[PATCH] genirq: doc: add design documentation

	    kernel-hacking.xml kernel-locking.xml deviceiobook.xml \

	    procfs-guide.xml writing_usb_driver.xml \

	    kernel-api.xml journal-api.xml lsm.xml usb.xml \

	    gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml

	    gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \

	    genericirq.xml

###

# The build process is as follows (targets):

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"

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

<book id="Generic-IRQ-Guide">

 <bookinfo>

  <title>Linux generic IRQ handling</title>

  <authorgroup>

   <author>

    <firstname>Thomas</firstname>

    <surname>Gleixner</surname>

    <affiliation>

     <address>

      <email>tglx@linutronix.de</email>

     </address>

    </affiliation>

   </author>

   <author>

    <firstname>Ingo</firstname>

    <surname>Molnar</surname>

    <affiliation>

     <address>

      <email>mingo@elte.hu</email>

     </address>

    </affiliation>

   </author>

  </authorgroup>

  <copyright>

   <year>2005-2006</year>

   <holder>Thomas Gleixner</holder>

  </copyright>

  <copyright>

   <year>2005-2006</year>

   <holder>Ingo Molnar</holder>

  </copyright>

  <legalnotice>

   <para>

     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.

   </para>

   <para>

     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.

   </para>

   <para>

     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

   </para>

   <para>

     For more details see the file COPYING in the source

     distribution of Linux.

   </para>

  </legalnotice>

 </bookinfo>

<toc></toc>

  <chapter id="intro">

    <title>Introduction</title>

    <para>

	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.

    </para>

    <para>

  	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.

    </para>

  </chapter>

  <chapter id="rationale">

    <title>Rationale</title>

	<para>

	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.

	</para>

	<para>

	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:

	<itemizedlist>

	  <listitem><para>Level type</para></listitem>

	  <listitem><para>Edge type</para></listitem>

	  <listitem><para>Simple type</para></listitem>

	</itemizedlist>

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

	was identified:

	<itemizedlist>

	  <listitem><para>Per CPU type</para></listitem>

	</itemizedlist>

	</para>

	<para>

	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.

	</para>

	<para>

	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.

	</para>

	<para>

	A more natural abstraction is the clean separation of the

	'irq flow' and the 'chip details'.

	</para>

	<para>

	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.

	</para>

	<para>

	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.)

	</para>

	<para>

	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.

	</para>

	<para>

	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.

	</para>

  </chapter>

  <chapter id="bugs">

    <title>Known Bugs And Assumptions</title>

    <para>

	None (knock on wood).

    </para>

  </chapter>

  <chapter id="Abstraction">

    <title>Abstraction layers</title>

    <para>

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

	<orderedlist>

	  <listitem><para>Highlevel driver API</para></listitem>

	  <listitem><para>Highlevel IRQ flow handlers</para></listitem>

	  <listitem><para>Chiplevel hardware encapsulation</para></listitem>

	</orderedlist>

    </para>

    <sect1>

	<title>Interrupt control flow</title>

	<para>

	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.

	</para>

	<para>

	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.

	</para>

    </sect1>

    <sect1>

	<title>Highlevel Driver API</title>

	<para>

	  The highlevel Driver API consists of following functions:

	  <itemizedlist>

	  <listitem><para>request_irq()</para></listitem>

	  <listitem><para>free_irq()</para></listitem>

	  <listitem><para>disable_irq()</para></listitem>

	  <listitem><para>enable_irq()</para></listitem>

	  <listitem><para>disable_irq_nosync() (SMP only)</para></listitem>

	  <listitem><para>synchronize_irq() (SMP only)</para></listitem>

	  <listitem><para>set_irq_type()</para></listitem>

	  <listitem><para>set_irq_wake()</para></listitem>

	  <listitem><para>set_irq_data()</para></listitem>

	  <listitem><para>set_irq_chip()</para></listitem>

	  <listitem><para>set_irq_chip_data()</para></listitem>

          </itemizedlist>

	  See the autogenerated function documentation for details.

	</para>

    </sect1>

    <sect1>

	<title>Highlevel IRQ flow handlers</title>

	<para>

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

	  <itemizedlist>

	  <listitem><para>handle_level_irq</para></listitem>

	  <listitem><para>handle_edge_irq</para></listitem>

	  <listitem><para>handle_simple_irq</para></listitem>

	  <listitem><para>handle_percpu_irq</para></listitem>

	  </itemizedlist>

	  The interrupt flow handlers (either predefined or architecture

	  specific) are assigned to specific interrupts by the architecture

	  either during bootup or during device initialization.

	</para>

	<sect2>

	<title>Default flow implementations</title>

	    <sect3>

	 	<title>Helper functions</title>

		<para>

		The helper functions call the chip primitives and

		are used by the default flow implementations.

		The following helper functions are implemented (simplified excerpt):

		<programlisting>

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)

{

}

		</programlisting>

	        </para>

	    </sect3>

	</sect2>

	<sect2>

	<title>Default flow handler implementations</title>

	    <sect3>

	 	<title>Default Level IRQ flow handler</title>

		<para>

		handle_level_irq provides a generic implementation

		for level-triggered interrupts.

		</para>

		<para>

		The following control flow is implemented (simplified excerpt):

		<programlisting>

desc->chip->start();

handle_IRQ_event(desc->action);

desc->chip->end();

		</programlisting>

		</para>

   	    </sect3>

	    <sect3>

	 	<title>Default Edge IRQ flow handler</title>

		<para>

		handle_edge_irq provides a generic implementation

		for edge-triggered interrupts.

		</para>

		<para>

		The following control flow is implemented (simplified excerpt):

		<programlisting>

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();

		</programlisting>

		</para>

   	    </sect3>

	    <sect3>

	 	<title>Default simple IRQ flow handler</title>

		<para>

		handle_simple_irq provides a generic implementation

		for simple interrupts.

		</para>

		<para>

		Note: The simple flow handler does not call any

		handler/chip primitives.

		</para>

		<para>

		The following control flow is implemented (simplified excerpt):

		<programlisting>

handle_IRQ_event(desc->action);

		</programlisting>

		</para>

   	    </sect3>

	    <sect3>

	 	<title>Default per CPU flow handler</title>

		<para>

		handle_percpu_irq provides a generic implementation

		for per CPU interrupts.

		</para>

		<para>

		Per CPU interrupts are only available on SMP and

		the handler provides a simplified version without

		locking.

		</para>

		<para>

		The following control flow is implemented (simplified excerpt):

		<programlisting>

desc->chip->start();

handle_IRQ_event(desc->action);

desc->chip->end();

		</programlisting>

		</para>

   	    </sect3>

	</sect2>

	<sect2>

	<title>Quirks and optimizations</title>

	<para>

	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.

	</para>

	</sect2>

	<sect2>

	<title>Delayed interrupt disable</title>

	<para>

	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.

	</para>

	</sect2>

    </sect1>

    <sect1>

	<title>Chiplevel hardware encapsulation</title>

	<para>

	The chip level hardware descriptor structure irq_chip

	contains all the direct chip relevant functions, which

	can be utilized by the irq flow implementations.

	  <itemizedlist>

	  <listitem><para>ack()</para></listitem>

	  <listitem><para>mask_ack() - Optional, recommended for performance</para></listitem>

	  <listitem><para>mask()</para></listitem>

	  <listitem><para>unmask()</para></listitem>

	  <listitem><para>retrigger() - Optional</para></listitem>

	  <listitem><para>set_type() - Optional</para></listitem>

	  <listitem><para>set_wake() - Optional</para></listitem>

	  </itemizedlist>

	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.

	</para>

    </sect1>

  </chapter>

  <chapter id="doirq">

     <title>__do_IRQ entry point</title>

     <para>

 	The original implementation __do_IRQ() is an alternative entry

	point for all types of interrupts.

     </para>

     <para>

	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.

      </para>

      <para>

	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.

     </para>

  </chapter>

  <chapter id="locking">

     <title>Locking on SMP</title>

     <para>

	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.

     </para>

  </chapter>

  <chapter id="structs">

     <title>Structures</title>

     <para>

     This chapter contains the autogenerated documentation of the structures which are

     used in the generic IRQ layer.

     </para>

!Iinclude/linux/irq.h

  </chapter>

  <chapter id="pubfunctions">

     <title>Public Functions Provided</title>

     <para>

     This chapter contains the autogenerated documentation of the kernel API functions

      which are exported.

     </para>

!Ekernel/irq/manage.c

!Ekernel/irq/chip.c

  </chapter>

  <chapter id="intfunctions">

     <title>Internal Functions Provided</title>

     <para>

     This chapter contains the autogenerated documentation of the internal functions.

     </para>

!Ikernel/irq/handle.c

!Ikernel/irq/chip.c

  </chapter>

  <chapter id="credits">

     <title>Credits</title>

	<para>

		The following people have contributed to this document:

		<orderedlist>

			<listitem><para>Thomas Gleixner<email>tglx@linutronix.de</email></para></listitem>

			<listitem><para>Ingo Molnar<email>mingo@elte.hu</email></para></listitem>

		</orderedlist>

	</para>

  </chapter>

</book>