source: rtems-docs/c_user/interrupt_manager.rst @ d389819

Interrupt Manager
#################

Introduction
============

Any real-time executive must provide a mechanism for
quick response to externally generated interrupts to satisfy the
critical time constraints of the application.  The interrupt
manager provides this mechanism for RTEMS.  This manager permits
quick interrupt response times by providing the critical ability
to alter task execution which allows a task to be preempted upon
exit from an ISR.  The interrupt manager includes the following
directive:

- ``rtems_interrupt_catch`` - Establish an ISR

- ``rtems_interrupt_disable`` - Disable Interrupts

- ``rtems_interrupt_enable`` - Enable Interrupts

- ``rtems_interrupt_flash`` - Flash Interrupt

- ``rtems_interrupt_local_disable`` - Disable Interrupts on Current Processor

- ``rtems_interrupt_local_enable`` - Enable Interrupts on Current Processor

- ``rtems_interrupt_lock_initialize`` - Initialize an ISR Lock

- ``rtems_interrupt_lock_acquire`` - Acquire an ISR Lock

- ``rtems_interrupt_lock_release`` - Release an ISR Lock

- ``rtems_interrupt_lock_acquire_isr`` - Acquire an ISR Lock from ISR

- ``rtems_interrupt_lock_release_isr`` - Release an ISR Lock from ISR

- ``rtems_interrupt_is_in_progress`` - Is an ISR in Progress

Background
==========

Processing an Interrupt
-----------------------
.. index:: interrupt processing

The interrupt manager allows the application to
connect a function to a hardware interrupt vector.  When an
interrupt occurs, the processor will automatically vector to
RTEMS.  RTEMS saves and restores all registers which are not
preserved by the normal C calling convention
for the target
processor and invokes the user's ISR.  The user's ISR is
responsible for processing the interrupt, clearing the interrupt
if necessary, and device specific manipulation... index:: rtems_vector_number

The ``rtems_interrupt_catch``
directive connects a procedure to
an interrupt vector.  The vector number is managed using
the ``rtems_vector_number`` data type.

The interrupt service routine is assumed
to abide by these conventions and have a prototype similar to
the following:.. index:: rtems_isr

.. code:: c

    rtems_isr user_isr(
    rtems_vector_number vector
    );

The vector number argument is provided by RTEMS to
allow the application to identify the interrupt source.  This
could be used to allow a single routine to service interrupts
from multiple instances of the same device.  For example, a
single routine could service interrupts from multiple serial
ports and use the vector number to identify which port requires
servicing.

To minimize the masking of lower or equal priority
level interrupts, the ISR should perform the minimum actions
required to service the interrupt.  Other non-essential actions
should be handled by application tasks.  Once the user's ISR has
completed, it returns control to the RTEMS interrupt manager
which will perform task dispatching and restore the registers
saved before the ISR was invoked.

The RTEMS interrupt manager guarantees that proper
task scheduling and dispatching are performed at the conclusion
of an ISR.  A system call made by the ISR may have readied a
task of higher priority than the interrupted task.  Therefore,
when the ISR completes, the postponed dispatch processing must
be performed.  No dispatch processing is performed as part of
directives which have been invoked by an ISR.

Applications must adhere to the following rule if
proper task scheduling and dispatching is to be performed:

- ** *The interrupt manager must be used for all ISRs which
  may be interrupted by the highest priority ISR which invokes an
  RTEMS directive.*

Consider a processor which allows a numerically low
interrupt level to interrupt a numerically greater interrupt
level.  In this example, if an RTEMS directive is used in a
level 4 ISR, then all ISRs which execute at levels 0 through 4
must use the interrupt manager.

Interrupts are nested whenever an interrupt occurs
during the execution of another ISR.  RTEMS supports efficient
interrupt nesting by allowing the nested ISRs to terminate
without performing any dispatch processing.  Only when the
outermost ISR terminates will the postponed dispatching occur.

RTEMS Interrupt Levels
----------------------
.. index:: interrupt levels

Many processors support multiple interrupt levels or
priorities.  The exact number of interrupt levels is processor
dependent.  RTEMS internally supports 256 interrupt levels which
are mapped to the processor's interrupt levels.  For specific
information on the mapping between RTEMS and the target
processor's interrupt levels, refer to the Interrupt Processing
chapter of the Applications Supplement document for a specific
target processor.

Disabling of Interrupts by RTEMS
--------------------------------
.. index:: disabling interrupts

During the execution of directive calls, critical
sections of code may be executed.  When these sections are
encountered, RTEMS disables all maskable interrupts before the
execution of the section and restores them to the previous level
upon completion of the section.  RTEMS has been optimized to
ensure that interrupts are disabled for a minimum length of
time.  The maximum length of time interrupts are disabled by
RTEMS is processor dependent and is detailed in the Timing
Specification chapter of the Applications Supplement document
for a specific target processor.

Non-maskable interrupts (NMI) cannot be disabled, and
ISRs which execute at this level MUST NEVER issue RTEMS system
calls.  If a directive is invoked, unpredictable results may
occur due to the inability of RTEMS to protect its critical
sections.  However, ISRs that make no system calls may safely
execute as non-maskable interrupts.

Operations
==========

Establishing an ISR
-------------------

The ``rtems_interrupt_catch``
directive establishes an ISR for
the system.  The address of the ISR and its associated CPU
vector number are specified to this directive.  This directive
installs the RTEMS interrupt wrapper in the processor's
Interrupt Vector Table and the address of the user's ISR in the
RTEMS' Vector Table.  This directive returns the previous
contents of the specified vector in the RTEMS' Vector Table.

Directives Allowed from an ISR
------------------------------

Using the interrupt manager ensures that RTEMS knows
when a directive is being called from an ISR.  The ISR may then
use system calls to synchronize itself with an application task.
The synchronization may involve messages, events or signals
being passed by the ISR to the desired task.  Directives invoked
by an ISR must operate only on objects which reside on the local
node.  The following is a list of RTEMS system calls that may be
made from an ISR:

- Task Management
  Although it is acceptable to operate on the RTEMS_SELF task (e.g.
  the currently executing task), while in an ISR, this will refer
  to the interrupted task.  Most of the time, it is an application
  implementation error to use RTEMS_SELF from an ISR.
  - rtems_task_suspend
  - rtems_task_resume

- Interrupt Management
  - rtems_interrupt_enable
  - rtems_interrupt_disable
  - rtems_interrupt_flash
  - rtems_interrupt_lock_acquire
  - rtems_interrupt_lock_release
  - rtems_interrupt_lock_acquire_isr
  - rtems_interrupt_lock_release_isr
  - rtems_interrupt_is_in_progress
  - rtems_interrupt_catch

- Clock Management
  - rtems_clock_set
  - rtems_clock_get
  - rtems_clock_get_tod
  - rtems_clock_get_tod_timeval
  - rtems_clock_get_seconds_since_epoch
  - rtems_clock_get_ticks_per_second
  - rtems_clock_get_ticks_since_boot
  - rtems_clock_get_uptime
  - rtems_clock_set_nanoseconds_extension
  - rtems_clock_tick

- Timer Management
  - rtems_timer_cancel
  - rtems_timer_reset
  - rtems_timer_fire_after
  - rtems_timer_fire_when
  - rtems_timer_server_fire_after
  - rtems_timer_server_fire_when

- Event Management
  - rtems_event_send
  - rtems_event_system_send
  - rtems_event_transient_send

- Semaphore Management
  - rtems_semaphore_release

- Message Management
  - rtems_message_queue_send
  - rtems_message_queue_urgent

- Signal Management
  - rtems_signal_send

- Dual-Ported Memory Management
  - rtems_port_external_to_internal
  - rtems_port_internal_to_external

- IO Management
  The following services are safe to call from an ISR if and only if
  the device driver service invoked is also safe.  The IO Manager itself
  is safe but the invoked driver entry point may or may not be.
  - rtems_io_initialize
  - rtems_io_open
  - rtems_io_close
  - rtems_io_read
  - rtems_io_write
  - rtems_io_control

- Fatal Error Management
  - rtems_fatal
  - rtems_fatal_error_occurred

- Multiprocessing
  - rtems_multiprocessing_announce

Directives
==========

This section details the interrupt manager's
directives.  A subsection is dedicated to each of this manager's
directives and describes the calling sequence, related
constants, usage, and status codes.

INTERRUPT_CATCH - Establish an ISR
----------------------------------
.. index:: establish an ISR
.. index:: install an ISR

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_catch

.. code:: c

    rtems_status_code rtems_interrupt_catch(
    rtems_isr_entry      new_isr_handler,
    rtems_vector_number  vector,
    rtems_isr_entry     \*old_isr_handler
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - ISR established successfully
``RTEMS_INVALID_NUMBER`` - illegal vector number
``RTEMS_INVALID_ADDRESS`` - illegal ISR entry point or invalid ``old_isr_handler``

**DESCRIPTION:**

This directive establishes an interrupt service
routine (ISR) for the specified interrupt vector number.  The``new_isr_handler`` parameter specifies the entry point of the ISR.
The entry point of the previous ISR for the specified vector is
returned in ``old_isr_handler``.

To release an interrupt vector, pass the old handler's address obtained
when the vector was first capture.

**NOTES:**

This directive will not cause the calling task to be preempted.

INTERRUPT_DISABLE - Disable Interrupts
--------------------------------------
.. index:: disable interrupts

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_disable

.. code:: c

    void rtems_interrupt_disable(
    rtems_interrupt_level  level
    );
    /* this is implemented as a macro and sets level as a side-effect \*/

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive disables all maskable interrupts and returns
the previous ``level``.  A later invocation of the``rtems_interrupt_enable`` directive should be used to
restore the interrupt level.

**NOTES:**

This directive will not cause the calling task to be preempted.

*This directive is implemented as a macro which modifies the ``level``
parameter.*

This directive is only available on uni-processor configurations.  The
directive ``rtems_interrupt_local_disable`` is available on all
configurations.

INTERRUPT_ENABLE - Enable Interrupts
------------------------------------
.. index:: enable interrupts

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_enable

.. code:: c

    void rtems_interrupt_enable(
    rtems_interrupt_level  level
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive enables maskable interrupts to the ``level``
which was returned by a previous call to``rtems_interrupt_disable``.
Immediately prior to invoking this directive, maskable interrupts should
be disabled by a call to ``rtems_interrupt_disable``
and will be enabled when this directive returns to the caller.

**NOTES:**

This directive will not cause the calling task to be preempted.

This directive is only available on uni-processor configurations.  The
directive ``rtems_interrupt_local_enable`` is available on all
configurations.

INTERRUPT_FLASH - Flash Interrupts
----------------------------------
.. index:: flash interrupts

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_flash

.. code:: c

    void rtems_interrupt_flash(
    rtems_interrupt_level level
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive temporarily enables maskable interrupts to the ``level``
which was returned by a previous call to``rtems_interrupt_disable``.
Immediately prior to invoking this directive, maskable interrupts should
be disabled by a call to ``rtems_interrupt_disable``
and will be redisabled when this directive returns to the caller.

**NOTES:**

This directive will not cause the calling task to be preempted.

This directive is only available on uni-processor configurations.  The
directives ``rtems_interrupt_local_disable`` and``rtems_interrupt_local_enable`` is available on all
configurations.

INTERRUPT_LOCAL_DISABLE - Disable Interrupts on Current Processor
-----------------------------------------------------------------
.. index:: disable interrupts

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_local_disable

.. code:: c

    void rtems_interrupt_local_disable(
    rtems_interrupt_level  level
    );
    /* this is implemented as a macro and sets level as a side-effect \*/

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive disables all maskable interrupts and returns
the previous ``level``.  A later invocation of the``rtems_interrupt_local_enable`` directive should be used to
restore the interrupt level.

**NOTES:**

This directive will not cause the calling task to be preempted.

*This directive is implemented as a macro which modifies the ``level``
parameter.*

On SMP configurations this will not ensure system wide mutual exclusion.  Use
interrupt locks instead.

INTERRUPT_LOCAL_ENABLE - Enable Interrupts on Current Processor
---------------------------------------------------------------
.. index:: enable interrupts

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_local_enable

.. code:: c

    void rtems_interrupt_local_enable(
    rtems_interrupt_level  level
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive enables maskable interrupts to the ``level``
which was returned by a previous call to``rtems_interrupt_local_disable``.
Immediately prior to invoking this directive, maskable interrupts should
be disabled by a call to ``rtems_interrupt_local_disable``
and will be enabled when this directive returns to the caller.

**NOTES:**

This directive will not cause the calling task to be preempted.

INTERRUPT_LOCK_INITIALIZE - Initialize an ISR Lock
--------------------------------------------------

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_lock_initialize

.. code:: c

    void rtems_interrupt_lock_initialize(
    rtems_interrupt_lock \*lock
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

Initializes an interrupt lock.

**NOTES:**

Concurrent initialization leads to unpredictable results.

INTERRUPT_LOCK_ACQUIRE - Acquire an ISR Lock
--------------------------------------------

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_lock_acquire

.. code:: c

    void rtems_interrupt_lock_acquire(
    rtems_interrupt_lock \*lock,
    rtems_interrupt_level level
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

Interrupts will be disabled.  On SMP configurations this directive acquires a
SMP lock.

**NOTES:**

This directive will not cause the calling thread to be preempted.  This
directive can be used in thread and interrupt context.

INTERRUPT_LOCK_RELEASE - Release an ISR Lock
--------------------------------------------

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_lock_release

.. code:: c

    void rtems_interrupt_lock_release(
    rtems_interrupt_lock \*lock,
    rtems_interrupt_level level
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

The interrupt status will be restored.  On SMP configurations this directive
releases a SMP lock.

**NOTES:**

This directive will not cause the calling thread to be preempted.  This
directive can be used in thread and interrupt context.

INTERRUPT_LOCK_ACQUIRE_ISR - Acquire an ISR Lock from ISR
---------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_lock_acquire_isr

.. code:: c

    void rtems_interrupt_lock_acquire_isr(
    rtems_interrupt_lock \*lock,
    rtems_interrupt_level level
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

The interrupt status will remain unchanged.  On SMP configurations this
directive acquires a SMP lock.

In case the corresponding interrupt service routine can be interrupted by
higher priority interrupts and these interrupts enter the critical section
protected by this lock, then the result is unpredictable.

**NOTES:**

This directive should be called from the corresponding interrupt service
routine.

INTERRUPT_LOCK_RELEASE_ISR - Release an ISR Lock from ISR
---------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_lock_release_isr

.. code:: c

    void rtems_interrupt_lock_release_isr(
    rtems_interrupt_lock \*lock,
    rtems_interrupt_level level
    );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

The interrupt status will remain unchanged.  On SMP configurations this
directive releases a SMP lock.

**NOTES:**

This directive should be called from the corresponding interrupt service
routine.

INTERRUPT_IS_IN_PROGRESS - Is an ISR in Progress
------------------------------------------------
.. index:: is interrupt in progress

**CALLING SEQUENCE:**

.. index:: rtems_interrupt_is_in_progress

.. code:: c

    bool rtems_interrupt_is_in_progress( void );

**DIRECTIVE STATUS CODES:**

NONE

**DESCRIPTION:**

This directive returns ``TRUE`` if the processor is currently
servicing an interrupt and ``FALSE`` otherwise.  A return value
of ``TRUE`` indicates that the caller is an interrupt service
routine, *NOT* a task.  The directives available to an interrupt
service routine are restricted.

**NOTES:**

This directive will not cause the calling task to be preempted.

.. COMMENT: COPYRIGHT (c) 1988-2008

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.