Documentation/DocBook/genericirq.tmpl

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   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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   4
   5 <book id="Generic-IRQ-Guide">
   6  <bookinfo>
   7   <title>Linux generic IRQ handling</title>
   8
   9   <authorgroup>
  10    <author>
  11     <firstname>Thomas</firstname>
  12     <surname>Gleixner</surname>
  13     <affiliation>
  14      <address>
  15       <email>tglx@linutronix.de</email>
  16      </address>
  17     </affiliation>
  18    </author>
  19    <author>
  20     <firstname>Ingo</firstname>
  21     <surname>Molnar</surname>
  22     <affiliation>
  23      <address>
  24       <email>mingo@elte.hu</email>
  25      </address>
  26     </affiliation>
  27    </author>
  28   </authorgroup>
  29
  30   <copyright>
  31    <year>2005-2010</year>
  32    <holder>Thomas Gleixner</holder>
  33   </copyright>
  34   <copyright>
  35    <year>2005-2006</year>
  36    <holder>Ingo Molnar</holder>
  37   </copyright>
  38
  39   <legalnotice>
  40    <para>
  41      This documentation is free software; you can redistribute
  42      it and/or modify it under the terms of the GNU General Public
  43      License version 2 as published by the Free Software Foundation.
  44    </para>
  45
  46    <para>
  47      This program is distributed in the hope that it will be
  48      useful, but WITHOUT ANY WARRANTY; without even the implied
  49      warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  50      See the GNU General Public License for more details.
  51    </para>
  52
  53    <para>
  54      You should have received a copy of the GNU General Public
  55      License along with this program; if not, write to the Free
  56      Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  57      MA 02111-1307 USA
  58    </para>
  59
  60    <para>
  61      For more details see the file COPYING in the source
  62      distribution of Linux.
  63    </para>
  64   </legalnotice>
  65  </bookinfo>
  66
  67 <toc></toc>
  68
  69   <chapter id="intro">
  70     <title>Introduction</title>
  71     <para>
  72         The generic interrupt handling layer is designed to provide a
  73         complete abstraction of interrupt handling for device drivers.
  74         It is able to handle all the different types of interrupt controller
  75         hardware. Device drivers use generic API functions to request, enable,
  76         disable and free interrupts. The drivers do not have to know anything
  77         about interrupt hardware details, so they can be used on different
  78         platforms without code changes.
  79     </para>
  80     <para>
  81         This documentation is provided to developers who want to implement
  82         an interrupt subsystem based for their architecture, with the help
  83         of the generic IRQ handling layer.
  84     </para>
  85   </chapter>
  86
  87   <chapter id="rationale">
  88     <title>Rationale</title>
  89         <para>
  90         The original implementation of interrupt handling in Linux uses
  91         the __do_IRQ() super-handler, which is able to deal with every
  92         type of interrupt logic.
  93         </para>
  94         <para>
  95         Originally, Russell King identified different types of handlers to
  96         build a quite universal set for the ARM interrupt handler
  97         implementation in Linux 2.5/2.6. He distinguished between:
  98         <itemizedlist>
  99           <listitem><para>Level type</para></listitem>
 100           <listitem><para>Edge type</para></listitem>
 101           <listitem><para>Simple type</para></listitem>
 102         </itemizedlist>
 103         During the implementation we identified another type:
 104         <itemizedlist>
 105           <listitem><para>Fast EOI type</para></listitem>
 106         </itemizedlist>
 107         In the SMP world of the __do_IRQ() super-handler another type
 108         was identified:
 109         <itemizedlist>
 110           <listitem><para>Per CPU type</para></listitem>
 111         </itemizedlist>
 112         </para>
 113         <para>
 114         This split implementation of high-level IRQ handlers allows us to
 115         optimize the flow of the interrupt handling for each specific
 116         interrupt type. This reduces complexity in that particular code path
 117         and allows the optimized handling of a given type.
 118         </para>
 119         <para>
 120         The original general IRQ implementation used hw_interrupt_type
 121         structures and their ->ack(), ->end() [etc.] callbacks to
 122         differentiate the flow control in the super-handler. This leads to
 123         a mix of flow logic and low-level hardware logic, and it also leads
 124         to unnecessary code duplication: for example in i386, there is an
 125         ioapic_level_irq and an ioapic_edge_irq IRQ-type which share many
 126         of the low-level details but have different flow handling.
 127         </para>
 128         <para>
 129         A more natural abstraction is the clean separation of the
 130         'irq flow' and the 'chip details'.
 131         </para>
 132         <para>
 133         Analysing a couple of architecture's IRQ subsystem implementations
 134         reveals that most of them can use a generic set of 'irq flow'
 135         methods and only need to add the chip-level specific code.
 136         The separation is also valuable for (sub)architectures
 137         which need specific quirks in the IRQ flow itself but not in the
 138         chip details - and thus provides a more transparent IRQ subsystem
 139         design.
 140         </para>
 141         <para>
 142         Each interrupt descriptor is assigned its own high-level flow
 143         handler, which is normally one of the generic
 144         implementations. (This high-level flow handler implementation also
 145         makes it simple to provide demultiplexing handlers which can be
 146         found in embedded platforms on various architectures.)
 147         </para>
 148         <para>
 149         The separation makes the generic interrupt handling layer more
 150         flexible and extensible. For example, an (sub)architecture can
 151         use a generic IRQ-flow implementation for 'level type' interrupts
 152         and add a (sub)architecture specific 'edge type' implementation.
 153         </para>
 154         <para>
 155         To make the transition to the new model easier and prevent the
 156         breakage of existing implementations, the __do_IRQ() super-handler
 157         is still available. This leads to a kind of duality for the time
 158         being. Over time the new model should be used in more and more
 159         architectures, as it enables smaller and cleaner IRQ subsystems.
 160         It's deprecated for three years now and about to be removed.
 161         </para>
 162   </chapter>
 163   <chapter id="bugs">
 164     <title>Known Bugs And Assumptions</title>
 165     <para>
 166         None (knock on wood).
 167     </para>
 168   </chapter>
 169
 170   <chapter id="Abstraction">
 171     <title>Abstraction layers</title>
 172     <para>
 173         There are three main levels of abstraction in the interrupt code:
 174         <orderedlist>
 175           <listitem><para>High-level driver API</para></listitem>
 176           <listitem><para>High-level IRQ flow handlers</para></listitem>
 177           <listitem><para>Chip-level hardware encapsulation</para></listitem>
 178         </orderedlist>
 179     </para>
 180     <sect1 id="Interrupt_control_flow">
 181         <title>Interrupt control flow</title>
 182         <para>
 183         Each interrupt is described by an interrupt descriptor structure
 184         irq_desc. The interrupt is referenced by an 'unsigned int' numeric
 185         value which selects the corresponding interrupt description structure
 186         in the descriptor structures array.
 187         The descriptor structure contains status information and pointers
 188         to the interrupt flow method and the interrupt chip structure
 189         which are assigned to this interrupt.
 190         </para>
 191         <para>
 192         Whenever an interrupt triggers, the low-level architecture code calls
 193         into the generic interrupt code by calling desc->handle_irq().
 194         This high-level IRQ handling function only uses desc->irq_data.chip
 195         primitives referenced by the assigned chip descriptor structure.
 196         </para>
 197     </sect1>
 198     <sect1 id="Highlevel_Driver_API">
 199         <title>High-level Driver API</title>
 200         <para>
 201           The high-level Driver API consists of following functions:
 202           <itemizedlist>
 203           <listitem><para>request_irq()</para></listitem>
 204           <listitem><para>free_irq()</para></listitem>
 205           <listitem><para>disable_irq()</para></listitem>
 206           <listitem><para>enable_irq()</para></listitem>
 207           <listitem><para>disable_irq_nosync() (SMP only)</para></listitem>
 208           <listitem><para>synchronize_irq() (SMP only)</para></listitem>
 209           <listitem><para>irq_set_irq_type()</para></listitem>
 210           <listitem><para>irq_set_irq_wake()</para></listitem>
 211           <listitem><para>irq_set_handler_data()</para></listitem>
 212           <listitem><para>irq_set_chip()</para></listitem>
 213           <listitem><para>irq_set_chip_data()</para></listitem>
 214           </itemizedlist>
 215           See the autogenerated function documentation for details.
 216         </para>
 217     </sect1>
 218     <sect1 id="Highlevel_IRQ_flow_handlers">
 219         <title>High-level IRQ flow handlers</title>
 220         <para>
 221           The generic layer provides a set of pre-defined irq-flow methods:
 222           <itemizedlist>
 223           <listitem><para>handle_level_irq</para></listitem>
 224           <listitem><para>handle_edge_irq</para></listitem>
 225           <listitem><para>handle_fasteoi_irq</para></listitem>
 226           <listitem><para>handle_simple_irq</para></listitem>
 227           <listitem><para>handle_percpu_irq</para></listitem>
 228           <listitem><para>handle_edge_eoi_irq</para></listitem>
 229           <listitem><para>handle_bad_irq</para></listitem>
 230           </itemizedlist>
 231           The interrupt flow handlers (either pre-defined or architecture
 232           specific) are assigned to specific interrupts by the architecture
 233           either during bootup or during device initialization.
 234         </para>
 235         <sect2 id="Default_flow_implementations">
 236         <title>Default flow implementations</title>
 237             <sect3 id="Helper_functions">
 238                 <title>Helper functions</title>
 239                 <para>
 240                 The helper functions call the chip primitives and
 241                 are used by the default flow implementations.
 242                 The following helper functions are implemented (simplified excerpt):
 243                 <programlisting>
 244 default_enable(struct irq_data *data)
 245 {
 246         desc->irq_data.chip->irq_unmask(data);
 247 }
 248
 249 default_disable(struct irq_data *data)
 250 {
 251         if (!delay_disable(data))
 252                 desc->irq_data.chip->irq_mask(data);
 253 }
 254
 255 default_ack(struct irq_data *data)
 256 {
 257         chip->irq_ack(data);
 258 }
 259
 260 default_mask_ack(struct irq_data *data)
 261 {
 262         if (chip->irq_mask_ack) {
 263                 chip->irq_mask_ack(data);
 264         } else {
 265                 chip->irq_mask(data);
 266                 chip->irq_ack(data);
 267         }
 268 }
 269
 270 noop(struct irq_data *data))
 271 {
 272 }
 273
 274                 </programlisting>
 275                 </para>
 276             </sect3>
 277         </sect2>
 278         <sect2 id="Default_flow_handler_implementations">
 279         <title>Default flow handler implementations</title>
 280             <sect3 id="Default_Level_IRQ_flow_handler">
 281                 <title>Default Level IRQ flow handler</title>
 282                 <para>
 283                 handle_level_irq provides a generic implementation
 284                 for level-triggered interrupts.
 285                 </para>
 286                 <para>
 287                 The following control flow is implemented (simplified excerpt):
 288                 <programlisting>
 289 desc->irq_data.chip->irq_mask_ack();
 290 handle_irq_event(desc->action);
 291 desc->irq_data.chip->irq_unmask();
 292                 </programlisting>
 293                 </para>
 294             </sect3>
 295             <sect3 id="Default_FASTEOI_IRQ_flow_handler">
 296                 <title>Default Fast EOI IRQ flow handler</title>
 297                 <para>
 298                 handle_fasteoi_irq provides a generic implementation
 299                 for interrupts, which only need an EOI at the end of
 300                 the handler.
 301                 </para>
 302                 <para>
 303                 The following control flow is implemented (simplified excerpt):
 304                 <programlisting>
 305 handle_irq_event(desc->action);
 306 desc->irq_data.chip->irq_eoi();
 307                 </programlisting>
 308                 </para>
 309             </sect3>
 310             <sect3 id="Default_Edge_IRQ_flow_handler">
 311                 <title>Default Edge IRQ flow handler</title>
 312                 <para>
 313                 handle_edge_irq provides a generic implementation
 314                 for edge-triggered interrupts.
 315                 </para>
 316                 <para>
 317                 The following control flow is implemented (simplified excerpt):
 318                 <programlisting>
 319 if (desc->status &amp; running) {
 320         desc->irq_data.chip->irq_mask_ack();
 321         desc->status |= pending | masked;
 322         return;
 323 }
 324 desc->irq_data.chip->irq_ack();
 325 desc->status |= running;
 326 do {
 327         if (desc->status &amp; masked)
 328                 desc->irq_data.chip->irq_unmask();
 329         desc->status &amp;= ~pending;
 330         handle_irq_event(desc->action);
 331 } while (status &amp; pending);
 332 desc->status &amp;= ~running;
 333                 </programlisting>
 334                 </para>
 335             </sect3>
 336             <sect3 id="Default_simple_IRQ_flow_handler">
 337                 <title>Default simple IRQ flow handler</title>
 338                 <para>
 339                 handle_simple_irq provides a generic implementation
 340                 for simple interrupts.
 341                 </para>
 342                 <para>
 343                 Note: The simple flow handler does not call any
 344                 handler/chip primitives.
 345                 </para>
 346                 <para>
 347                 The following control flow is implemented (simplified excerpt):
 348                 <programlisting>
 349 handle_irq_event(desc->action);
 350                 </programlisting>
 351                 </para>
 352             </sect3>
 353             <sect3 id="Default_per_CPU_flow_handler">
 354                 <title>Default per CPU flow handler</title>
 355                 <para>
 356                 handle_percpu_irq provides a generic implementation
 357                 for per CPU interrupts.
 358                 </para>
 359                 <para>
 360                 Per CPU interrupts are only available on SMP and
 361                 the handler provides a simplified version without
 362                 locking.
 363                 </para>
 364                 <para>
 365                 The following control flow is implemented (simplified excerpt):
 366                 <programlisting>
 367 if (desc->irq_data.chip->irq_ack)
 368         desc->irq_data.chip->irq_ack();
 369 handle_irq_event(desc->action);
 370 if (desc->irq_data.chip->irq_eoi)
 371         desc->irq_data.chip->irq_eoi();
 372                 </programlisting>
 373                 </para>
 374             </sect3>
 375             <sect3 id="EOI_Edge_IRQ_flow_handler">
 376                 <title>EOI Edge IRQ flow handler</title>
 377                 <para>
 378                 handle_edge_eoi_irq provides an abnomination of the edge
 379                 handler which is solely used to tame a badly wreckaged
 380                 irq controller on powerpc/cell.
 381                 </para>
 382             </sect3>
 383             <sect3 id="BAD_IRQ_flow_handler">
 384                 <title>Bad IRQ flow handler</title>
 385                 <para>
 386                 handle_bad_irq is used for spurious interrupts which
 387                 have no real handler assigned..
 388                 </para>
 389             </sect3>
 390         </sect2>
 391         <sect2 id="Quirks_and_optimizations">
 392         <title>Quirks and optimizations</title>
 393         <para>
 394         The generic functions are intended for 'clean' architectures and chips,
 395         which have no platform-specific IRQ handling quirks. If an architecture
 396         needs to implement quirks on the 'flow' level then it can do so by
 397         overriding the high-level irq-flow handler.
 398         </para>
 399         </sect2>
 400         <sect2 id="Delayed_interrupt_disable">
 401         <title>Delayed interrupt disable</title>
 402         <para>
 403         This per interrupt selectable feature, which was introduced by Russell
 404         King in the ARM interrupt implementation, does not mask an interrupt
 405         at the hardware level when disable_irq() is called. The interrupt is
 406         kept enabled and is masked in the flow handler when an interrupt event
 407         happens. This prevents losing edge interrupts on hardware which does
 408         not store an edge interrupt event while the interrupt is disabled at
 409         the hardware level. When an interrupt arrives while the IRQ_DISABLED
 410         flag is set, then the interrupt is masked at the hardware level and
 411         the IRQ_PENDING bit is set. When the interrupt is re-enabled by
 412         enable_irq() the pending bit is checked and if it is set, the
 413         interrupt is resent either via hardware or by a software resend
 414         mechanism. (It's necessary to enable CONFIG_HARDIRQS_SW_RESEND when
 415         you want to use the delayed interrupt disable feature and your
 416         hardware is not capable of retriggering an interrupt.)
 417         The delayed interrupt disable is not configurable.
 418         </para>
 419         </sect2>
 420     </sect1>
 421     <sect1 id="Chiplevel_hardware_encapsulation">
 422         <title>Chip-level hardware encapsulation</title>
 423         <para>
 424         The chip-level hardware descriptor structure irq_chip
 425         contains all the direct chip relevant functions, which
 426         can be utilized by the irq flow implementations.
 427           <itemizedlist>
 428           <listitem><para>irq_ack()</para></listitem>
 429           <listitem><para>irq_mask_ack() - Optional, recommended for performance</para></listitem>
 430           <listitem><para>irq_mask()</para></listitem>
 431           <listitem><para>irq_unmask()</para></listitem>
 432           <listitem><para>irq_eoi() - Optional, required for EOI flow handlers</para></listitem>
 433           <listitem><para>irq_retrigger() - Optional</para></listitem>
 434           <listitem><para>irq_set_type() - Optional</para></listitem>
 435           <listitem><para>irq_set_wake() - Optional</para></listitem>
 436           </itemizedlist>
 437         These primitives are strictly intended to mean what they say: ack means
 438         ACK, masking means masking of an IRQ line, etc. It is up to the flow
 439         handler(s) to use these basic units of low-level functionality.
 440         </para>
 441     </sect1>
 442   </chapter>
 443
 444   <chapter id="doirq">
 445      <title>__do_IRQ entry point</title>
 446      <para>
 447         The original implementation __do_IRQ() was an alternative entry
 448         point for all types of interrupts. It no longer exists.
 449      </para>
 450      <para>
 451         This handler turned out to be not suitable for all
 452         interrupt hardware and was therefore reimplemented with split
 453         functionality for edge/level/simple/percpu interrupts. This is not
 454         only a functional optimization. It also shortens code paths for
 455         interrupts.
 456       </para>
 457   </chapter>
 458
 459   <chapter id="locking">
 460      <title>Locking on SMP</title>
 461      <para>
 462         The locking of chip registers is up to the architecture that
 463         defines the chip primitives. The per-irq structure is
 464         protected via desc->lock, by the generic layer.
 465      </para>
 466   </chapter>
 467
 468   <chapter id="genericchip">
 469      <title>Generic interrupt chip</title>
 470      <para>
 471        To avoid copies of identical implementations of IRQ chips the
 472        core provides a configurable generic interrupt chip
 473        implementation. Developers should check carefully whether the
 474        generic chip fits their needs before implementing the same
 475        functionality slightly differently themselves.
 476      </para>
 477 !Ekernel/irq/generic-chip.c
 478   </chapter>
 479
 480   <chapter id="structs">
 481      <title>Structures</title>
 482      <para>
 483      This chapter contains the autogenerated documentation of the structures which are
 484      used in the generic IRQ layer.
 485      </para>
 486 !Iinclude/linux/irq.h
 487 !Iinclude/linux/interrupt.h
 488   </chapter>
 489
 490   <chapter id="pubfunctions">
 491      <title>Public Functions Provided</title>
 492      <para>
 493      This chapter contains the autogenerated documentation of the kernel API functions
 494       which are exported.
 495      </para>
 496 !Ekernel/irq/manage.c
 497 !Ekernel/irq/chip.c
 498   </chapter>
 499
 500   <chapter id="intfunctions">
 501      <title>Internal Functions Provided</title>
 502      <para>
 503      This chapter contains the autogenerated documentation of the internal functions.
 504      </para>
 505 !Ikernel/irq/irqdesc.c
 506 !Ikernel/irq/handle.c
 507 !Ikernel/irq/chip.c
 508   </chapter>
 509
 510   <chapter id="credits">
 511      <title>Credits</title>
 512         <para>
 513                 The following people have contributed to this document:
 514                 <orderedlist>
 515                         <listitem><para>Thomas Gleixner<email>tglx@linutronix.de</email></para></listitem>
 516                         <listitem><para>Ingo Molnar<email>mingo@elte.hu</email></para></listitem>
 517                 </orderedlist>
 518         </para>
 519   </chapter>
 520 </book>