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+Trusted Firmware-A EL3 runtime service writer's guide
+=====================================================
+
+
+
+.. contents::
+
+Introduction
+------------
+
+This document describes how to add a runtime service to the EL3 Runtime
+Firmware component of Trusted Firmware-A (TF-A), BL31.
+
+Software executing in the normal world and in the trusted world at exception
+levels lower than EL3 will request runtime services using the Secure Monitor
+Call (SMC) instruction. These requests will follow the convention described in
+the SMC Calling Convention PDD (`SMCCC`_). The `SMCCC`_ assigns function
+identifiers to each SMC request and describes how arguments are passed and
+results are returned.
+
+SMC Functions are grouped together based on the implementor of the service, for
+example a subset of the Function IDs are designated as "OEM Calls" (see `SMCCC`_
+for full details). The EL3 runtime services framework in BL31 enables the
+independent implementation of services for each group, which are then compiled
+into the BL31 image. This simplifies the integration of common software from
+Arm to support `PSCI`_, Secure Monitor for a Trusted OS and SoC specific
+software. The common runtime services framework ensures that SMC Functions are
+dispatched to their respective service implementation - the `Firmware Design`_
+provides details of how this is achieved.
+
+The interface and operation of the runtime services depends heavily on the
+concepts and definitions described in the `SMCCC`_, in particular SMC Function
+IDs, Owning Entity Numbers (OEN), Fast and Standard calls, and the SMC32 and
+SMC64 calling conventions. Please refer to that document for a full explanation
+of these terms.
+
+Owning Entities, Call Types and Function IDs
+--------------------------------------------
+
+The SMC Function Identifier includes a OEN field. These values and their
+meaning are described in `SMCCC`_ and summarized in table 1 below. Some entities
+are allocated a range of of OENs. The OEN must be interpreted in conjunction
+with the SMC call type, which is either *Fast* or *Yielding*. Fast calls are
+uninterruptible whereas Yielding calls can be pre-empted. The majority of
+Owning Entities only have allocated ranges for Fast calls: Yielding calls are
+reserved exclusively for Trusted OS providers or for interoperability with
+legacy 32-bit software that predates the `SMCCC`_.
+
+::
+
+    Type       OEN     Service
+    Fast        0      Arm Architecture calls
+    Fast        1      CPU Service calls
+    Fast        2      SiP Service calls
+    Fast        3      OEM Service calls
+    Fast        4      Standard Service calls
+    Fast       5-47    Reserved for future use
+    Fast      48-49    Trusted Application calls
+    Fast      50-63    Trusted OS calls
+
+    Yielding   0- 1    Reserved for existing Armv7-A calls
+    Yielding   2-63    Trusted OS Standard Calls
+
+*Table 1: Service types and their corresponding Owning Entity Numbers*
+
+Each individual entity can allocate the valid identifiers within the entity
+range as they need - it is not necessary to coordinate with other entities of
+the same type. For example, two SoC providers can use the same Function ID
+within the SiP Service calls OEN range to mean different things - as these
+calls should be specific to the SoC. The Standard Runtime Calls OEN is used for
+services defined by Arm standards, such as `PSCI`_.
+
+The SMC Function ID also indicates whether the call has followed the SMC32
+calling convention, where all parameters are 32-bit, or the SMC64 calling
+convention, where the parameters are 64-bit. The framework identifies and
+rejects invalid calls that use the SMC64 calling convention but that originate
+from an AArch32 caller.
+
+The EL3 runtime services framework uses the call type and OEN to identify a
+specific handler for each SMC call, but it is expected that an individual
+handler will be responsible for all SMC Functions within a given service type.
+
+Getting started
+---------------
+
+TF-A has a `services`_ directory in the source tree under which
+each owning entity can place the implementation of its runtime service. The
+`PSCI`_ implementation is located here in the `lib/psci`_ directory.
+
+Runtime service sources will need to include the `runtime_svc.h`_ header file.
+
+Registering a runtime service
+-----------------------------
+
+A runtime service is registered using the ``DECLARE_RT_SVC()`` macro, specifying
+the name of the service, the range of OENs covered, the type of service and
+initialization and call handler functions.
+
+::
+
+    #define DECLARE_RT_SVC(_name, _start, _end, _type, _setup, _smch)
+
+-  ``_name`` is used to identify the data structure declared by this macro, and
+   is also used for diagnostic purposes
+
+-  ``_start`` and ``_end`` values must be based on the ``OEN_*`` values defined in
+   `smccc.h`_
+
+-  ``_type`` must be one of ``SMC_TYPE_FAST`` or ``SMC_TYPE_YIELD``
+
+-  ``_setup`` is the initialization function with the ``rt_svc_init`` signature:
+
+   .. code:: c
+
+       typedef int32_t (*rt_svc_init)(void);
+
+-  ``_smch`` is the SMC handler function with the ``rt_svc_handle`` signature:
+
+   .. code:: c
+
+       typedef uintptr_t (*rt_svc_handle_t)(uint32_t smc_fid,
+                                         u_register_t x1, u_register_t x2,
+                                         u_register_t x3, u_register_t x4,
+                                         void *cookie,
+                                         void *handle,
+                                         u_register_t flags);
+
+Details of the requirements and behavior of the two callbacks is provided in
+the following sections.
+
+During initialization the services framework validates each declared service
+to ensure that the following conditions are met:
+
+#. The ``_start`` OEN is not greater than the ``_end`` OEN
+#. The ``_end`` OEN does not exceed the maximum OEN value (63)
+#. The ``_type`` is one of ``SMC_TYPE_FAST`` or ``SMC_TYPE_YIELD``
+#. ``_setup`` and ``_smch`` routines have been specified
+
+`std_svc_setup.c`_ provides an example of registering a runtime service:
+
+.. code:: c
+
+    /* Register Standard Service Calls as runtime service */
+    DECLARE_RT_SVC(
+            std_svc,
+            OEN_STD_START,
+            OEN_STD_END,
+            SMC_TYPE_FAST,
+            std_svc_setup,
+            std_svc_smc_handler
+    );
+
+Initializing a runtime service
+------------------------------
+
+Runtime services are initialized once, during cold boot, by the primary CPU
+after platform and architectural initialization is complete. The framework
+performs basic validation of the declared service before calling
+the service initialization function (``_setup`` in the declaration). This
+function must carry out any essential EL3 initialization prior to receiving a
+SMC Function call via the handler function.
+
+On success, the initialization function must return ``0``. Any other return value
+will cause the framework to issue a diagnostic:
+
+::
+
+    Error initializing runtime service <name of the service>
+
+and then ignore the service - the system will continue to boot but SMC calls
+will not be passed to the service handler and instead return the *Unknown SMC
+Function ID* result ``0xFFFFFFFF``.
+
+If the system must not be allowed to proceed without the service, the
+initialization function must itself cause the firmware boot to be halted.
+
+If the service uses per-CPU data this must either be initialized for all CPUs
+during this call, or be done lazily when a CPU first issues an SMC call to that
+service.
+
+Handling runtime service requests
+---------------------------------
+
+SMC calls for a service are forwarded by the framework to the service's SMC
+handler function (``_smch`` in the service declaration). This function must have
+the following signature:
+
+.. code:: c
+
+    typedef uintptr_t (*rt_svc_handle_t)(uint32_t smc_fid,
+                                       u_register_t x1, u_register_t x2,
+                                       u_register_t x3, u_register_t x4,
+                                       void *cookie,
+                                       void *handle,
+                                       u_register_t flags);
+
+The handler is responsible for:
+
+#. Determining that ``smc_fid`` is a valid and supported SMC Function ID,
+   otherwise completing the request with the *Unknown SMC Function ID*:
+
+   .. code:: c
+
+       SMC_RET1(handle, SMC_UNK);
+
+#. Determining if the requested function is valid for the calling security
+   state. SMC Calls can be made from both the normal and trusted worlds and
+   the framework will forward all calls to the service handler.
+
+   The ``flags`` parameter to this function indicates the caller security state
+   in bit[0], where a value of ``1`` indicates a non-secure caller. The
+   ``is_caller_secure(flags)`` and ``is_caller_non_secure(flags)`` can be used to
+   test this condition.
+
+   If invalid, the request should be completed with:
+
+   .. code:: c
+
+       SMC_RET1(handle, SMC_UNK);
+
+#. Truncating parameters for calls made using the SMC32 calling convention.
+   Such calls can be determined by checking the CC field in bit[30] of the
+   ``smc_fid`` parameter, for example by using:
+
+   ::
+
+       if (GET_SMC_CC(smc_fid) == SMC_32) ...
+
+   For such calls, the upper bits of the parameters x1-x4 and the saved
+   parameters X5-X7 are UNDEFINED and must be explicitly ignored by the
+   handler. This can be done by truncating the values to a suitable 32-bit
+   integer type before use, for example by ensuring that functions defined
+   to handle individual SMC Functions use appropriate 32-bit parameters.
+
+#. Providing the service requested by the SMC Function, utilizing the
+   immediate parameters x1-x4 and/or the additional saved parameters X5-X7.
+   The latter can be retrieved using the ``SMC_GET_GP(handle, ref)`` function,
+   supplying the appropriate ``CTX_GPREG_Xn`` reference, e.g.
+
+   .. code:: c
+
+       uint64_t x6 = SMC_GET_GP(handle, CTX_GPREG_X6);
+
+#. Implementing the standard SMC32 Functions that provide information about
+   the implementation of the service. These are the Call Count, Implementor
+   UID and Revision Details for each service documented in section 6 of the
+   `SMCCC`_.
+
+   TF-A expects owning entities to follow this recommendation.
+
+#. Returning the result to the caller. The `SMCCC`_ allows for up to 256 bits
+   of return value in SMC64 using X0-X3 and 128 bits in SMC32 using W0-W3. The
+   framework provides a family of macros to set the multi-register return
+   value and complete the handler:
+
+   .. code:: c
+
+       SMC_RET1(handle, x0);
+       SMC_RET2(handle, x0, x1);
+       SMC_RET3(handle, x0, x1, x2);
+       SMC_RET4(handle, x0, x1, x2, x3);
+
+The ``cookie`` parameter to the handler is reserved for future use and can be
+ignored. The ``handle`` is returned by the SMC handler - completion of the
+handler function must always be via one of the ``SMC_RETn()`` macros.
+
+NOTE: The PSCI and Test Secure-EL1 Payload Dispatcher services do not follow
+all of the above requirements yet.
+
+Services that contain multiple sub-services
+-------------------------------------------
+
+It is possible that a single owning entity implements multiple sub-services. For
+example, the Standard calls service handles ``0x84000000``-``0x8400FFFF`` and
+``0xC4000000``-``0xC400FFFF`` functions. Within that range, the `PSCI`_ service
+handles the ``0x84000000``-``0x8400001F`` and ``0xC4000000``-``0xC400001F`` functions.
+In that respect, `PSCI`_ is a 'sub-service' of the Standard calls service. In
+future, there could be additional such sub-services in the Standard calls
+service which perform independent functions.
+
+In this situation it may be valuable to introduce a second level framework to
+enable independent implementation of sub-services. Such a framework might look
+very similar to the current runtime services framework, but using a different
+part of the SMC Function ID to identify the sub-service. TF-A does not provide
+such a framework at present.
+
+Secure-EL1 Payload Dispatcher service (SPD)
+-------------------------------------------
+
+Services that handle SMC Functions targeting a Trusted OS, Trusted Application,
+or other Secure-EL1 Payload are special. These services need to manage the
+Secure-EL1 context, provide the *Secure Monitor* functionality of switching
+between the normal and secure worlds, deliver SMC Calls through to Secure-EL1
+and generally manage the Secure-EL1 Payload through CPU power-state transitions.
+
+TODO: Provide details of the additional work required to implement a SPD and
+the BL31 support for these services. Or a reference to the document that will
+provide this information....
+
+--------------
+
+*Copyright (c) 2014-2018, Arm Limited and Contributors. All rights reserved.*
+
+.. _SMCCC: http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
+.. _PSCI: http://infocenter.arm.com/help/topic/com.arm.doc.den0022c/DEN0022C_Power_State_Coordination_Interface.pdf
+.. _Firmware Design: ../designb_documents/firmware-design.rst
+.. _services: ../../services
+.. _lib/psci: ../../lib/psci
+.. _runtime_svc.h: ../../include/common/runtime_svc.h
+.. _smccc.h: ../../include/lib/smccc.h
+.. _std_svc_setup.c: ../../services/std_svc/std_svc_setup.c