bebd04cc45
This patch updates the IRQ docs to match reality. Signed-off-by: Krzysztof Halasa <khc@pm.waw.pl> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
475 lines
16 KiB
XML
475 lines
16 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="Generic-IRQ-Guide">
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<bookinfo>
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<title>Linux generic IRQ handling</title>
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<authorgroup>
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<author>
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<firstname>Thomas</firstname>
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<surname>Gleixner</surname>
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<affiliation>
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<address>
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<email>tglx@linutronix.de</email>
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</address>
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</affiliation>
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</author>
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<author>
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<firstname>Ingo</firstname>
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<surname>Molnar</surname>
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<affiliation>
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<address>
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<email>mingo@elte.hu</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2005-2006</year>
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<holder>Thomas Gleixner</holder>
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</copyright>
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<copyright>
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<year>2005-2006</year>
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<holder>Ingo Molnar</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License version 2 as published by the Free Software Foundation.
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</para>
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<para>
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This program is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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</legalnotice>
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</bookinfo>
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<toc></toc>
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<chapter id="intro">
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<title>Introduction</title>
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<para>
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The generic interrupt handling layer is designed to provide a
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complete abstraction of interrupt handling for device drivers.
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It is able to handle all the different types of interrupt controller
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hardware. Device drivers use generic API functions to request, enable,
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disable and free interrupts. The drivers do not have to know anything
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about interrupt hardware details, so they can be used on different
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platforms without code changes.
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</para>
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<para>
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This documentation is provided to developers who want to implement
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an interrupt subsystem based for their architecture, with the help
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of the generic IRQ handling layer.
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</para>
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</chapter>
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<chapter id="rationale">
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<title>Rationale</title>
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<para>
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The original implementation of interrupt handling in Linux is using
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the __do_IRQ() super-handler, which is able to deal with every
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type of interrupt logic.
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</para>
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<para>
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Originally, Russell King identified different types of handlers to
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build a quite universal set for the ARM interrupt handler
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implementation in Linux 2.5/2.6. He distinguished between:
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<itemizedlist>
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<listitem><para>Level type</para></listitem>
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<listitem><para>Edge type</para></listitem>
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<listitem><para>Simple type</para></listitem>
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</itemizedlist>
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In the SMP world of the __do_IRQ() super-handler another type
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was identified:
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<itemizedlist>
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<listitem><para>Per CPU type</para></listitem>
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</itemizedlist>
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</para>
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<para>
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This split implementation of highlevel IRQ handlers allows us to
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optimize the flow of the interrupt handling for each specific
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interrupt type. This reduces complexity in that particular codepath
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and allows the optimized handling of a given type.
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</para>
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<para>
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The original general IRQ implementation used hw_interrupt_type
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structures and their ->ack(), ->end() [etc.] callbacks to
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differentiate the flow control in the super-handler. This leads to
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a mix of flow logic and lowlevel hardware logic, and it also leads
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to unnecessary code duplication: for example in i386, there is a
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ioapic_level_irq and a ioapic_edge_irq irq-type which share many
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of the lowlevel details but have different flow handling.
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</para>
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<para>
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A more natural abstraction is the clean separation of the
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'irq flow' and the 'chip details'.
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</para>
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<para>
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Analysing a couple of architecture's IRQ subsystem implementations
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reveals that most of them can use a generic set of 'irq flow'
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methods and only need to add the chip level specific code.
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The separation is also valuable for (sub)architectures
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which need specific quirks in the irq flow itself but not in the
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chip-details - and thus provides a more transparent IRQ subsystem
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design.
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</para>
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<para>
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Each interrupt descriptor is assigned its own highlevel flow
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handler, which is normally one of the generic
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implementations. (This highlevel flow handler implementation also
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makes it simple to provide demultiplexing handlers which can be
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found in embedded platforms on various architectures.)
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</para>
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<para>
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The separation makes the generic interrupt handling layer more
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flexible and extensible. For example, an (sub)architecture can
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use a generic irq-flow implementation for 'level type' interrupts
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and add a (sub)architecture specific 'edge type' implementation.
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</para>
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<para>
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To make the transition to the new model easier and prevent the
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breakage of existing implementations, the __do_IRQ() super-handler
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is still available. This leads to a kind of duality for the time
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being. Over time the new model should be used in more and more
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architectures, as it enables smaller and cleaner IRQ subsystems.
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</para>
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</chapter>
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<chapter id="bugs">
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<title>Known Bugs And Assumptions</title>
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<para>
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None (knock on wood).
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</para>
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</chapter>
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<chapter id="Abstraction">
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<title>Abstraction layers</title>
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<para>
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There are three main levels of abstraction in the interrupt code:
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<orderedlist>
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<listitem><para>Highlevel driver API</para></listitem>
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<listitem><para>Highlevel IRQ flow handlers</para></listitem>
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<listitem><para>Chiplevel hardware encapsulation</para></listitem>
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</orderedlist>
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</para>
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<sect1 id="Interrupt_control_flow">
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<title>Interrupt control flow</title>
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<para>
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Each interrupt is described by an interrupt descriptor structure
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irq_desc. The interrupt is referenced by an 'unsigned int' numeric
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value which selects the corresponding interrupt decription structure
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in the descriptor structures array.
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The descriptor structure contains status information and pointers
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to the interrupt flow method and the interrupt chip structure
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which are assigned to this interrupt.
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</para>
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<para>
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Whenever an interrupt triggers, the lowlevel arch code calls into
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the generic interrupt code by calling desc->handle_irq().
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This highlevel IRQ handling function only uses desc->chip primitives
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referenced by the assigned chip descriptor structure.
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</para>
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</sect1>
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<sect1 id="Highlevel_Driver_API">
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<title>Highlevel Driver API</title>
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<para>
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The highlevel Driver API consists of following functions:
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<itemizedlist>
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<listitem><para>request_irq()</para></listitem>
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<listitem><para>free_irq()</para></listitem>
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<listitem><para>disable_irq()</para></listitem>
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<listitem><para>enable_irq()</para></listitem>
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<listitem><para>disable_irq_nosync() (SMP only)</para></listitem>
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<listitem><para>synchronize_irq() (SMP only)</para></listitem>
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<listitem><para>set_irq_type()</para></listitem>
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<listitem><para>set_irq_wake()</para></listitem>
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<listitem><para>set_irq_data()</para></listitem>
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<listitem><para>set_irq_chip()</para></listitem>
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<listitem><para>set_irq_chip_data()</para></listitem>
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</itemizedlist>
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See the autogenerated function documentation for details.
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</para>
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</sect1>
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<sect1 id="Highlevel_IRQ_flow_handlers">
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<title>Highlevel IRQ flow handlers</title>
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<para>
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The generic layer provides a set of pre-defined irq-flow methods:
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<itemizedlist>
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<listitem><para>handle_level_irq</para></listitem>
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<listitem><para>handle_edge_irq</para></listitem>
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<listitem><para>handle_simple_irq</para></listitem>
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<listitem><para>handle_percpu_irq</para></listitem>
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</itemizedlist>
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The interrupt flow handlers (either predefined or architecture
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specific) are assigned to specific interrupts by the architecture
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either during bootup or during device initialization.
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</para>
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<sect2 id="Default_flow_implementations">
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<title>Default flow implementations</title>
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<sect3 id="Helper_functions">
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<title>Helper functions</title>
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<para>
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The helper functions call the chip primitives and
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are used by the default flow implementations.
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The following helper functions are implemented (simplified excerpt):
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<programlisting>
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default_enable(irq)
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{
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desc->chip->unmask(irq);
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}
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default_disable(irq)
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{
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if (!delay_disable(irq))
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desc->chip->mask(irq);
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}
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default_ack(irq)
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{
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chip->ack(irq);
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}
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default_mask_ack(irq)
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{
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if (chip->mask_ack) {
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chip->mask_ack(irq);
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} else {
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chip->mask(irq);
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chip->ack(irq);
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}
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}
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noop(irq)
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{
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}
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</programlisting>
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</para>
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</sect3>
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</sect2>
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<sect2 id="Default_flow_handler_implementations">
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<title>Default flow handler implementations</title>
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<sect3 id="Default_Level_IRQ_flow_handler">
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<title>Default Level IRQ flow handler</title>
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<para>
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handle_level_irq provides a generic implementation
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for level-triggered interrupts.
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</para>
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<para>
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The following control flow is implemented (simplified excerpt):
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<programlisting>
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desc->chip->start();
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handle_IRQ_event(desc->action);
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desc->chip->end();
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</programlisting>
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</para>
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</sect3>
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<sect3 id="Default_Edge_IRQ_flow_handler">
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<title>Default Edge IRQ flow handler</title>
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<para>
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handle_edge_irq provides a generic implementation
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for edge-triggered interrupts.
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</para>
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<para>
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The following control flow is implemented (simplified excerpt):
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<programlisting>
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if (desc->status & running) {
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desc->chip->hold();
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desc->status |= pending | masked;
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return;
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}
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desc->chip->start();
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desc->status |= running;
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do {
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if (desc->status & masked)
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desc->chip->enable();
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desc->status &= ~pending;
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handle_IRQ_event(desc->action);
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} while (status & pending);
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desc->status &= ~running;
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desc->chip->end();
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</programlisting>
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</para>
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</sect3>
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<sect3 id="Default_simple_IRQ_flow_handler">
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<title>Default simple IRQ flow handler</title>
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<para>
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handle_simple_irq provides a generic implementation
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for simple interrupts.
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</para>
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<para>
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Note: The simple flow handler does not call any
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handler/chip primitives.
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</para>
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<para>
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The following control flow is implemented (simplified excerpt):
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<programlisting>
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handle_IRQ_event(desc->action);
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</programlisting>
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</para>
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</sect3>
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<sect3 id="Default_per_CPU_flow_handler">
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<title>Default per CPU flow handler</title>
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<para>
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handle_percpu_irq provides a generic implementation
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for per CPU interrupts.
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</para>
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<para>
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Per CPU interrupts are only available on SMP and
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the handler provides a simplified version without
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locking.
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</para>
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<para>
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The following control flow is implemented (simplified excerpt):
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<programlisting>
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desc->chip->start();
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handle_IRQ_event(desc->action);
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desc->chip->end();
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</programlisting>
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</para>
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</sect3>
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</sect2>
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<sect2 id="Quirks_and_optimizations">
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<title>Quirks and optimizations</title>
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<para>
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The generic functions are intended for 'clean' architectures and chips,
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which have no platform-specific IRQ handling quirks. If an architecture
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needs to implement quirks on the 'flow' level then it can do so by
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overriding the highlevel irq-flow handler.
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</para>
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</sect2>
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<sect2 id="Delayed_interrupt_disable">
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<title>Delayed interrupt disable</title>
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<para>
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This per interrupt selectable feature, which was introduced by Russell
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King in the ARM interrupt implementation, does not mask an interrupt
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at the hardware level when disable_irq() is called. The interrupt is
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kept enabled and is masked in the flow handler when an interrupt event
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happens. This prevents losing edge interrupts on hardware which does
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not store an edge interrupt event while the interrupt is disabled at
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the hardware level. When an interrupt arrives while the IRQ_DISABLED
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flag is set, then the interrupt is masked at the hardware level and
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the IRQ_PENDING bit is set. When the interrupt is re-enabled by
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enable_irq() the pending bit is checked and if it is set, the
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interrupt is resent either via hardware or by a software resend
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mechanism. (It's necessary to enable CONFIG_HARDIRQS_SW_RESEND when
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you want to use the delayed interrupt disable feature and your
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hardware is not capable of retriggering an interrupt.)
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The delayed interrupt disable can be runtime enabled, per interrupt,
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by setting the IRQ_DELAYED_DISABLE flag in the irq_desc status field.
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</para>
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</sect2>
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</sect1>
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<sect1 id="Chiplevel_hardware_encapsulation">
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<title>Chiplevel hardware encapsulation</title>
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<para>
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The chip level hardware descriptor structure irq_chip
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contains all the direct chip relevant functions, which
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can be utilized by the irq flow implementations.
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<itemizedlist>
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<listitem><para>ack()</para></listitem>
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<listitem><para>mask_ack() - Optional, recommended for performance</para></listitem>
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<listitem><para>mask()</para></listitem>
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<listitem><para>unmask()</para></listitem>
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<listitem><para>retrigger() - Optional</para></listitem>
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<listitem><para>set_type() - Optional</para></listitem>
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<listitem><para>set_wake() - Optional</para></listitem>
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</itemizedlist>
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These primitives are strictly intended to mean what they say: ack means
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ACK, masking means masking of an IRQ line, etc. It is up to the flow
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handler(s) to use these basic units of lowlevel functionality.
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</para>
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</sect1>
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</chapter>
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<chapter id="doirq">
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<title>__do_IRQ entry point</title>
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<para>
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The original implementation __do_IRQ() is an alternative entry
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point for all types of interrupts.
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</para>
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<para>
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This handler turned out to be not suitable for all
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interrupt hardware and was therefore reimplemented with split
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functionality for egde/level/simple/percpu interrupts. This is not
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only a functional optimization. It also shortens code paths for
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interrupts.
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</para>
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<para>
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To make use of the split implementation, replace the call to
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__do_IRQ by a call to desc->handle_irq() and associate
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the appropriate handler function to desc->handle_irq().
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In most cases the generic handler implementations should
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be sufficient.
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</para>
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</chapter>
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<chapter id="locking">
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<title>Locking on SMP</title>
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<para>
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The locking of chip registers is up to the architecture that
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defines the chip primitives. There is a chip->lock field that can be used
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for serialization, but the generic layer does not touch it. The per-irq
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structure is protected via desc->lock, by the generic layer.
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</para>
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</chapter>
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<chapter id="structs">
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<title>Structures</title>
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<para>
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This chapter contains the autogenerated documentation of the structures which are
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used in the generic IRQ layer.
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</para>
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!Iinclude/linux/irq.h
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!Iinclude/linux/interrupt.h
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</chapter>
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<chapter id="pubfunctions">
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<title>Public Functions Provided</title>
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<para>
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This chapter contains the autogenerated documentation of the kernel API functions
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which are exported.
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</para>
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!Ekernel/irq/manage.c
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!Ekernel/irq/chip.c
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</chapter>
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<chapter id="intfunctions">
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<title>Internal Functions Provided</title>
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<para>
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This chapter contains the autogenerated documentation of the internal functions.
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</para>
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!Ikernel/irq/handle.c
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!Ikernel/irq/chip.c
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</chapter>
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<chapter id="credits">
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<title>Credits</title>
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<para>
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The following people have contributed to this document:
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<orderedlist>
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<listitem><para>Thomas Gleixner<email>tglx@linutronix.de</email></para></listitem>
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<listitem><para>Ingo Molnar<email>mingo@elte.hu</email></para></listitem>
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</orderedlist>
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</para>
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</chapter>
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</book>
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