[PATCH] genirq: doc: add design documentation

Add docbook file - includes API documentation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 474 insertions, 0 deletions

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+<?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 &amp; running) {
+	desc->chip->hold();
+	desc->status |= pending | masked;
+	return;
+}
+desc->chip->start();
+desc->status |= running;
+do {
+	if (desc->status &amp; masked)
+		desc->chip->enable();
+	desc-status &amp;= ~pending;
+	handle_IRQ_event(desc->action);
+} while (status &amp; pending);
+desc-status &amp;= ~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>