/xlat_tables/) is provided to help in this setup.

Note that although this library supports non-identity mappings, this is intended only for re-mapping peripheral physical addresses and allows platforms with high I/O addresses to reduce their virtual address space. All other addresses corresponding to code and data must currently use an identity mapping.

Also, the only translation granule size supported in TF-A is 4KB, as various parts of the code assume that is the case. It is not possible to switch to 16 KB or 64 KB granule sizes at the moment.

In Arm standard platforms, each BL stage configures the MMU in the platform-specific architecture setup function, blX_plat_arch_setup(), and uses an identity mapping for all addresses.

If the build option USE_COHERENT_MEM is enabled, each platform can allocate a block of identity mapped secure memory with Device-nGnRE attributes aligned to page boundary (4K) for each BL stage. All sections which allocate coherent memory are grouped under coherent_ram. For ex: Bakery locks are placed in a section identified by name bakery_lock inside coherent_ram so that its possible for the firmware to place variables in it using the following C code directive:

__section("bakery_lock")

Or alternatively the following assembler code directive:

.section bakery_lock

The coherent_ram section is a sum of all sections like bakery_lock which are 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.

3.1.   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 will require updating the list of PLAT_INCLUDES in the platform.mk file.

Platform ports may optionally use the file include/plat/common/common_def.h, which provides typical values for some of the constants below. These values are likely to be suitable for all platform ports.

  • #define : PLATFORM_LINKER_FORMAT

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

  • #define : PLATFORM_LINKER_ARCH

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

  • #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 : CACHE_WRITEBACK_GRANULE

    Defines the size in bits of the largest cache line across all the cache levels in the platform.

  • #define : FIRMWARE_WELCOME_STR

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

  • #define : PLATFORM_CORE_COUNT

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

  • #define : PLAT_NUM_PWR_DOMAINS

    Defines the total number of nodes in the power domain topology tree at all the power domain levels used by the platform. This macro is used by the PSCI implementation to allocate data structures to represent power domain topology.

  • #define : PLAT_MAX_PWR_LVL

    Defines the maximum power domain level that the power management operations should apply to. More often, but not always, the power domain level corresponds to affinity level. This macro allows the PSCI implementation to know the highest power domain level that it should consider for power management operations in the system that the platform implements. For example, the Base AEM FVP implements two clusters with a configurable number of CPUs and it reports the maximum power domain level as 1.

  • #define : PLAT_MAX_OFF_STATE

    Defines the local power state corresponding to the deepest power down possible at every power domain level in the platform. The local power states for each level may be sparsely allocated between 0 and this value with 0 being reserved for the RUN state. The PSCI implementation uses this value to initialize the local power states of the power domain nodes and to specify the requested power state for a PSCI_CPU_OFF call.

  • #define : PLAT_MAX_RET_STATE

    Defines the local power state corresponding to the deepest retention state possible at every power domain level in the platform. This macro should be a value less than PLAT_MAX_OFF_STATE and greater than 0. It is used by the PSCI implementation to distinguish between retention and power down local power states within PSCI_CPU_SUSPEND call.

  • #define : PLAT_MAX_PWR_LVL_STATES

    Defines the maximum number of local power states per power domain level that the platform supports. The default value of this macro is 2 since most platforms just support a maximum of two local power states at each power domain level (power-down and retention). If the platform needs to account for more local power states, then it must redefine this macro.

    Currently, this macro is used by the Generic PSCI implementation to size the array used for PSCI_STAT_COUNT/RESIDENCY accounting.

  • #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. This constant is not applicable when BL2_IN_XIP_MEM is set to '1'.

  • #define : BL2_LIMIT

    Defines the maximum address in secure RAM that the BL2 image can occupy. This constant is not applicable when BL2_IN_XIP_MEM is set to '1'.

  • #define : BL2_RO_BASE

    Defines the base address in secure XIP memory where BL2 RO section originally lives. Must be aligned on a page-size boundary. This constant is only needed when BL2_IN_XIP_MEM is set to '1'.

  • #define : BL2_RO_LIMIT

    Defines the maximum address in secure XIP memory that BL2's actual content (i.e. excluding any data section allocated at runtime) can occupy. This constant is only needed when BL2_IN_XIP_MEM is set to '1'.

  • #define : BL2_RW_BASE

    Defines the base address in secure RAM where BL2's read-write data will live at runtime. Must be aligned on a page-size boundary. This constant is only needed when BL2_IN_XIP_MEM is set to '1'.

  • #define : BL2_RW_LIMIT

    Defines the maximum address in secure RAM that BL2's read-write data can occupy at runtime. This constant is only needed when BL2_IN_XIP_MEM is set to '1'.

  • #define : BL31_BASE

    Defines the base address in secure RAM where BL2 loads the BL31 binary image. Must be aligned on a page-size boundary.

  • #define : BL31_LIMIT

    Defines the maximum address in secure RAM that the BL31 image can occupy.

For every image, the platform must define individual identifiers that will be used by BL1 or BL2 to load the corresponding image into memory from non-volatile storage. For the sake of performance, integer numbers will be used as identifiers. The platform will use those identifiers to return the relevant information about the image to be loaded (file handler, load address, authentication information, etc.). The following image identifiers are mandatory:

  • #define : BL2_IMAGE_ID

    BL2 image identifier, used by BL1 to load BL2.

  • #define : BL31_IMAGE_ID

    BL31 image identifier, used by BL2 to load BL31.

  • #define : BL33_IMAGE_ID

    BL33 image identifier, used by BL2 to load BL33.

If Trusted Board Boot is enabled, the following certificate identifiers must also be defined:

  • #define : TRUSTED_BOOT_FW_CERT_ID

    BL2 content certificate identifier, used by BL1 to load the BL2 content certificate.

  • #define : TRUSTED_KEY_CERT_ID

    Trusted key certificate identifier, used by BL2 to load the trusted key certificate.

  • #define : SOC_FW_KEY_CERT_ID

    BL31 key certificate identifier, used by BL2 to load the BL31 key certificate.

  • #define : SOC_FW_CONTENT_CERT_ID

    BL31 content certificate identifier, used by BL2 to load the BL31 content certificate.

  • #define : NON_TRUSTED_FW_KEY_CERT_ID

    BL33 key certificate identifier, used by BL2 to load the BL33 key certificate.

  • #define : NON_TRUSTED_FW_CONTENT_CERT_ID

    BL33 content certificate identifier, used by BL2 to load the BL33 content certificate.

  • #define : FWU_CERT_ID

    Firmware Update (FWU) certificate identifier, used by NS_BL1U to load the FWU content certificate.

  • #define : PLAT_CRYPTOCELL_BASE

    This defines the base address of Arm® TrustZone® CryptoCell and must be defined if CryptoCell crypto driver is used for Trusted Board Boot. For capable Arm platforms, this driver is used if ARM_CRYPTOCELL_INTEG is set.

If the AP Firmware Updater Configuration image, BL2U is used, the following must also be defined:

  • #define : BL2U_BASE

    Defines the base address in secure memory where BL1 copies the BL2U binary image. Must be aligned on a page-size boundary.

  • #define : BL2U_LIMIT

    Defines the maximum address in secure memory that the BL2U image can occupy.

  • #define : BL2U_IMAGE_ID

    BL2U image identifier, used by BL1 to fetch an image descriptor corresponding to BL2U.

If the SCP Firmware Update Configuration Image, SCP_BL2U is used, the following must also be defined:

  • #define : SCP_BL2U_IMAGE_ID

    SCP_BL2U image identifier, used by BL1 to fetch an image descriptor corresponding to SCP_BL2U. NOTE: TF-A does not provide source code for this image.

If the Non-Secure Firmware Updater ROM, NS_BL1U is used, the following must also be defined:

  • #define : NS_BL1U_BASE

    Defines the base address in non-secure ROM where NS_BL1U executes. Must be aligned on a page-size boundary. NOTE: TF-A does not provide source code for this image.

  • #define : NS_BL1U_IMAGE_ID

    NS_BL1U image identifier, used by BL1 to fetch an image descriptor corresponding to NS_BL1U.

If the Non-Secure Firmware Updater, NS_BL2U is used, the following must also be defined:

  • #define : NS_BL2U_BASE

    Defines the base address in non-secure memory where NS_BL2U executes. Must be aligned on a page-size boundary. NOTE: TF-A does not provide source code for this image.

  • #define : NS_BL2U_IMAGE_ID

    NS_BL2U image identifier, used by BL1 to fetch an image descriptor corresponding to NS_BL2U.

For the the Firmware update capability of TRUSTED BOARD BOOT, the following macros may also be defined:

  • #define : PLAT_FWU_MAX_SIMULTANEOUS_IMAGES

    Total number of images that can be loaded simultaneously. If the platform doesn't specify any value, it defaults to 10.

If a SCP_BL2 image is supported by the platform, the following constants must also be defined:

  • #define : SCP_BL2_IMAGE_ID

    SCP_BL2 image identifier, used by BL2 to load SCP_BL2 into secure memory from platform storage before being transferred to the SCP.

  • #define : SCP_FW_KEY_CERT_ID

    SCP_BL2 key certificate identifier, used by BL2 to load the SCP_BL2 key certificate (mandatory when Trusted Board Boot is enabled).

  • #define : SCP_FW_CONTENT_CERT_ID

    SCP_BL2 content certificate identifier, used by BL2 to load the SCP_BL2 content certificate (mandatory when Trusted Board Boot is enabled).

If a BL32 image is supported by the platform, the following constants must also be defined:

  • #define : BL32_IMAGE_ID

    BL32 image identifier, used by BL2 to load BL32.

  • #define : TRUSTED_OS_FW_KEY_CERT_ID

    BL32 key certificate identifier, used by BL2 to load the BL32 key certificate (mandatory when Trusted Board Boot is enabled).

  • #define : TRUSTED_OS_FW_CONTENT_CERT_ID

    BL32 content certificate identifier, used by BL2 to load the BL32 content certificate (mandatory when Trusted Board Boot is enabled).

  • #define : BL32_BASE

    Defines the base address in secure memory where BL2 loads the BL32 binary image. Must be aligned on a page-size boundary.

  • #define : BL32_LIMIT

    Defines the maximum address that the BL32 image can occupy.

If the Test Secure-EL1 Payload (TSP) instantiation of BL32 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 BL32 image on the platform. TSP_SEC_MEM_BASE and TSP_SEC_MEM_SIZE must fully accommodate the memory required by the BL32 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 translation table library code, the following constants must also be defined:

  • #define : PLAT_XLAT_TABLES_DYNAMIC

    Optional flag that can be set per-image to enable the dynamic allocation of regions even when the MMU is enabled. If not defined, only static functionality will be available, if defined and set to 1 it will also include the dynamic functionality.

  • #define : MAX_XLAT_TABLES

    Defines the maximum number of translation tables that are allocated by the translation table library code. To minimize the amount of runtime memory used, choose the smallest value needed to map the required virtual addresses for each BL stage. If PLAT_XLAT_TABLES_DYNAMIC flag is enabled for a BL image, MAX_XLAT_TABLES must be defined to accommodate the dynamic regions as well.

  • #define : MAX_MMAP_REGIONS

    Defines the maximum number of regions that are allocated by the translation table library code. A region consists of physical base address, virtual base address, size and attributes (Device/Memory, RO/RW, Secure/Non-Secure), as defined in the mmap_region_t structure. The platform defines the regions that should be mapped. Then, the translation table library will create the corresponding tables and descriptors at runtime. To minimize the amount of runtime memory used, choose the smallest value needed to register the required regions for each BL stage. If PLAT_XLAT_TABLES_DYNAMIC flag is enabled for a BL image, MAX_MMAP_REGIONS must be defined to accommodate the dynamic regions as well.

  • #define : PLAT_VIRT_ADDR_SPACE_SIZE

    Defines the total size of the virtual address space in bytes. For example, for a 32 bit virtual address space, this value should be (1ULL << 32).

  • #define : PLAT_PHY_ADDR_SPACE_SIZE

    Defines the total size of the physical address space in bytes. For example, for a 32 bit physical address space, this value should be (1ULL << 32).

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 -ENOMEM.

  • #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 -ENOMEM.

  • #define : MAX_IO_BLOCK_DEVICES

    Defines the maximum number of registered IO block devices. Attempting to register more devices this value using io_dev_open() will fail with -ENOMEM. MAX_IO_BLOCK_DEVICES should be less than MAX_IO_DEVICES. With this macro, multiple block devices could be supported at the same time.

If the platform needs to allocate data within the per-cpu data framework in BL31, 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 in Arm standard platforms.

  • #define : BL31_PROGBITS_LIMIT

    Defines the maximum address in secure RAM that the BL31's progbits sections can occupy.

  • #define : TSP_PROGBITS_LIMIT

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

If the platform port uses the PL061 GPIO driver, the following constant may optionally be defined:

  • PLAT_PL061_MAX_GPIOS Maximum number of GPIOs required by the platform. This allows control how much memory is allocated for PL061 GPIO controllers. The default value is
    1. $(eval $(call add_define,PLAT_PL061_MAX_GPIOS))

If the platform port uses the partition driver, the following constant may optionally be defined:

  • PLAT_PARTITION_MAX_ENTRIES Maximum number of partition entries required by the platform. This allows control how much memory is allocated for partition entries. The default value is 128. For example, define the build flag in platform.mk: PLAT_PARTITION_MAX_ENTRIES := 12 $(eval $(call add_define,PLAT_PARTITION_MAX_ENTRIES))

The following constant is optional. It should be defined to override the default behaviour of the assert() function (for example, to save memory).

  • PLAT_LOG_LEVEL_ASSERT If PLAT_LOG_LEVEL_ASSERT is higher or equal than LOG_LEVEL_VERBOSE, assert() prints the name of the file, the line number and the asserted expression. Else if it is higher than LOG_LEVEL_INFO, it prints the file name and the line number. Else if it is lower than LOG_LEVEL_INFO, it doesn't print anything to the console. If PLAT_LOG_LEVEL_ASSERT isn't defined, it defaults to LOG_LEVEL.

If the platform port uses the Activity Monitor Unit, the following constants may be defined:

  • PLAT_AMU_GROUP1_COUNTERS_MASK This mask reflects the set of group counters that should be enabled. The maximum number of group 1 counters supported by AMUv1 is 16 so the mask can be at most 0xffff. If the platform does not define this mask, no group 1 counters are enabled. If the platform defines this mask, the following constant needs to also be defined.
  • PLAT_AMU_GROUP1_NR_COUNTERS This value is used to allocate an array to save and restore the counters specified by PLAT_AMU_GROUP1_COUNTERS_MASK on CPU suspend. This value should be equal to the highest bit position set in the mask, plus 1. The maximum number of group 1 counters in AMUv1 is 16.

3.2.   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 development platforms, this file is found in plat/arm/board/<plat_name>/include/plat_macros.S.

  • Macro : plat_crash_print_regs

    This macro allows the crash reporting routine to print relevant platform registers in case of an unhandled exception in BL31. This aids in debugging and this macro can be defined to be empty in case register reporting is not desired.

    For instance, GIC or interconnect registers may be helpful for troubleshooting.

4.   Handling Reset

BL1 by default implements the reset vector where execution starts from a cold or warm boot. BL31 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 BL31 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.

4.1.   Function : plat_get_my_entrypoint() [mandatory when PROGRAMMABLE_RESET_ADDRESS == 0]

Argument : void
Return   : uintptr_t

This function is called with the MMU and caches disabled (SCTLR_EL3.M = 0 and SCTLR_EL3.C = 0). The function is responsible for distinguishing between a warm and cold reset for the current CPU using platform-specific means. If it's a warm reset, then it returns the warm reset entrypoint point provided to plat_setup_psci_ops() during BL31 initialization. If it's a cold reset then this function must return zero.

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.

Note that for platforms that support programming the reset address, it is expected that a CPU will start executing code directly at the right address, both on a cold and warm reset. In this case, there is no need to identify the type of reset nor to query the warm reset entrypoint. Therefore, implementing this function is not required on such platforms.

4.2.   Function : plat_secondary_cold_boot_setup() [mandatory when COLD_BOOT_SINGLE_CPU == 0]

Argument : 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. This function must not return.

In the Arm FVP port, when using the normal boot flow, each secondary CPU powers itself off. The primary CPU is responsible for powering up the secondary CPUs when normal world software requires them. When booting an EL3 payload instead, they stay powered on and are put in a holding pen until their mailbox gets populated.

This function fulfills requirement 2 above.

Note that for platforms that can't release secondary CPUs out of reset, only the primary CPU will execute the cold boot code. Therefore, implementing this function is not required on such platforms.

4.3.   Function : plat_is_my_cpu_primary() [mandatory when COLD_BOOT_SINGLE_CPU == 0]

Argument : void
Return   : unsigned int

This function identifies whether the current 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.

Note that for platforms that can't release secondary CPUs out of reset, only the primary CPU will execute the cold boot code. Therefore, there is no need to distinguish between primary and secondary CPUs and implementing this function is not required.

4.4.   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.

4.5.   Function: plat_get_rotpk_info()

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

This function is mandatory when Trusted Board Boot is enabled. It returns a pointer to the ROTPK stored in the platform (or a hash of it) and its length. The ROTPK must be encoded in DER format according to the following ASN.1 structure:

AlgorithmIdentifier  ::=  SEQUENCE  {
    algorithm         OBJECT IDENTIFIER,
    parameters        ANY DEFINED BY algorithm OPTIONAL
}

SubjectPublicKeyInfo  ::=  SEQUENCE  {
    algorithm         AlgorithmIdentifier,
    subjectPublicKey  BIT STRING
}

In case the function returns a hash of the key:

DigestInfo ::= SEQUENCE {
    digestAlgorithm   AlgorithmIdentifier,
    digest            OCTET STRING
}

The function returns 0 on success. Any other value is treated as error by the Trusted Board Boot. The function also reports extra information related to the ROTPK in the flags parameter:

ROTPK_IS_HASH      : Indicates that the ROTPK returned by the platform is a
                     hash.
ROTPK_NOT_DEPLOYED : This allows the platform to skip certificate ROTPK
                     verification while the platform ROTPK is not deployed.
                     When this flag is set, the function does not need to
                     return a platform ROTPK, and the authentication
                     framework uses the ROTPK in the certificate without
                     verifying it against the platform value. This flag
                     must not be used in a deployed production environment.

4.6.   Function: plat_get_nv_ctr()

Argument : void *, unsigned int *
Return   : int

This function is mandatory when Trusted Board Boot is enabled. It returns the non-volatile counter value stored in the platform in the second argument. The cookie in the first argument may be used to select the counter in case the platform provides more than one (for example, on platforms that use the default TBBR CoT, the cookie will correspond to the OID values defined in TRUSTED_FW_NVCOUNTER_OID or NON_TRUSTED_FW_NVCOUNTER_OID).

The function returns 0 on success. Any other value means the counter value could not be retrieved from the platform.

4.7.   Function: plat_set_nv_ctr()

Argument : void *, unsigned int
Return   : int

This function is mandatory when Trusted Board Boot is enabled. It sets a new counter value in the platform. The cookie in the first argument may be used to select the counter (as explained in plat_get_nv_ctr()). The second argument is the updated counter value to be written to the NV counter.

The function returns 0 on success. Any other value means the counter value could not be updated.

4.8.   Function: plat_set_nv_ctr2()

Argument : void *, const auth_img_desc_t *, unsigned int
Return   : int

This function is optional when Trusted Board Boot is enabled. If this interface is defined, then plat_set_nv_ctr() need not be defined. The first argument passed is a cookie and is typically used to differentiate between a Non Trusted NV Counter and a Trusted NV Counter. The second argument is a pointer to an authentication image descriptor and may be used to decide if the counter is allowed to be updated or not. The third argument is the updated counter value to be written to the NV counter.

The function returns 0 on success. Any other value means the counter value either could not be updated or the authentication image descriptor indicates that it is not allowed to be updated.

5.   Common mandatory function modifications

The following functions are mandatory functions which need to be implemented by the platform port.

5.1.   Function : plat_my_core_pos()

Argument : void
Return   : unsigned int

This function returns the index of the calling CPU which is used as a CPU-specific linear index into blocks of memory (for example while allocating per-CPU stacks). This function will be invoked very early in the initialization sequence which mandates that this function should be implemented in assembly and should not rely on the availability of a C runtime environment. This function can clobber x0 - x8 and must preserve x9 - x29.

This function plays a crucial role in the power domain topology framework in PSCI and details of this can be found in Power Domain Topology Design.

5.2.   Function : plat_core_pos_by_mpidr()

Argument : u_register_t
Return   : int

This function validates the MPIDR of a CPU and converts it to an index, which can be used as a CPU-specific linear index into blocks of memory. In case the MPIDR is invalid, this function returns -1. This function will only be invoked by BL31 after the power domain topology is initialized and can utilize the C runtime environment. For further details about how TF-A represents the power domain topology and how this relates to the linear CPU index, please refer Power Domain Topology Design.

6.   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.

6.1.   Function : plat_set_my_stack()

Argument : void
Return   : void

This function sets the current stack pointer to the normal memory stack that has been allocated for the current CPU. For BL images that only require a stack for the primary CPU, the UP version of the function is used. 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

6.2.   Function : plat_get_my_stack()

Argument : void
Return   : uintptr_t

This function returns the base address of the normal memory stack that has been allocated for the current CPU. For BL images that only require a stack for the primary CPU, the UP version of the function is used. 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

6.3.   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.

For AArch64, this function receives the exception type as its argument. Possible values for exceptions types are listed in the include/common/bl_common.h header file. Note that these constants are not related to any architectural exception code; they are just a TF-A convention.

For AArch32, this function receives the exception mode as its argument. Possible values for exception modes are listed in the include/lib/aarch32/arch.h header file.

6.4.   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 for general guidelines.

6.5.   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.

6.6.   Function : plat_error_handler()

Argument : int
Return   : void

This API is called when the generic code encounters an error situation from which it cannot continue. It allows the platform to perform error reporting or recovery actions (for example, reset the system). This function must not return.

The parameter indicates the type of error using standard codes from errno.h. Possible errors reported by the generic code are:

  • -EAUTH: a certificate or image could not be authenticated (when Trusted Board Boot is enabled)
  • -ENOENT: the requested image or certificate could not be found or an IO error was detected
  • -ENOMEM: resources exhausted. TF-A does not use dynamic memory, so this error is usually an indication of an incorrect array size

The default implementation simply spins.

6.7.   Function : plat_panic_handler()

Argument : void
Return   : void

This API is called when the generic code encounters an unexpected error situation from which it cannot recover. This function must not return, and must be implemented in assembly because it may be called before the C environment is initialized.

Note: The address from where it was called is stored in x30 (Link Register). The default implementation simply spins.

6.8.   Function : plat_get_bl_image_load_info()

Argument : void
Return   : bl_load_info_t *

This function returns pointer to the list of images that the platform has populated to load. This function is invoked in BL2 to load the BL3xx images.

6.9.   Function : plat_get_next_bl_params()

Argument : void
Return   : bl_params_t *

This function returns a pointer to the shared memory that the platform has kept aside to pass TF-A related information that next BL image needs. This function is invoked in BL2 to pass this information to the next BL image.

6.10.   Function : plat_get_stack_protector_canary()

Argument : void
Return   : u_register_t

This function returns a random value that is used to initialize the canary used when the stack protector is enabled with ENABLE_STACK_PROTECTOR. A predictable value will weaken the protection as the attacker could easily write the right value as part of the attack most of the time. Therefore, it should return a true random number.

Note: For the protection to be effective, the global data need to be placed at a lower address than the stack bases. Failure to do so would allow an attacker to overwrite the canary as part of the stack buffer overflow attack.

6.11.   Function : plat_flush_next_bl_params()

Argument : void
Return   : void

This function flushes to main memory all the image params that are passed to next image. This function is invoked in BL2 to flush this information to the next BL image.

6.12.   Function : plat_log_get_prefix()

Argument : unsigned int
Return   : const char *

This function defines the prefix string corresponding to the log_level to be prepended to all the log output from TF-A. The log_level (argument) will correspond to one of the standard log levels defined in debug.h. The platform can override the common implementation to define a different prefix string for the log output. The implementation should be robust to future changes that increase the number of log levels.

6.13.   Function : plat_get_mbedtls_heap()

Arguments : void **heap_addr, size_t *heap_size
Return    : int

This function is invoked during Mbed TLS library initialisation to get a heap, by means of a starting address and a size. This heap will then be used internally by the Mbed TLS library. The heap is requested from the current BL stage, i.e. the current BL image inside which Mbed TLS is used.

In the default implementation a heap is statically allocated inside every image (i.e. every BL stage) that utilises Mbed TLS. So, in this case, the function simply returns the address and size of this "pre-allocated" heap. However, by overriding the default implementation, platforms have the potential to optimise memory usage. For example, on some Arm platforms, the Mbed TLS heap is shared between BL1 and BL2 stages and, thus, the necessary space is not reserved twice.

On success the function should return 0 and a negative error code otherwise.

7.   Modifications specific to a Boot Loader stage

8.   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. Identifying and starting the Firmware Update process (if required).

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

  5. 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

    By default, BL1 places this meminfo structure at the end of secure memory visible to BL2.

    It is possible for the platform to decide where it wants to place the meminfo structure for BL2 or restrict the amount of memory visible to BL2 by overriding the weak default implementation of bl1_plat_handle_post_image_load API.

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

8.1.   Function : bl1_early_platform_setup() [mandatory]

Argument : void
Return   : void

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

On Arm standard platforms, this function:

  • Enables a secure instance of SP805 to act as the Trusted Watchdog.
  • Initializes a UART (PL011 console), which enables access to the printf family of functions in BL1.
  • Enables issuing of snoop and DVM (Distributed Virtual Memory) requests to the CCI slave interface corresponding to the cluster that includes the primary CPU.

8.2.   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 and the interconnect.

In Arm standard platforms, this function enables the MMU.

This function helps fulfill requirement 2 above.

8.3.   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.

if support for multiple boot sources is required, it initializes the boot sequence used by plat_try_next_boot_source().

In Arm standard platforms, this function initializes the storage abstraction layer used to load the next bootloader image.

This function helps fulfill requirement 4 above.

8.4.   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

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 requirements 4 and 5 above.

8.5.   Function : bl1_plat_prepare_exit() [optional]

Argument : entry_point_info_t *
Return   : void

This function is called prior to exiting BL1 in response to the BL1_SMC_RUN_IMAGE SMC request raised by BL2. It should be used to perform platform specific clean up or bookkeeping operations before transferring control to the next image. It receives the address of the entry_point_info_t structure passed from BL2. This function runs with MMU disabled.

8.6.   Function : bl1_plat_set_ep_info() [optional]

Argument : unsigned int image_id, entry_point_info_t *ep_info
Return   : void

This function allows platforms to override ep_info for the given image_id.

The default implementation just returns.

8.7.   Function : bl1_plat_get_next_image_id() [optional]

Argument : void
Return   : unsigned int

This and the following function must be overridden to enable the FWU feature.

BL1 calls this function after platform setup to identify the next image to be loaded and executed. If the platform returns BL2_IMAGE_ID then BL1 proceeds with the normal boot sequence, which loads and executes BL2. If the platform returns a different image id, BL1 assumes that Firmware Update is required.

The default implementation always returns BL2_IMAGE_ID. The Arm development platforms override this function to detect if firmware update is required, and if so, return the first image in the firmware update process.

8.8.   Function : bl1_plat_get_image_desc() [optional]

Argument : unsigned int image_id
Return   : image_desc_t *

BL1 calls this function to get the image descriptor information image_desc_t for the provided image_id from the platform.

The default implementation always returns a common BL2 image descriptor. Arm standard platforms return an image descriptor corresponding to BL2 or one of the firmware update images defined in the Trusted Board Boot Requirements specification.

8.9.   Function : bl1_plat_handle_pre_image_load() [optional]

Argument : unsigned int image_id
Return   : int

This function can be used by the platforms to update/use image information corresponding to image_id. This function is invoked in BL1, both in cold boot and FWU code path, before loading the image.

8.10.   Function : bl1_plat_handle_post_image_load() [optional]

Argument : unsigned int image_id
Return   : int

This function can be used by the platforms to update/use image information corresponding to image_id. This function is invoked in BL1, both in cold boot and FWU code path, after loading and authenticating the image.

The default weak implementation of this function calculates the amount of Trusted SRAM that can be used by BL2 and allocates a meminfo_t structure at the beginning of this free memory and populates it. The address of meminfo_t structure is updated in arg1 of the entrypoint information to BL2.

8.11.   Function : bl1_plat_fwu_done() [optional]

Argument : unsigned int image_id, uintptr_t image_src,
           unsigned int image_size
Return   : void

BL1 calls this function when the FWU process is complete. It must not return. The platform may override this function to take platform specific action, for example to initiate the normal boot flow.

The default implementation spins forever.

8.12.   Function : bl1_plat_mem_check() [mandatory]

Argument : uintptr_t mem_base, unsigned int mem_size,
           unsigned int flags
Return   : int

BL1 calls this function while handling FWU related SMCs, more specifically when copying or authenticating an image. Its responsibility is to ensure that the region of memory identified by mem_base and mem_size is mapped in BL1, and that this memory corresponds to either a secure or non-secure memory region as indicated by the security state of the flags argument.

This function can safely assume that the value resulting from the addition of mem_base and mem_size fits into a uintptr_t type variable and does not overflow.

This function must return 0 on success, a non-null error code otherwise.

The default implementation of this function asserts therefore platforms must override it when using the FWU feature.

9.   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 and invokes plat_get_bl_image_load_info() to retrieve the list of images to load from non-volatile storage to secure/non-secure RAM. After all the images are loaded then BL2 invokes plat_get_next_bl_params() to get the list of executable images to be passed to the next BL image.

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

9.1.   Function : bl2_early_platform_setup2() [mandatory]

Argument : u_register_t, u_register_t, u_register_t, u_register_t
Return   : void

This function executes with the MMU and data caches disabled. It is only called by the primary CPU. The 4 arguments are passed by BL1 to BL2 and these arguments are platform specific.

On Arm standard platforms, the arguments received are :

arg0 - Points to load address of HW_CONFIG if present

arg1 - meminfo structure populated by BL1. The platform copies the contents of meminfo as it may be subsequently overwritten by BL2.

On Arm standard platforms, this function also:

  • Initializes a UART (PL011 console), which enables access to the printf family of functions in BL2.
  • Initializes the storage abstraction layer used to load further bootloader images. It is necessary to do this early on platforms with a SCP_BL2 image, since the later bl2_platform_setup must be done after SCP_BL2 is loaded.

9.2.   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.

On Arm standard platforms, this function enables the MMU.

9.3.   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.

In Arm standard platforms, this function performs security setup, including configuration of the TrustZone controller to allow non-secure masters access to most of DRAM. Part of DRAM is reserved for secure world use.

9.4.   Function : bl2_plat_handle_pre_image_load() [optional]

Argument : unsigned int
Return   : int

This function can be used by the platforms to update/use image information for given image_id. This function is currently invoked in BL2 before loading each image.

9.5.   Function : bl2_plat_handle_post_image_load() [optional]

Argument : unsigned int
Return   : int

This function can be used by the platforms to update/use image information for given image_id. This function is currently invoked in BL2 after loading each image.

9.6.   Function : bl2_plat_preload_setup [optional]

Argument : void
Return   : void

This optional function performs any BL2 platform initialization required before image loading, that is not done later in bl2_platform_setup(). Specifically, if support for multiple boot sources is required, it initializes the boot sequence used by plat_try_next_boot_source().

9.7.   Function : plat_try_next_boot_source() [optional]

Argument : void
Return   : int

This optional function passes to the next boot source in the redundancy sequence.

This function moves the current boot redundancy source to the next element in the boot sequence. If there are no more boot sources then it must return 0, otherwise it must return 1. The default implementation of this always returns 0.

10.   Boot Loader Stage 2 (BL2) at EL3

When the platform has a non-TF-A Boot ROM it is desirable to jump directly to BL2 instead of TF-A BL1. In this case BL2 is expected to execute at EL3 instead of executing at EL1. Refer to the Firmware Design for more information.

All mandatory functions of BL2 must be implemented, except the functions bl2_early_platform_setup and bl2_el3_plat_arch_setup, because their work is done now by bl2_el3_early_platform_setup and bl2_el3_plat_arch_setup. These functions should generally implement the bl1_plat_xxx() and bl2_plat_xxx() functionality combined.

10.1.   Function : bl2_el3_early_platform_setup() [mandatory]

Argument : u_register_t, u_register_t, u_register_t, u_register_t
Return   : void

This function executes with the MMU and data caches disabled. It is only called by the primary CPU. This function receives four parameters which can be used by the platform to pass any needed information from the Boot ROM to BL2.

On Arm standard platforms, this function does the following:

  • Initializes a UART (PL011 console), which enables access to the printf family of functions in BL2.
  • Initializes the storage abstraction layer used to load further bootloader images. It is necessary to do this early on platforms with a SCP_BL2 image, since the later bl2_platform_setup must be done after SCP_BL2 is loaded.
  • Initializes the private variables that define the memory layout used.

10.2.   Function : bl2_el3_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.

On Arm standard platforms, this function enables the MMU.

10.3.   Function : bl2_el3_plat_prepare_exit() [optional]

Argument : void
Return   : void

This function is called prior to exiting BL2 and run the next image. It should be used to perform platform specific clean up or bookkeeping operations before transferring control to the next image. This function runs with MMU disabled.

11.   FWU Boot Loader Stage 2 (BL2U)

The AP Firmware Updater Configuration, BL2U, is an optional part of the FWU process and is executed only by the primary CPU. BL1 passes control to BL2U at BL2U_BASE. BL2U executes in Secure-EL1 and is responsible for:

  1. (Optional) Transferring the optional SCP_BL2U binary image from AP secure memory to SCP RAM. BL2U uses the SCP_BL2U image_info passed by BL1. SCP_BL2U_BASE defines the address in AP secure memory where SCP_BL2U should be copied from. Subsequent handling of the SCP_BL2U image is implemented by the platform specific bl2u_plat_handle_scp_bl2u() function. If SCP_BL2U_BASE is not defined then this step is not performed.
  2. Any platform specific setup required to perform the FWU process. For example, Arm standard platforms initialize the TZC controller so that the normal world can access DDR memory.

The following functions must be implemented by the platform port to enable BL2U to perform the tasks mentioned above.

11.1.   Function : bl2u_early_platform_setup() [mandatory]

Argument : meminfo *mem_info, void *plat_info
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 and platform specific info provided by BL1.

The platform may copy the contents of the mem_info and plat_info into private storage as the original memory may be subsequently overwritten by BL2U.

On Arm CSS platforms plat_info is interpreted as an image_info_t structure, to extract SCP_BL2U image information, which is then copied into a private variable.

11.2.   Function : bl2u_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).

11.3.   Function : bl2u_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 bl2u_plat_arch_setup(). It is only called by the primary CPU.

The purpose of this function is to perform any platform initialization specific to BL2U.

In Arm standard platforms, this function performs security setup, including configuration of the TrustZone controller to allow non-secure masters access to most of DRAM. Part of DRAM is reserved for secure world use.

11.4.   Function : bl2u_plat_handle_scp_bl2u() [optional]

Argument : void
Return   : int

This function 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 BL2U execution to continue.

This function returns 0 on success, a negative error code otherwise. This function is included if SCP_BL2U_BASE is defined.

12.   Boot Loader Stage 3-1 (BL31)

During cold boot, the BL31 stage is executed only by the primary CPU. This is determined in BL1 using the platform_is_primary_cpu() function. BL1 passes control to BL31 at BL31_BASE. During warm boot, BL31 is executed by all CPUs. BL31 executes at EL3 and is responsible for:

  1. Re-initializing all architectural and platform state. Although BL1 performs some of this initialization, BL31 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. On ARM platforms, BL31 uses the bl_params list populated by BL2 in memory to do this.
  3. Providing runtime firmware services. Currently, BL31 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 BL32 image, pre-loaded at a platform- specific address by BL2. BL31 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. On ARM platforms, BL31 uses the bl_params list populated by BL2 in memory to do this.

If BL31 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 BL31 to perform the above tasks.

12.1.   Function : bl31_early_platform_setup2() [mandatory]

Argument : u_register_t, u_register_t, u_register_t, u_register_t
Return   : void

This function executes with the MMU and data caches disabled. It is only called by the primary CPU. BL2 can pass 4 arguments to BL31 and these arguments are platform specific.

In Arm standard platforms, the arguments received are :

arg0 - The pointer to the head of bl_params_t list which is list of executable images following BL31,

arg1 - Points to load address of SOC_FW_CONFIG if present

arg2 - Points to load address of HW_CONFIG if present

arg3 - A special value to verify platform parameters from BL2 to BL31. Not used in release builds.

The function runs through the bl_param_t list and extracts the entry point information for BL32 and BL33. It also performs the following:

  • Initialize a UART (PL011 console), which enables access to the printf family of functions in BL31.
  • Enable issuing of snoop and DVM (Distributed Virtual Memory) requests to the CCI slave interface corresponding to the cluster that includes the primary CPU.

12.2.   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.

On Arm standard platforms, this function enables the MMU.

12.3.   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 BL31 runtime services and normal world software can function correctly.

On Arm standard platforms, this function does the following:

  • Initialize the generic interrupt controller.

    Depending on the GIC driver selected by the platform, the appropriate GICv2 or GICv3 initialization will be done, which mainly consists of:

    • Enable secure interrupts in the GIC CPU interface.
    • 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 and the other secure interrupts on the platform as secure.
    • Target all secure SPIs to CPU0.
    • Enable these secure interrupts in the GIC distributor.
    • Configure all other interrupts as non-secure.
    • Enable signaling of secure interrupts in the GIC distributor.

  • Enable system-level implementation of the generic timer counter through the memory mapped interface.

  • Grant access to the system counter timer module

  • Initialize the power controller device.

    In particular, initialise the locks that prevent concurrent accesses to the power controller device.

12.4.   Function : bl31_plat_runtime_setup() [optional]

Argument : void
Return   : void

The purpose of this function is allow the platform to perform any BL31 runtime setup just prior to BL31 exit during cold boot. The default weak implementation of this function will invoke console_switch_state() to switch console output to consoles marked for use in the runtime state.

12.5.   Function : bl31_plat_get_next_image_ep_info() [mandatory]

Argument : uint32_t
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. BL31 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.

12.6.   Function : bl31_plat_enable_mmu [optional]

Argument : uint32_t
Return   : void

This function enables the MMU. The boot code calls this function with MMU and caches disabled. This function should program necessary registers to enable translation, and upon return, the MMU on the calling PE must be enabled.

The function must honor flags passed in the first argument. These flags are defined by the translation library, and can be found in the file include/lib/xlat_tables/xlat_mmu_helpers.h.

On DynamIQ systems, this function must not use stack while enabling MMU, which is how the function in xlat table library version 2 is implemented.

12.7.   Function : plat_get_syscnt_freq2() [mandatory]

Argument : void
Return   : unsigned int

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 Arm standard platforms, it returns the base frequency of the system counter, which is retrieved from the first entry in the frequency modes table.

12.8.   #define : PLAT_PERCPU_BAKERY_LOCK_SIZE [optional]

When USE_COHERENT_MEM = 0, this constant defines the total memory (in bytes) aligned to the cache line boundary that should be allocated per-cpu to accommodate all the bakery locks.

If this constant is not defined when USE_COHERENT_MEM = 0, the linker calculates the size of the bakery_lock input section, aligns it to the nearest CACHE_WRITEBACK_GRANULE, multiplies it with PLATFORM_CORE_COUNT and stores the result in a linker symbol. This constant prevents a platform from relying on the linker and provide a more efficient mechanism for accessing per-cpu bakery lock information.

If this constant is defined and its value is not equal to the value calculated by the linker then a link time assertion is raised. A compile time assertion is raised if the value of the constant is not aligned to the cache line boundary.

12.9.   SDEI porting requirements

The SDEI dispatcher requires the platform to provide the following macros and functions, of which some are optional, and some others mandatory.

12.9.1.   Macros

12.9.1.1.   Macro: PLAT_SDEI_NORMAL_PRI [mandatory]

This macro must be defined to the EL3 exception priority level associated with Normal SDEI events on the platform. This must have a higher value (therefore of lower priority) than PLAT_SDEI_CRITICAL_PRI.

12.9.1.2.   Macro: PLAT_SDEI_CRITICAL_PRI [mandatory]

This macro must be defined to the EL3 exception priority level associated with Critical SDEI events on the platform. This must have a lower value (therefore of higher priority) than PLAT_SDEI_NORMAL_PRI.

Note: SDEI exception priorities must be the lowest among Secure priorities. Among the SDEI exceptions, Critical SDEI priority must be higher than Normal SDEI priority.

12.9.2.   Functions

12.9.2.1.   Function: int plat_sdei_validate_entry_point(uintptr_t ep) [optional]
Argument: uintptr_t
Return: int

This function validates the address of client entry points provided for both event registration and Complete and Resume SDEI calls. The function takes one argument, which is the address of the handler the SDEI client requested to register. The function must return 0 for successful validation, or -1 upon failure.

The default implementation always returns 0. On Arm platforms, this function is implemented to translate the entry point to physical address, and further to ensure that the address is located in Non-secure DRAM.

12.9.2.2.   Function: void plat_sdei_handle_masked_trigger(uint64_t mpidr, unsigned int intr) [optional]
Argument: uint64_t
Argument: unsigned int
Return: void

SDEI specification requires that a PE comes out of reset with the events masked. The client therefore is expected to call PE_UNMASK to unmask SDEI events on the PE. No SDEI events can be dispatched until such time.

Should a PE receive an interrupt that was bound to an SDEI event while the events are masked on the PE, the dispatcher implementation invokes the function plat_sdei_handle_masked_trigger. The MPIDR of the PE that received the interrupt and the interrupt ID are passed as parameters.

The default implementation only prints out a warning message.

13.   Power State Coordination Interface (in BL31)

The TF-A implementation of the PSCI API is based around the concept of a power domain. A power domain is a CPU or a logical group of CPUs which share some state on which power management operations can be performed as specified by PSCI. Each CPU in the system is assigned a cpu index which is a unique number between 0 and PLATFORM_CORE_COUNT - 1. The power domains are arranged in a hierarchical tree structure and each power domain can be identified in a system by the cpu index of any CPU that is part of that domain and a power domain level. A processing element (for example, a CPU) is at level 0. If the power domain node above a CPU is a logical grouping of CPUs that share some state, then level 1 is that group of CPUs (for example, a cluster), and level 2 is a group of clusters (for example, the system). More details on the power domain topology and its organization can be found in Power Domain Topology Design.

BL31'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_psci_ops structure. The PSCI implementation calls members of the plat_psci_ops structure for performing power management operations on the power domains. For example, the target CPU is specified by its MPIDR in a PSCI CPU_ON call. The pwr_domain_on() handler (if present) is called for the CPU power domain.

The power-state parameter of a PSCI CPU_SUSPEND call can be used to describe composite power states specific to a platform. The PSCI implementation defines a generic representation of the power-state parameter viz which is an array of local power states where each index corresponds to a power domain level. Each entry contains the local power state the power domain at that power level could enter. It depends on the validate_power_state() handler to convert the power-state parameter (possibly encoding a composite power state) passed in a PSCI CPU_SUSPEND call to this representation.

The following functions form part of platform port of PSCI functionality.

13.1.   Function : plat_psci_stat_accounting_start() [optional]

Argument : const psci_power_state_t *
Return   : void

This is an optional hook that platforms can implement for residency statistics accounting before entering a low power state. The pwr_domain_state field of state_info (first argument) can be inspected if stat accounting is done differently at CPU level versus higher levels. As an example, if the element at index 0 (CPU power level) in the pwr_domain_state array indicates a power down state, special hardware logic may be programmed in order to keep track of the residency statistics. For higher levels (array indices > 0), the residency statistics could be tracked in software using PMF. If ENABLE_PMF is set, the default implementation will use PMF to capture timestamps.

13.2.   Function : plat_psci_stat_accounting_stop() [optional]

Argument : const psci_power_state_t *
Return   : void

This is an optional hook that platforms can implement for residency statistics accounting after exiting from a low power state. The pwr_domain_state field of state_info (first argument) can be inspected if stat accounting is done differently at CPU level versus higher levels. As an example, if the element at index 0 (CPU power level) in the pwr_domain_state array indicates a power down state, special hardware logic may be programmed in order to keep track of the residency statistics. For higher levels (array indices > 0), the residency statistics could be tracked in software using PMF. If ENABLE_PMF is set, the default implementation will use PMF to capture timestamps.

13.3.   Function : plat_psci_stat_get_residency() [optional]

Argument : unsigned int, const psci_power_state_t *, int
Return   : u_register_t

This is an optional interface that is is invoked after resuming from a low power state and provides the time spent resident in that low power state by the power domain at a particular power domain level. When a CPU wakes up from suspend, all its parent power domain levels are also woken up. The generic PSCI code invokes this function for each parent power domain that is resumed and it identified by the lvl (first argument) parameter. The state_info (second argument) describes the low power state that the power domain has resumed from. The current CPU is the first CPU in the power domain to resume from the low power state and the last_cpu_idx (third parameter) is the index of the last CPU in the power domain to suspend and may be needed to calculate the residency for that power domain.

13.4.   Function : plat_get_target_pwr_state() [optional]

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

The PSCI generic code uses this function to let the platform participate in state coordination during a power management operation. The function is passed a pointer to an array of platform specific local power state states (second argument) which contains the requested power state for each CPU at a particular power domain level lvl (first argument) within the power domain. The function is expected to traverse this array of upto ncpus (third argument) and return a coordinated target power state by the comparing all the requested power states. The target power state should not be deeper than any of the requested power states.

A weak definition of this API is provided by default wherein it assumes that the platform assigns a local state value in order of increasing depth of the power state i.e. for two power states X & Y, if X < Y then X represents a shallower power state than Y. As a result, the coordinated target local power state for a power domain will be the minimum of the requested local power state values.

13.5.   Function : plat_get_power_domain_tree_desc() [mandatory]

Argument : void
Return   : const unsigned char *

This function returns a pointer to the byte array containing the power domain topology tree description. The format and method to construct this array are described in Power Domain Topology Design. The BL31 PSCI initialization code requires this array to be described by the platform, either statically or dynamically, to initialize the power domain topology tree. In case the array is populated dynamically, then plat_core_pos_by_mpidr() and plat_my_core_pos() should also be implemented suitably so that the topology tree description matches the CPU indices returned by these APIs. These APIs together form the platform interface for the PSCI topology framework.

13.6.   Function : plat_setup_psci_ops() [mandatory]

Argument : uintptr_t, const plat_psci_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 let the platform layer know about the warm boot entrypoint through the sec_entrypoint (first argument) and to export handler routines for platform-specific psci power management actions by populating the passed pointer with a pointer to BL31's private plat_psci_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/arm/board/fvp/fvp_pm.c as an example. For each PSCI function that the platform wants to support, the associated operation or operations in this structure must be provided and implemented (Refer section 4 of Firmware Design for the PSCI API supported in TF-A). To disable a PSCI function in a platform port, the operation should be removed from this structure instead of providing an empty implementation.

13.6.1.   plat_psci_ops.cpu_standby()

Perform the platform-specific actions to enter the standby state for a cpu indicated by the passed argument. This provides a fast path for CPU standby wherein overheads of PSCI state management and lock acquisition is avoided. For this handler to be invoked by the PSCI CPU_SUSPEND API implementation, the suspend state type specified in the power-state parameter should be STANDBY and the target power domain level specified should be the CPU. The handler should put the CPU into a low power retention state (usually by issuing a wfi instruction) and ensure that it can be woken up from that state by a normal interrupt. The generic code expects the handler to succeed.

13.6.2.   plat_psci_ops.pwr_domain_on()

Perform the platform specific actions to power on a CPU, specified by the MPIDR (first argument). The generic code expects the platform to return PSCI_E_SUCCESS on success or PSCI_E_INTERN_FAIL for any failure.

13.6.3.   plat_psci_ops.pwr_domain_off()

Perform the platform specific actions to prepare to power off the calling CPU and its higher parent power domain levels as indicated by the target_state (first argument). It is called by the PSCI CPU_OFF API implementation.

The target_state encodes the platform coordinated target local power states for the CPU power domain and its parent power domain levels. The handler needs to perform power management operation corresponding to the local state at each power level.

For this handler, the local power state for the CPU power domain will be a power down state where as it could be either power down, retention or run state for the higher power domain levels depending on the result of state coordination. The generic code expects the handler to succeed.

13.6.4.   plat_psci_ops.pwr_domain_suspend_pwrdown_early() [optional]

This optional function may be used as a performance optimization to replace or complement pwr_domain_suspend() on some platforms. Its calling semantics are identical to pwr_domain_suspend(), except the PSCI implementation only calls this function when suspending to a power down state, and it guarantees that data caches are enabled.

When HW_ASSISTED_COHERENCY = 0, the PSCI implementation disables data caches before calling pwr_domain_suspend(). If the target_state corresponds to a power down state and it is safe to perform some or all of the platform specific actions in that function with data caches enabled, it may be more efficient to move those actions to this function. When HW_ASSISTED_COHERENCY = 1, data caches remain enabled throughout, and so there is no advantage to moving platform specific actions to this function.

13.6.5.   plat_psci_ops.pwr_domain_suspend()

Perform the platform specific actions to prepare to suspend the calling CPU and its higher parent power domain levels as indicated by the target_state (first argument). It is called by the PSCI CPU_SUSPEND API implementation.

The target_state has a similar meaning as described in the pwr_domain_off() operation. It encodes the platform coordinated target local power states for the CPU power domain and its parent power domain levels. The handler needs to perform power management operation corresponding to the local state at each power level. The generic code expects the handler to succeed.

The difference between turning a power domain off versus suspending it is that in the former case, the power domain is expected to re-initialize its state when it is next powered on (see pwr_domain_on_finish()). In the latter case, the power domain is expected to save enough state so that it can resume execution by restoring this state when its powered on (see pwr_domain_suspend_finish()).

When suspending a core, the platform can also choose to power off the GICv3 Redistributor and ITS through an implementation-defined sequence. To achieve this safely, the ITS context must be saved first. The architectural part is implemented by the gicv3_its_save_disable() helper, but most of the needed sequence is implementation defined and it is therefore the responsibility of the platform code to implement the necessary sequence. Then the GIC Redistributor context can be saved using the gicv3_rdistif_save() helper. Powering off the Redistributor requires the implementation to support it and it is the responsibility of the platform code to execute the right implementation defined sequence.

When a system suspend is requested, the platform can also make use of the gicv3_distif_save() helper to save the context of the GIC Distributor after it has saved the context of the Redistributors and ITS of all the cores in the system. The context of the Distributor can be large and may require it to be allocated in a special area if it cannot fit in the platform's global static data, for example in DRAM. The Distributor can then be powered down using an implementation-defined sequence.

13.6.6.   plat_psci_ops.pwr_domain_pwr_down_wfi()

This is an optional function and, if implemented, is expected to perform platform specific actions including the wfi invocation which allows the CPU to powerdown. Since this function is invoked outside the PSCI locks, the actions performed in this hook must be local to the CPU or the platform must ensure that races between multiple CPUs cannot occur.

The target_state has a similar meaning as described in the pwr_domain_off() operation and it encodes the platform coordinated target local power states for the CPU power domain and its parent power domain levels. This function must not return back to the caller.

If this function is not implemented by the platform, PSCI generic implementation invokes psci_power_down_wfi() for power down.

13.6.7.   plat_psci_ops.pwr_domain_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 target_state (first argument) is the prior state of the power domains immediately before the CPU was turned on. It indicates which power domains above the CPU might require initialization due to having previously been in low power states. The generic code expects the handler to succeed.

13.6.8.   plat_psci_ops.pwr_domain_suspend_finish()

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 or SYSTEM_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 target_state (first argument) has a similar meaning as described in the pwr_domain_on_finish() operation. The generic code expects the platform to succeed.

If the Distributor, Redistributors or ITS have been powered off as part of a suspend, their context must be restored in this function in the reverse order to how they were saved during suspend sequence.

13.6.9.   plat_psci_ops.system_off()

This function is called by PSCI implementation in response to a SYSTEM_OFF call. It performs the platform-specific system poweroff sequence after notifying the Secure Payload Dispatcher.

13.6.10.   plat_psci_ops.system_reset()

This function is called by PSCI implementation in response to a SYSTEM_RESET call. It performs the platform-specific system reset sequence after notifying the Secure Payload Dispatcher.

13.6.11.   plat_psci_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 and if valid, populate it in req_state (second argument) array as power domain level specific local states. If the power_state is invalid, the platform must return PSCI_E_INVALID_PARAMS as error, which is propagated back to the normal world PSCI client.

13.6.12.   plat_psci_ops.validate_ns_entrypoint()

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

13.6.13.   plat_psci_ops.get_sys_suspend_power_state()

This function is called by the PSCI implementation during the SYSTEM_SUSPEND call to get the req_state parameter from platform which encodes the power domain level specific local states to suspend to system affinity level. The req_state will be utilized to do the PSCI state coordination and pwr_domain_suspend() will be invoked with the coordinated target state to enter system suspend.

13.6.14.   plat_psci_ops.get_pwr_lvl_state_idx()

This is an optional function and, if implemented, is invoked by the PSCI implementation to convert the local_state (first argument) at a specified pwr_lvl (second argument) to an index between 0 and PLAT_MAX_PWR_LVL_STATES - 1. This function is only needed if the platform supports more than two local power states at each power domain level, that is PLAT_MAX_PWR_LVL_STATES is greater than 2, and needs to account for these local power states.

13.6.15.   plat_psci_ops.translate_power_state_by_mpidr()

This is an optional function and, if implemented, verifies the power_state (second argument) parameter of the PSCI API corresponding to a target power domain. The target power domain is identified by using both MPIDR (first argument) and the power domain level encoded in power_state. The power domain level specific local states are to be extracted from power_state and be populated in the output_state (third argument) array. The functionality is similar to the validate_power_state function described above and is envisaged to be used in case the validity of power_state depend on the targeted power domain. If the power_state is invalid for the targeted power domain, the platform must return PSCI_E_INVALID_PARAMS as error. If this function is not implemented, then the generic implementation relies on validate_power_state function to translate the power_state.

This function can also be used in case the platform wants to support local power state encoding for power_state parameter of PSCI_STAT_COUNT/RESIDENCY APIs as described in Section 5.18 of PSCI.

13.6.16.   plat_psci_ops.get_node_hw_state()

This is an optional function. If implemented this function is intended to return the power state of a node (identified by the first parameter, the MPIDR) in the power domain topology (identified by the second parameter, power_level), as retrieved from a power controller or equivalent component on the platform. Upon successful completion, the implementation must map and return the final status among HW_ON, HW_OFF or HW_STANDBY. Upon encountering failures, it must return either PSCI_E_INVALID_PARAMS or PSCI_E_NOT_SUPPORTED as appropriate.

Implementations are not expected to handle power_levels greater than PLAT_MAX_PWR_LVL.

13.6.17.   plat_psci_ops.system_reset2()

This is an optional function. If implemented this function is called during the SYSTEM_RESET2 call to perform a reset based on the first parameter reset_type as specified in PSCI. The parameter cookie can be used to pass additional reset information. If the reset_type is not supported, the function must return PSCI_E_NOT_SUPPORTED. For architectural resets, all failures must return PSCI_E_INVALID_PARAMETERS and vendor reset can return other PSCI error codes as defined in PSCI. On success this function will not return.

13.6.18.   plat_psci_ops.write_mem_protect()

This is an optional function. If implemented it enables or disables the MEM_PROTECT functionality based on the value of val. A non-zero value enables MEM_PROTECT and a value of zero disables it. Upon encountering failures it must return a negative value and on success it must return 0.

13.6.19.   plat_psci_ops.read_mem_protect()

This is an optional function. If implemented it returns the current state of MEM_PROTECT via the val parameter. Upon encountering failures it must return a negative value and on success it must return 0.

13.6.20.   plat_psci_ops.mem_protect_chk()

This is an optional function. If implemented it checks if a memory region defined by a base address base and with a size of length bytes is protected by MEM_PROTECT. If the region is protected then it must return 0, otherwise it must return a negative number.

14.   Interrupt Management framework (in BL31)

BL31 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 in Arm standard platforms. The API implementation depends upon the type of interrupt controller present in the platform. Arm standard platform layer supports both Arm Generic Interrupt Controller version 2.0 (GICv2) and 3.0 (GICv3). Juno builds the Arm platform layer to use GICv2 and the FVP can be configured to use either GICv2 or GICv3 depending on the build flag FVP_USE_GIC_DRIVER (See FVP platform specific build options in User Guide for more details).

See also: Interrupt Controller Abstraction APIs.

14.1.   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. This API must be invoked at EL3.

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.

In the case of Arm standard platforms using GICv2, S-EL1 interrupts are configured as FIQs and Non-secure interrupts as IRQs from either security state.

In the case of Arm standard platforms using GICv3, the interrupt line to be configured depends on the security state of the execution context when the interrupt is signalled and are as follows:

  • The S-EL1 interrupts are signaled as IRQ in S-EL0/1 context and as FIQ in NS-EL0/1/2 context.
  • The Non secure interrupts are signaled as FIQ in S-EL0/1 context and as IRQ in the NS-EL0/1/2 context.
  • The EL3 interrupts are signaled as FIQ in both S-EL0/1 and NS-EL0/1/2 context.

14.2.   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. This API must be invoked at EL3.

In the case of Arm standard platforms using GICv2, the Highest Priority Pending Interrupt Register (GICC_HPPIR) is read 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.

In the case of Arm standard platforms using GICv3, the system register ICC_HPPIR0_EL1, Highest Priority Pending group 0 Interrupt Register, is read to determine the id of the pending interrupt. The type of interrupt depends upon the id value as follows.

  1. id = PENDING_G1S_INTID (1020) is reported as a S-EL1 interrupt
  2. id = PENDING_G1NS_INTID (1021) is reported as a Non-secure interrupt.
  3. id = GIC_SPURIOUS_INTERRUPT (1023) is reported as an invalid interrupt type.
  4. All other interrupt id's are reported as EL3 interrupt.

14.3.   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. INTR_ID_UNAVAILABLE is returned when there is no interrupt pending.

In the case of Arm standard platforms using GICv2, the Highest Priority Pending Interrupt Register (GICC_HPPIR) is read 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.

In the case of Arm standard platforms using GICv3, if the API is invoked from EL3, the system register ICC_HPPIR0_EL1, Highest Priority Pending Interrupt group 0 Register, is read 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 < PENDING_G1S_INTID (1020). id is returned as is.
  2. id = PENDING_G1S_INTID (1020) or PENDING_G1NS_INTID (1021). The system register ICC_HPPIR1_EL1, Highest Priority Pending Interrupt group 1 Register is read to determine the id of the group 1 interrupt. This id is returned by the API as long as it is a valid interrupt id
  3. If the id is any of the special interrupt identifiers, INTR_ID_UNAVAILABLE is returned.

When the API invoked from S-EL1 for GICv3 systems, the id read from system register ICC_HPPIR1_EL1, Highest Priority Pending group 1 Interrupt Register, is returned if is not equal to GIC_SPURIOUS_INTERRUPT (1023) else INTR_ID_UNAVAILABLE is returned.

14.4.   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 raw, unmodified value obtained from the interrupt controller when acknowledging an interrupt. The actual interrupt number shall be extracted from this raw value using the API plat_ic_get_interrupt_id().

This function in Arm standard platforms using GICv2, 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, unmodified.

In the case of Arm standard platforms using GICv3, if the API is invoked from EL3, the function reads the system register ICC_IAR0_EL1, Interrupt Acknowledge Register group 0. If the API is invoked from S-EL1, the function reads the system register ICC_IAR1_EL1, Interrupt Acknowledge Register group 1. The read changes the state of the highest pending interrupt from pending to active in the interrupt controller. The value read is returned unmodified.

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

14.5.   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.

Arm standard platforms write the id to the End of Interrupt Register (GICC_EOIR) in case of GICv2, and to ICC_EOIR0_EL1 or ICC_EOIR1_EL1 system register in case of GICv3 depending on where the API is invoked from, EL3 or S-EL1. This deactivates the corresponding interrupt in the interrupt controller.

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

14.6.   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. This API must be invoked at EL3.

Arm standard platforms using GICv2 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.

In the case of Arm standard platforms using GICv3, both the Interrupt Group Register (GICD_IGROUPRn) and Interrupt Group Modifier Register (GICD_IGRPMODRn) is read to figure out whether the interrupt is configured as Group 0 secure interrupt, Group 1 secure interrupt or Group 1 NS interrupt.

15.   Crash Reporting mechanism (in BL31)

BL31 implements a crash reporting mechanism which prints the various registers of the CPU to enable quick crash analysis and debugging. This mechanism relies on the platform implementing plat_crash_console_init, plat_crash_console_putc and plat_crash_console_flush.

The file plat/common/aarch64/crash_console_helpers.S contains sample implementation of all of them. Platforms may include this file to their makefiles in order to benefit from them. By default, they will cause the crash output to be routed over the normal console infrastructure and get printed on consoles configured to output in crash state. console_set_scope() can be used to control whether a console is used for crash output. NOTE: Platforms are responsible for making sure that they only mark consoles for use in the crash scope that are able to support this, i.e. that are written in assembly and conform with the register clobber rules for putc() (x0-x2, x16-x17) and flush() (x0-x3, x16-x17) crash callbacks.

In some cases (such as debugging very early crashes that happen before the normal boot console can be set up), platforms may want to control crash output more explicitly. These platforms may instead provide custom implementations for these. They are executed outside of a C environment and without a stack. Many console drivers provide functions named console_xxx_core_init/putc/flush that are designed to be used by these functions. See Arm platforms (like juno) for an example of this.

15.1.   Function : plat_crash_console_init [mandatory]

Argument : void
Return   : int

This API is used by the crash reporting mechanism to initialize the crash console. It must only use the general purpose registers x0 through x7 to do the initialization and returns 1 on success.

15.2.   Function : plat_crash_console_putc [mandatory]

Argument : int
Return   : int

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

15.3.   Function : plat_crash_console_flush [mandatory]

Argument : void
Return   : int

This API is used by the crash reporting mechanism to force write of all buffered data on the designated crash console. It should only use general purpose registers x0 through x5 to do its work. The return value is 0 on successful completion; otherwise the return value is -1.

16.   External Abort handling and RAS Support

16.1.   Function : plat_ea_handler

Argument : int
Argument : uint64_t
Argument : void *
Argument : void *
Argument : uint64_t
Return   : void

This function is invoked by the RAS framework for the platform to handle an External Abort received at EL3. The intention of the function is to attempt to resolve the cause of External Abort and return; if that's not possible, to initiate orderly shutdown of the system.

The first parameter (int ea_reason) indicates the reason for External Abort. Its value is one of ERROR_EA_* constants defined in ea_handle.h.

The second parameter (uint64_t syndrome) is the respective syndrome presented to EL3 after having received the External Abort. Depending on the nature of the abort (as can be inferred from the ea_reason parameter), this can be the content of either ESR_EL3 or DISR_EL1.

The third parameter (void *cookie) is unused for now. The fourth parameter (void *handle) is a pointer to the preempted context. The fifth parameter (uint64_t flags) indicates the preempted security state. These parameters are received from the top-level exception handler.

If RAS_EXTENSION is set to 1, the default implementation of this function iterates through RAS handlers registered by the platform. If any of the RAS handlers resolve the External Abort, no further action is taken.

If RAS_EXTENSION is set to 0, or if none of the platform RAS handlers could resolve the External Abort, the default implementation prints an error message, and panics.

16.2.   Function : plat_handle_uncontainable_ea

Argument : int
Argument : uint64_t
Return   : void

This function is invoked by the RAS framework when an External Abort of Uncontainable type is received at EL3. Due to the critical nature of Uncontainable errors, the intention of this function is to initiate orderly shutdown of the system, and is not expected to return.

This function must be implemented in assembly.

The first and second parameters are the same as that of plat_ea_handler.

The default implementation of this function calls report_unhandled_exception.

16.3.   Function : plat_handle_double_fault

Argument : int
Argument : uint64_t
Return   : void

This function is invoked by the RAS framework when another External Abort is received at EL3 while one is already being handled. I.e., a call to plat_ea_handler is outstanding. Due to its critical nature, the intention of this function is to initiate orderly shutdown of the system, and is not expected recover or return.

This function must be implemented in assembly.

The first and second parameters are the same as that of plat_ea_handler.

The default implementation of this function calls report_unhandled_exception.

16.4.   Function : plat_handle_el3_ea

Return   : void

This function is invoked when an External Abort is received while executing in EL3. Due to its critical nature, the intention of this function is to initiate orderly shutdown of the system, and is not expected recover or return.

This function must be implemented in assembly.

The default implementation of this function calls report_unhandled_exception.

17.   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_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 BL33 image in the fip image by defining this flag to no. If any of the options EL3_PAYLOAD_BASE or PRELOADED_BL33_BASE are used, this flag will be set to no automatically.

18.   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 TF-A 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.

Some C headers have been obtained from FreeBSD and SCC, while others have been written specifically for TF-A. Fome implementation files have been obtained from FreeBSD, others have been written specifically for TF-A as well. The files can be found in include/lib/libc and lib/libc.

SCC can be found in `http://www.simple-cc.org/`_. A copy of the FreeBSD sources can be obtained from `http://github.com/freebsd/freebsd`_.

19.   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 Arm development platforms 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 identifiers, as defined in [include/plat/common/platform_def.h] (or a separate header file included from there). 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.

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