docs: Add SDEI dispatcher documentation
The document includes SDEI sequence diagrams that are generated using
PlantUML [1].
A shell script is introduced to generate SVG files from PlantUML files
supplied in arguments.
[1] http://plantuml.com/PlantUML_Language_Reference_Guide.pdf
Change-Id: I433897856810bf1927f2800a7b2b1d81827c69b2
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
diff --git a/docs/plantuml/plantuml_to_svg.sh b/docs/plantuml/plantuml_to_svg.sh
new file mode 100755
index 0000000..0bf8588
--- /dev/null
+++ b/docs/plantuml/plantuml_to_svg.sh
@@ -0,0 +1,13 @@
+#!/bin/bash
+
+# Convert all PlantUML files in this directory to SVG files. The plantuml_jar
+# environment variable must be set to the path to PlantUML JAR file.
+
+if [ -z "$plantuml_jar" ]; then
+ echo "Usage: plantuml_jar=/path/to/plantuml.jar $0 *.puml" >&2
+ exit 1
+fi
+
+java -jar "$plantuml_jar" -nometadata -tsvg "$@"
+
+# vim:set noet sts=8 tw=80:
diff --git a/docs/plantuml/sdei_explicit_dispatch.puml b/docs/plantuml/sdei_explicit_dispatch.puml
new file mode 100644
index 0000000..51214f5
--- /dev/null
+++ b/docs/plantuml/sdei_explicit_dispatch.puml
@@ -0,0 +1,45 @@
+/'
+ ' Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ '
+ ' SPDX-License-Identifier: BSD-3-Clause
+ '/
+
+@startuml
+
+autonumber "<b>[#]</b>"
+participant "SDEI client" as EL2
+participant EL3
+participant SEL1
+
+activate EL2
+EL2->EL3: **SDEI_EVENT_REGISTER**(ev, handler, ...)
+EL3->EL2: success
+EL2->EL3: **SDEI_EVENT_ENABLE**(ev)
+EL3->EL2: success
+EL2->EL3: **SDEI_PE_UNMASK**()
+EL3->EL2: 1
+
+... <<Business as usual>> ...
+
+EL3<--]: **CRITICAL EVENT**
+activate EL3 #red
+note over EL3: Critical event triage
+EL3->SEL1: dispatch
+activate SEL1 #salmon
+note over SEL1: Critical event handling
+SEL1->EL3: done
+deactivate SEL1
+EL3-->EL3: sdei_dispatch_event(ev)
+note over EL3: Prepare SDEI dispatch
+EL3->EL2: dispatch
+activate EL2 #salmon
+note over EL2: SDEI handler
+EL2->EL3: **SDEI_EVENT_COMPLETE()**
+deactivate EL2
+note over EL3: Complete SDEI dispatch
+EL3->EL2: resumes preempted execution
+deactivate EL3
+
+... <<Normal execution resumes>> ...
+
+@enduml
diff --git a/docs/plantuml/sdei_explicit_dispatch.svg b/docs/plantuml/sdei_explicit_dispatch.svg
new file mode 100644
index 0000000..b33944e
--- /dev/null
+++ b/docs/plantuml/sdei_explicit_dispatch.svg
@@ -0,0 +1 @@
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\ No newline at end of file
diff --git a/docs/plantuml/sdei_general.puml b/docs/plantuml/sdei_general.puml
new file mode 100644
index 0000000..ab6929a
--- /dev/null
+++ b/docs/plantuml/sdei_general.puml
@@ -0,0 +1,43 @@
+/'
+ ' Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ '
+ ' SPDX-License-Identifier: BSD-3-Clause
+ '/
+
+@startuml
+
+autonumber "<b>[#]</b>"
+participant "SDEI client" as EL2
+participant EL3
+participant "SDEI interrupt source" as SDEI
+
+activate EL2
+EL2->EL3: **SDEI_INTERRUPT_BIND**(irq)
+EL3->EL2: event number: ev
+EL2->EL3: **SDEI_EVENT_REGISTER**(ev, handler, ...)
+EL3->EL2: success
+EL2->EL3: **SDEI_EVENT_ENABLE**(ev)
+EL3->EL2: success
+EL2->EL3: **SDEI_PE_UNMASK**()
+EL3->EL2: 1
+
+... <<Business as usual>> ...
+
+SDEI-->EL3: SDEI interrupt
+activate SDEI #salmon
+activate EL3 #red
+note over EL3: Prepare SDEI dispatch
+EL3->EL2: dispatch
+activate EL2 #salmon
+note over EL2: SDEI handler
+EL2->EL3: **SDEI_EVENT_COMPLETE()**
+deactivate EL2
+note over EL3: Complete SDEI dispatch
+EL3-->SDEI: EOI
+deactivate SDEI
+EL3->EL2: resumes preempted execution
+deactivate EL3
+
+... <<Normal execution resumes>> ...
+
+@enduml
diff --git a/docs/plantuml/sdei_general.svg b/docs/plantuml/sdei_general.svg
new file mode 100644
index 0000000..e172112
--- /dev/null
+++ b/docs/plantuml/sdei_general.svg
@@ -0,0 +1 @@
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+Software Delegated Exception Interface
+======================================
+
+
+.. section-numbering::
+ :suffix: .
+
+.. contents::
+ :depth: 2
+
+This document provides an overview of the SDEI dispatcher implementation in ARM
+Trusted Firmware.
+
+Introduction
+------------
+
+`Software Delegated Exception Interface`_ (SDEI) is an ARM specification for
+Non-secure world to register handlers with firmware to receive notifications
+about system events. Firmware will first receive the system events by way of
+asynchronous exceptions and, in response, arranges for the registered handler to
+execute in the Non-secure EL.
+
+Normal world software that interacts with the SDEI dispatcher (makes SDEI
+requests and receives notifications) is referred to as the *SDEI Client*. A
+client receives the event notification at the registered handler even when it
+was executing with exceptions masked. The list of SDEI events available to the
+client are specific to the platform [#std-event]_. See also `Determining client
+EL`_.
+
+.. _general SDEI dispatch:
+
+The following figure depicts a general sequence involving SDEI client executing
+at EL2 and an event dispatch resulting from the triggering of a bound interrupt.
+A commentary is provided below:
+
+.. image:: plantuml/sdei_general.svg
+
+As part of initialisation, the SDEI client binds a Non-secure interrupt [1], and
+the SDEI dispatcher returns a platform dynamic event number [2]. The client then
+registers a handler for that event [3], enables the event [5], and unmasks all
+events on the current PE [7]. This sequence is typical of an SDEI client, but it
+may involve additional SDEI calls.
+
+At a later point in time, when the bound interrupt triggers [9], it's trapped to
+EL3. The interrupt is handed over to the SDEI dispatcher, which then arranges to
+execute the registered handler [10]. The client terminates its execution with
+``SDEI_EVENT_COMPLETE`` [11], following which the dispatcher resumes the
+original EL2 execution [13]. Note that the SDEI interrupt remains active until
+the client handler completes, at which point EL3 does EOI [12].
+
+SDEI events can be explicitly dispatched in response to other asynchronous
+exceptions. See `Explicit dispatch of events`_.
+
+The remainder of this document only discusses the design and implementation of
+SDEI dispatcher in ARM Trusted Firmware, and assumes that the reader is familiar
+with the SDEI specification, the interfaces, and their requirements.
+
+.. [#std-event] Except event 0, which is defined by the SDEI specification as a
+ standard event.
+
+Defining events
+---------------
+
+A platform choosing to include the SDEI dispatcher must also define the events
+available on the platform, along with their attributes.
+
+The platform is expected to provide two arrays of event descriptors: one for
+private events, and another for shared events. The SDEI dispatcher provides
+``SDEI_PRIVATE_EVENT()`` and ``SDEI_SHARED_EVENT()`` macros to populate the
+event descriptors. Both macros take 3 arguments:
+
+- The event number: this must be a positive 32-bit integer.
+
+- The interrupt number the event is bound to:
+
+ - If it's not applicable to an event, this shall be left as ``0``.
+
+ - If the event is dynamic, this should be specified as ``SDEI_DYN_IRQ``.
+
+- A bit map of `Event flags`_.
+
+To define event 0, the macro ``SDEI_DEFINE_EVENT_0()`` should be used. This
+macro takes only one parameter: an SGI number to signal other PEs.
+
+Once the event descriptor arrays are defined, they should be exported to the
+SDEI dispatcher using the ``REGISTER_SDEI_MAP()`` macro, passing it the pointers
+to the private and shared event descriptor arrays, respectively. Note that the
+``REGISTER_SDEI_MAP()`` macro must be used in the same file where the arrays are
+defined.
+
+Regarding event descriptors:
+
+- For Event 0:
+
+ - There must be exactly one descriptor in the private array, and none in the
+ shared array.
+
+ - The event should be defined using ``SDEI_DEFINE_EVENT_0()``.
+
+ - Must be bound to a Secure SGI on the platform.
+
+- Statically bound shared and private interrupts must be bound to shared and
+ private interrupts on the platform, respectively. See the section on
+ `interrupt configuration`__.
+
+ .. __: `Configuration within Exception Handling Framework`_
+
+- Both arrays should be one-dimensional. The ``REGISTER_SDEI_MAP()`` macro
+ takes care of replicating private events for each PE on the platform.
+
+- Both arrays must be sorted in the increasing order of event number.
+
+The SDEI specification doesn't have provisions for discovery of available events
+on the platform. The list of events made available to the client, along with
+their semantics, have to be communicated out of band; for example, through
+Device Trees or firmware configuration tables.
+
+See also `Event definition example`_.
+
+Event flags
+~~~~~~~~~~~
+
+Event flags describe the properties of the event. They are bit maps that can be
+``OR``\ ed to form parameters to macros that `define events`__.
+
+.. __: `Defining events`_
+
+- ``SDEI_MAPF_DYNAMIC``: Marks the event as dynamic. Dynamic events can be
+ bound to (or released from) any Non-secure interrupt at runtime via. the
+ ``SDEI_INTERRUPT_BIND`` and ``SDEI_INTERRUPT_RELEASE`` calls.
+
+- ``SDEI_MAPF_BOUND``: Marks the event as statically bound to an interrupt.
+ These events cannot be re-bound at runtime.
+
+- ``SDEI_MAPF_CRITICAL``: Marks the event as having *Critical* priority.
+ Without this flag, the event is assumed to have *Normal* priority.
+
+Event definition example
+------------------------
+
+.. code:: c
+
+ static sdei_ev_map_t plat_private_sdei[] = {
+ /* Event 0 definition */
+ SDEI_DEFINE_EVENT_0(8),
+
+ /* PPI */
+ SDEI_PRIVATE_EVENT(8, 23, SDEI_MAPF_BOUND),
+
+ /* Dynamic private events */
+ SDEI_PRIVATE_EVENT(100, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC),
+ SDEI_PRIVATE_EVENT(101, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC)
+ };
+
+ /* Shared event mappings */
+ static sdei_ev_map_t plat_shared_sdei[] = {
+ SDEI_SHARED_EVENT(804, 0, SDEI_MAPF_DYNAMIC),
+
+ /* Dynamic shared events */
+ SDEI_SHARED_EVENT(3000, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC),
+ SDEI_SHARED_EVENT(3001, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC)
+ };
+
+ /* Export SDEI events */
+ REGISTER_SDEI_MAP(plat_private_sdei, plat_shared_sdei);
+
+Configuration within Exception Handling Framework
+-------------------------------------------------
+
+The SDEI dispatcher functions alongside the Exception Handling Framework. This
+means that the platform must assign priorities to both Normal and Critical SDEI
+interrupts for the platform:
+
+- Install priority descriptors for Normal and Critical SDEI interrupts.
+
+- For those interrupts that are statically bound (i.e. events defined as having
+ the ``SDEI_MAPF_BOUND`` property), enumerate their properties for the GIC
+ driver to configure interrupts accordingly.
+
+ The interrupts must be configured to target EL3. This means that they should
+ be configured as *Group 0*. Additionally, on GICv2 systems, the build option
+ ``GICV2_G0_FOR_EL3`` must be set to ``1``.
+
+See also `SDEI porting requirements`_.
+
+Determining client EL
+---------------------
+
+The SDEI specification requires that the *physical* SDEI client executes in the
+highest Non-secure EL implemented on the system. This means that the dispatcher
+will only allow SDEI calls to be made from:
+
+- EL2, if EL2 is implemented. The Hypervisor is expected to implement a
+ *virtual* SDEI dispatcher to support SDEI clients in Guest Operating Systems
+ executing in Non-secure EL1.
+
+- Non-secure EL1, if EL2 is not implemented or disabled.
+
+See the function ``sdei_client_el()`` in ``sdei_private.h``.
+
+Explicit dispatch of events
+---------------------------
+
+Typically, an SDEI event dispatch is caused by the PE receiving interrupts that
+are bound to an SDEI event. However, there are cases where the Secure world
+requires dispatch of an SDEI event as a direct or indirect result of a past
+activity, viz. receiving a Secure interrupt or an exception.
+
+The SDEI dispatcher implementation provides ``sdei_dispatch_event()`` API for
+this purpose. The API has the following signature:
+
+::
+
+ int sdei_dispatch_event(int ev_num, unsigned int preempted_sec_state);
+
+- The parameter ``ev_num`` is the event number to dispatch;
+
+- The parameter ``preempted_sec_state`` indicates the context that was
+ preempted. This must be either ``SECURE`` or ``NON_SECURE``.
+
+The API returns ``0`` on success, or ``-1`` on failure.
+
+The following figure depicts a scenario involving explicit dispatch of SDEI
+event. A commentary is provided below:
+
+.. image:: plantuml/sdei_explicit_dispatch.svg
+
+As part of initialisation, the SDEI client registers a handler for a platform
+event [1], enables the event [3], and unmasks the current PE [5]. Note that,
+unlike in `general SDEI dispatch`_, this doesn't involve interrupt binding, as
+bound or dynamic events can't be explicitly dispatched (see the section below).
+
+At a later point in time, a critical event [#critical-event]_ is trapped into
+EL3 [7]. EL3 performs a first-level triage of the event, and decides to dispatch
+to Secure EL1 for further handling [8]. The dispatch completes, but intends to
+involve Non-secure world in further handling, and therefore decides to
+explicitly dispatch an event [10] (which the client had already registered for
+[1]). The rest of the sequence is similar to that in the `general SDEI
+dispatch`_: the requested event is dispatched to the client (assuming all the
+conditions are met), and when the handler completes, the preempted execution
+resumes.
+
+Conditions for event dispatch
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+All of the following requirements must be met for the API to return ``0`` and
+event to be dispatched:
+
+- SDEI events must be unmasked on the PE. I.e. the client must have called
+ ``PE_UNMASK`` beforehand.
+
+- Event 0 can't be dispatched.
+
+- The event must neither be a dynamic event nor be bound to an interrupt.
+
+- The event must be private to the PE.
+
+- The event must have been registered for and enabled.
+
+- A dispatch for the same event must not be outstanding. I.e. it hasn't already
+ been dispatched and is yet to be completed.
+
+- The priority of the event (either Critical or Normal, as configured by the
+ platform at build-time) shouldn't cause priority inversion. This means:
+
+ - If it's of Normal priority, neither Normal nor Critical priority dispatch
+ must be outstanding on the PE.
+
+ - If it's of a Critical priority, no Critical priority dispatch must be
+ outstanding on the PE.
+
+Further, the caller should be aware of the following assumptions made by the
+dispatcher:
+
+- The caller of the API is a component running in EL3; for example, the *Secure
+ Partition Manager*.
+
+- The requested dispatch will be permitted by the Exception Handling Framework.
+ I.e. the caller must make sure that the requested dispatch has sufficient
+ priority so as not to cause priority level inversion within Exception
+ Handling Framework.
+
+- At the time of the call, the active context is Secure, and it has been saved.
+
+- Upon returning success, the Non-secure context will be restored and setup for
+ the event dispatch, and it will be the active context. The Non-secure context
+ should not be modified further by the caller.
+
+- The API returning success only means that the dispatch is scheduled at the
+ next ``ERET``, and not immediately performed. Also, the caller must be
+ prepared for this API to return failure and handle accordingly.
+
+- Upon completing the event (i.e. when the client calls either
+ ``SDEI_EVENT_COMPLETE`` or ``SDEI_COMPLETE_AND_RESUME``), the preempted
+ context is resumed (as indicated by the ``preempted_sec_state`` parameter of
+ the API).
+
+.. [#critical-event] Examples of critical event are *SError*, *Synchronous
+ External Abort*, *Fault Handling interrupt*, or *Error
+ Recovery interrupt* from one of RAS nodes in the system.
+
+Porting requirements
+--------------------
+
+The porting requirements of the SDEI dispatcher are outlined in the `porting
+guide`__.
+
+.. __: `SDEI porting requirements`_
+
+Note on writing SDEI event handlers
+-----------------------------------
+
+*This section pertains to SDEI event handlers in general, not just when using
+ARM Trusted Firmware SDEI dispatcher.*
+
+The SDEI specification requires that event handlers preserve the contents of all
+registers except ``x0`` to ``x17``. This has significance if event handler is
+written in C: compilers typically adjust the stack frame at the beginning and
+end of C functions. For example, AArch64 GCC typically produces the following
+function prologue and epilogue:
+
+::
+
+ c_event_handler:
+ stp x29, x30, [sp,#-32]!
+ mov x29, sp
+
+ ...
+
+ bl ...
+
+ ...
+
+ ldp x29, x30, [sp],#32
+ ret
+
+The register ``x29`` is used as frame pointer in the prologue. Because neither a
+valid ``SDEI_EVENT_COMPLETE`` nor ``SDEI_EVENT_COMPLETE_AND_RESUME`` calls
+return to the handler, the epilogue never gets executed, and registers ``x29``
+and ``x30`` (in the case above) are inadvertently corrupted. This violates the
+SDEI specification, and the normal execution thereafter will result in
+unexpected behaviour.
+
+To work this around, it's advised that the top-level event handlers are
+implemented in assembly, following a similar pattern as below:
+
+::
+
+ asm_event_handler:
+ /* Save link register whilst maintaining stack alignment */
+ stp xzr, x30, [sp, #-16]!
+ bl c_event_handler
+
+ /* Restore link register */
+ ldp xzr, x30, [sp], #16
+
+ /* Complete call */
+ ldr x0, =SDEI_EVENT_COMPLETE
+ smc #0
+ b .
+
+----
+
+*Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.*
+
+.. _SDEI specification: http://infocenter.arm.com/help/topic/com.arm.doc.den0054a/ARM_DEN0054A_Software_Delegated_Exception_Interface.pdf
+.. _SDEI porting requirements: porting-guide.rst#sdei-porting-requirements
