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+.. include:: <isonum.txt>
+
+==========================
+Linux generic IRQ handling
+==========================
+
+:Copyright: |copy| 2005-2010: Thomas Gleixner
+:Copyright: |copy| 2005-2006:  Ingo Molnar
+
+Introduction
+============
+
+The generic interrupt handling layer is designed to provide a complete
+abstraction of interrupt handling for device drivers. It is able to
+handle all the different types of interrupt controller hardware. Device
+drivers use generic API functions to request, enable, disable and free
+interrupts. The drivers do not have to know anything about interrupt
+hardware details, so they can be used on different platforms without
+code changes.
+
+This documentation is provided to developers who want to implement an
+interrupt subsystem based for their architecture, with the help of the
+generic IRQ handling layer.
+
+Rationale
+=========
+
+The original implementation of interrupt handling in Linux uses the
+:c:func:`__do_IRQ` super-handler, which is able to deal with every type of
+interrupt logic.
+
+Originally, Russell King identified different types of handlers to build
+a quite universal set for the ARM interrupt handler implementation in
+Linux 2.5/2.6. He distinguished between:
+
+-  Level type
+
+-  Edge type
+
+-  Simple type
+
+During the implementation we identified another type:
+
+-  Fast EOI type
+
+In the SMP world of the :c:func:`__do_IRQ` super-handler another type was
+identified:
+
+-  Per CPU type
+
+This split implementation of high-level IRQ handlers allows us to
+optimize the flow of the interrupt handling for each specific interrupt
+type. This reduces complexity in that particular code path and allows
+the optimized handling of a given type.
+
+The original general IRQ implementation used hw_interrupt_type
+structures and their ``->ack``, ``->end`` [etc.] callbacks to differentiate
+the flow control in the super-handler. This leads to a mix of flow logic
+and low-level hardware logic, and it also leads to unnecessary code
+duplication: for example in i386, there is an ``ioapic_level_irq`` and an
+``ioapic_edge_irq`` IRQ-type which share many of the low-level details but
+have different flow handling.
+
+A more natural abstraction is the clean separation of the 'irq flow' and
+the 'chip details'.
+
+Analysing a couple of architecture's IRQ subsystem implementations
+reveals that most of them can use a generic set of 'irq flow' methods
+and only need to add the chip-level specific code. The separation is
+also valuable for (sub)architectures which need specific quirks in the
+IRQ flow itself but not in the chip details - and thus provides a more
+transparent IRQ subsystem design.
+
+Each interrupt descriptor is assigned its own high-level flow handler,
+which is normally one of the generic implementations. (This high-level
+flow handler implementation also makes it simple to provide
+demultiplexing handlers which can be found in embedded platforms on
+various architectures.)
+
+The separation makes the generic interrupt handling layer more flexible
+and extensible. For example, an (sub)architecture can use a generic
+IRQ-flow implementation for 'level type' interrupts and add a
+(sub)architecture specific 'edge type' implementation.
+
+To make the transition to the new model easier and prevent the breakage
+of existing implementations, the :c:func:`__do_IRQ` super-handler is still
+available. This leads to a kind of duality for the time being. Over time
+the new model should be used in more and more architectures, as it
+enables smaller and cleaner IRQ subsystems. It's deprecated for three
+years now and about to be removed.
+
+Known Bugs And Assumptions
+==========================
+
+None (knock on wood).
+
+Abstraction layers
+==================
+
+There are three main levels of abstraction in the interrupt code:
+
+1. High-level driver API
+
+2. High-level IRQ flow handlers
+
+3. Chip-level hardware encapsulation
+
+Interrupt control flow
+----------------------
+
+Each interrupt is described by an interrupt descriptor structure
+irq_desc. The interrupt is referenced by an 'unsigned int' numeric
+value which selects the corresponding interrupt description structure in
+the descriptor structures array. The descriptor structure contains
+status information and pointers to the interrupt flow method and the
+interrupt chip structure which are assigned to this interrupt.
+
+Whenever an interrupt triggers, the low-level architecture code calls
+into the generic interrupt code by calling :c:func:`desc->handle_irq`. This
+high-level IRQ handling function only uses desc->irq_data.chip
+primitives referenced by the assigned chip descriptor structure.
+
+High-level Driver API
+---------------------
+
+The high-level Driver API consists of following functions:
+
+-  :c:func:`request_irq`
+
+-  :c:func:`free_irq`
+
+-  :c:func:`disable_irq`
+
+-  :c:func:`enable_irq`
+
+-  :c:func:`disable_irq_nosync` (SMP only)
+
+-  :c:func:`synchronize_irq` (SMP only)
+
+-  :c:func:`irq_set_irq_type`
+
+-  :c:func:`irq_set_irq_wake`
+
+-  :c:func:`irq_set_handler_data`
+
+-  :c:func:`irq_set_chip`
+
+-  :c:func:`irq_set_chip_data`
+
+See the autogenerated function documentation for details.
+
+High-level IRQ flow handlers
+----------------------------
+
+The generic layer provides a set of pre-defined irq-flow methods:
+
+-  :c:func:`handle_level_irq`
+
+-  :c:func:`handle_edge_irq`
+
+-  :c:func:`handle_fasteoi_irq`
+
+-  :c:func:`handle_simple_irq`
+
+-  :c:func:`handle_percpu_irq`
+
+-  :c:func:`handle_edge_eoi_irq`
+
+-  :c:func:`handle_bad_irq`
+
+The interrupt flow handlers (either pre-defined or architecture
+specific) are assigned to specific interrupts by the architecture either
+during bootup or during device initialization.
+
+Default flow implementations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Helper functions
+^^^^^^^^^^^^^^^^
+
+The helper functions call the chip primitives and are used by the
+default flow implementations. The following helper functions are
+implemented (simplified excerpt)::
+
+    default_enable(struct irq_data *data)
+    {
+        desc->irq_data.chip->irq_unmask(data);
+    }
+
+    default_disable(struct irq_data *data)
+    {
+        if (!delay_disable(data))
+            desc->irq_data.chip->irq_mask(data);
+    }
+
+    default_ack(struct irq_data *data)
+    {
+        chip->irq_ack(data);
+    }
+
+    default_mask_ack(struct irq_data *data)
+    {
+        if (chip->irq_mask_ack) {
+            chip->irq_mask_ack(data);
+        } else {
+            chip->irq_mask(data);
+            chip->irq_ack(data);
+        }
+    }
+
+    noop(struct irq_data *data))
+    {
+    }
+
+
+
+Default flow handler implementations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Default Level IRQ flow handler
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+handle_level_irq provides a generic implementation for level-triggered
+interrupts.
+
+The following control flow is implemented (simplified excerpt)::
+
+    desc->irq_data.chip->irq_mask_ack();
+    handle_irq_event(desc->action);
+    desc->irq_data.chip->irq_unmask();
+
+
+Default Fast EOI IRQ flow handler
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+handle_fasteoi_irq provides a generic implementation for interrupts,
+which only need an EOI at the end of the handler.
+
+The following control flow is implemented (simplified excerpt)::
+
+    handle_irq_event(desc->action);
+    desc->irq_data.chip->irq_eoi();
+
+
+Default Edge IRQ flow handler
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+handle_edge_irq provides a generic implementation for edge-triggered
+interrupts.
+
+The following control flow is implemented (simplified excerpt)::
+
+    if (desc->status & running) {
+        desc->irq_data.chip->irq_mask_ack();
+        desc->status |= pending | masked;
+        return;
+    }
+    desc->irq_data.chip->irq_ack();
+    desc->status |= running;
+    do {
+        if (desc->status & masked)
+            desc->irq_data.chip->irq_unmask();
+        desc->status &= ~pending;
+        handle_irq_event(desc->action);
+    } while (status & pending);
+    desc->status &= ~running;
+
+
+Default simple IRQ flow handler
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+handle_simple_irq provides a generic implementation for simple
+interrupts.
+
+.. note::
+
+   The simple flow handler does not call any handler/chip primitives.
+
+The following control flow is implemented (simplified excerpt)::
+
+    handle_irq_event(desc->action);
+
+
+Default per CPU flow handler
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+handle_percpu_irq provides a generic implementation for per CPU
+interrupts.
+
+Per CPU interrupts are only available on SMP and the handler provides a
+simplified version without locking.
+
+The following control flow is implemented (simplified excerpt)::
+
+    if (desc->irq_data.chip->irq_ack)
+        desc->irq_data.chip->irq_ack();
+    handle_irq_event(desc->action);
+    if (desc->irq_data.chip->irq_eoi)
+        desc->irq_data.chip->irq_eoi();
+
+
+EOI Edge IRQ flow handler
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+handle_edge_eoi_irq provides an abnomination of the edge handler
+which is solely used to tame a badly wreckaged irq controller on
+powerpc/cell.
+
+Bad IRQ flow handler
+^^^^^^^^^^^^^^^^^^^^
+
+handle_bad_irq is used for spurious interrupts which have no real
+handler assigned..
+
+Quirks and optimizations
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The generic functions are intended for 'clean' architectures and chips,
+which have no platform-specific IRQ handling quirks. If an architecture
+needs to implement quirks on the 'flow' level then it can do so by
+overriding the high-level irq-flow handler.
+
+Delayed interrupt disable
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This per interrupt selectable feature, which was introduced by Russell
+King in the ARM interrupt implementation, does not mask an interrupt at
+the hardware level when :c:func:`disable_irq` is called. The interrupt is kept
+enabled and is masked in the flow handler when an interrupt event
+happens. This prevents losing edge interrupts on hardware which does not
+store an edge interrupt event while the interrupt is disabled at the
+hardware level. When an interrupt arrives while the IRQ_DISABLED flag
+is set, then the interrupt is masked at the hardware level and the
+IRQ_PENDING bit is set. When the interrupt is re-enabled by
+:c:func:`enable_irq` the pending bit is checked and if it is set, the interrupt
+is resent either via hardware or by a software resend mechanism. (It's
+necessary to enable CONFIG_HARDIRQS_SW_RESEND when you want to use
+the delayed interrupt disable feature and your hardware is not capable
+of retriggering an interrupt.) The delayed interrupt disable is not
+configurable.
+
+Chip-level hardware encapsulation
+---------------------------------
+
+The chip-level hardware descriptor structure :c:type:`irq_chip` contains all
+the direct chip relevant functions, which can be utilized by the irq flow
+implementations.
+
+-  ``irq_ack``
+
+-  ``irq_mask_ack`` - Optional, recommended for performance
+
+-  ``irq_mask``
+
+-  ``irq_unmask``
+
+-  ``irq_eoi`` - Optional, required for EOI flow handlers
+
+-  ``irq_retrigger`` - Optional
+
+-  ``irq_set_type`` - Optional
+
+-  ``irq_set_wake`` - Optional
+
+These primitives are strictly intended to mean what they say: ack means
+ACK, masking means masking of an IRQ line, etc. It is up to the flow
+handler(s) to use these basic units of low-level functionality.
+
+__do_IRQ entry point
+====================
+
+The original implementation :c:func:`__do_IRQ` was an alternative entry point
+for all types of interrupts. It no longer exists.
+
+This handler turned out to be not suitable for all interrupt hardware
+and was therefore reimplemented with split functionality for
+edge/level/simple/percpu interrupts. This is not only a functional
+optimization. It also shortens code paths for interrupts.
+
+Locking on SMP
+==============
+
+The locking of chip registers is up to the architecture that defines the
+chip primitives. The per-irq structure is protected via desc->lock, by
+the generic layer.
+
+Generic interrupt chip
+======================
+
+To avoid copies of identical implementations of IRQ chips the core
+provides a configurable generic interrupt chip implementation.
+Developers should check carefully whether the generic chip fits their
+needs before implementing the same functionality slightly differently
+themselves.
+
+.. kernel-doc:: kernel/irq/generic-chip.c
+   :export:
+
+Structures
+==========
+
+This chapter contains the autogenerated documentation of the structures
+which are used in the generic IRQ layer.
+
+.. kernel-doc:: include/linux/irq.h
+   :internal:
+
+.. kernel-doc:: include/linux/interrupt.h
+   :internal:
+
+Public Functions Provided
+=========================
+
+This chapter contains the autogenerated documentation of the kernel API
+functions which are exported.
+
+.. kernel-doc:: kernel/irq/manage.c
+
+.. kernel-doc:: kernel/irq/chip.c
+
+Internal Functions Provided
+===========================
+
+This chapter contains the autogenerated documentation of the internal
+functions.
+
+.. kernel-doc:: kernel/irq/irqdesc.c
+
+.. kernel-doc:: kernel/irq/handle.c
+
+.. kernel-doc:: kernel/irq/chip.c
+
+Credits
+=======
+
+The following people have contributed to this document:
+
+1. Thomas Gleixner tglx@linutronix.de
+
+2. Ingo Molnar mingo@elte.hu