Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

        "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>

    Thomas

    Gleixner

      tglx@linutronix.de

    Ingo

    Molnar

      mingo@elte.hu

   2005-2006

   Thomas Gleixner

   2005-2006

   Ingo Molnar

     This documentation is free software; you can redistribute

     it and/or modify it under the terms of the GNU General Public

     License version 2 as published by the Free Software Foundation.

     This program is distributed in the hope that it will be

     useful, but WITHOUT ANY WARRANTY; without even the implied

     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

     See the GNU General Public License for more details.

     You should have received a copy of the GNU General Public

     License along with this program; if not, write to the Free

     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,

     MA 02111-1307 USA

     For more details see the file COPYING in the source

     distribution of Linux.

        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.

        The original implementation of interrupt handling in Linux is using

        the __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

        In the SMP world of the __do_IRQ() super-handler another type

        was identified:

          Per CPU type

        This split implementation of highlevel IRQ handlers allows us to

        optimize the flow of the interrupt handling for each specific

        interrupt type. This reduces complexity in that particular codepath

        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 lowlevel hardware logic, and it also leads

        to unnecessary code duplication: for example in i386, there is a

        ioapic_level_irq and a ioapic_edge_irq irq-type which share many

        of the lowlevel 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 highlevel flow

        handler, which is normally one of the generic

        implementations. (This highlevel 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 __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.

        None (knock on wood).

        There are three main levels of abstraction in the interrupt code:

          Highlevel driver API

          Highlevel IRQ flow handlers

          Chiplevel hardware encapsulation

        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 decription 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 lowlevel arch code calls into

        the generic interrupt code by calling desc->handle_irq().

        This highlevel IRQ handling function only uses desc->chip primitives

        referenced by the assigned chip descriptor structure.

          The highlevel Driver API consists of following functions:

          request_irq()

          free_irq()

          disable_irq()

          enable_irq()

          disable_irq_nosync() (SMP only)

          synchronize_irq() (SMP only)

          set_irq_type()

          set_irq_wake()

          set_irq_data()

          set_irq_chip()

          set_irq_chip_data()

          See the autogenerated function documentation for details.

          The generic layer provides a set of pre-defined irq-flow methods:

          handle_level_irq

          handle_edge_irq

          handle_simple_irq

          handle_percpu_irq

          The interrupt flow handlers (either predefined or architecture

          specific) are assigned to specific interrupts by the architecture

          either during bootup or during device initialization.

                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(irq)

{

        desc->chip->unmask(irq);

}

default_disable(irq)

{

        if (!delay_disable(irq))

                desc->chip->mask(irq);

}

default_ack(irq)

{

        chip->ack(irq);

}

default_mask_ack(irq)

{

        if (chip->mask_ack) {

                chip->mask_ack(irq);

        } else {

                chip->mask(irq);

                chip->ack(irq);

        }

}

noop(irq)

{

}

                handle_level_irq provides a generic implementation

                for level-triggered interrupts.

                The following control flow is implemented (simplified excerpt):

desc->chip->start();

handle_IRQ_event(desc->action);

desc->chip->end();

                handle_edge_irq provides a generic implementation

                for edge-triggered interrupts.

                The following control flow is implemented (simplified excerpt):

if (desc->status & running) {

        desc->chip->hold();

        desc->status |= pending | masked;

        return;

}

desc->chip->start();

desc->status |= running;

do {

        if (desc->status & masked)

                desc->chip->enable();

        desc->status &= ~pending;

        handle_IRQ_event(desc->action);

} while (status & pending);

desc->status &= ~running;

desc->chip->end();

                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);

                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):

desc->chip->start();

handle_IRQ_event(desc->action);

desc->chip->end();

        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 highlevel irq-flow handler.

        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 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

        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 can be runtime enabled, per interrupt,

        by setting the IRQ_DELAYED_DISABLE flag in the irq_desc status field.

        The chip level hardware descriptor structure irq_chip

        contains all the direct chip relevant functions, which

        can be utilized by the irq flow implementations.

          ack()

          mask_ack() - Optional, recommended for performance

          mask()

          unmask()

          retrigger() - Optional

          set_type() - Optional

          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 lowlevel functionality.

        The original implementation __do_IRQ() is an alternative entry

        point for all types of interrupts.

        This handler turned out to be not suitable for all

        interrupt hardware and was therefore reimplemented with split

        functionality for egde/level/simple/percpu interrupts. This is not

        only a functional optimization. It also shortens code paths for

        interrupts.

        To make use of the split implementation, replace the call to

        __do_IRQ by a call to desc->chip->handle_irq() and associate

        the appropriate handler function to desc->chip->handle_irq().

        In most cases the generic handler implementations should

        be sufficient.

        The locking of chip registers is up to the architecture that

        defines the chip primitives. There is a chip->lock field that can be used

        for serialization, but the generic layer does not touch it. The per-irq

        structure is protected via desc->lock, by the generic layer.

     This chapter contains the autogenerated documentation of the structures which are

     used in the generic IRQ layer.

!Iinclude/linux/irq.h

     This chapter contains the autogenerated documentation of the kernel API functions

      which are exported.

!Ekernel/irq/manage.c

!Ekernel/irq/chip.c

     This chapter contains the autogenerated documentation of the internal functions.

!Ikernel/irq/handle.c

!Ikernel/irq/chip.c

                The following people have contributed to this document:

                        Thomas Gleixnertglx@linutronix.de

                        Ingo Molnarmingo@elte.hu