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>2005-2006</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 is using
  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         In the SMP world of the __do_IRQ() super-handler another type
 104         was identified:
 105         <itemizedlist>
 106           <listitem><para>Per CPU type</para></listitem>
 107         </itemizedlist>
 108         </para>
 109         <para>
 110         This split implementation of highlevel IRQ handlers allows us to
 111         optimize the flow of the interrupt handling for each specific
 112         interrupt type. This reduces complexity in that particular codepath
 113         and allows the optimized handling of a given type.
 114         </para>
 115         <para>
 116         The original general IRQ implementation used hw_interrupt_type
 117         structures and their ->ack(), ->end() [etc.] callbacks to
 118         differentiate the flow control in the super-handler. This leads to
 119         a mix of flow logic and lowlevel hardware logic, and it also leads
 120         to unnecessary code duplication: for example in i386, there is a
 121         ioapic_level_irq and a ioapic_edge_irq irq-type which share many
 122         of the lowlevel details but have different flow handling.
 123         </para>
 124         <para>
 125         A more natural abstraction is the clean separation of the
 126         'irq flow' and the 'chip details'.
 127         </para>
 128         <para>
 129         Analysing a couple of architecture's IRQ subsystem implementations
 130         reveals that most of them can use a generic set of 'irq flow'
 131         methods and only need to add the chip level specific code.
 132         The separation is also valuable for (sub)architectures
 133         which need specific quirks in the irq flow itself but not in the
 134         chip-details - and thus provides a more transparent IRQ subsystem
 135         design.
 136         </para>
 137         <para>
 138         Each interrupt descriptor is assigned its own highlevel flow
 139         handler, which is normally one of the generic
 140         implementations. (This highlevel flow handler implementation also
 141         makes it simple to provide demultiplexing handlers which can be
 142         found in embedded platforms on various architectures.)
 143         </para>
 144         <para>
 145         The separation makes the generic interrupt handling layer more
 146         flexible and extensible. For example, an (sub)architecture can
 147         use a generic irq-flow implementation for 'level type' interrupts
 148         and add a (sub)architecture specific 'edge type' implementation.
 149         </para>
 150         <para>
 151         To make the transition to the new model easier and prevent the
 152         breakage of existing implementations, the __do_IRQ() super-handler
 153         is still available. This leads to a kind of duality for the time
 154         being. Over time the new model should be used in more and more
 155         architectures, as it enables smaller and cleaner IRQ subsystems.
 156         </para>
 157   </chapter>
 158   <chapter id="bugs">
 159     <title>Known Bugs And Assumptions</title>
 160     <para>
 161         None (knock on wood).
 162     </para>
 163   </chapter>
 164
 165   <chapter id="Abstraction">
 166     <title>Abstraction layers</title>
 167     <para>
 168         There are three main levels of abstraction in the interrupt code:
 169         <orderedlist>
 170           <listitem><para>Highlevel driver API</para></listitem>
 171           <listitem><para>Highlevel IRQ flow handlers</para></listitem>
 172           <listitem><para>Chiplevel hardware encapsulation</para></listitem>
 173         </orderedlist>
 174     </para>
 175     <sect1 id="Interrupt_control_flow">
 176         <title>Interrupt control flow</title>
 177         <para>
 178         Each interrupt is described by an interrupt descriptor structure
 179         irq_desc. The interrupt is referenced by an 'unsigned int' numeric
 180         value which selects the corresponding interrupt decription structure
 181         in the descriptor structures array.
 182         The descriptor structure contains status information and pointers
 183         to the interrupt flow method and the interrupt chip structure
 184         which are assigned to this interrupt.
 185         </para>
 186         <para>
 187         Whenever an interrupt triggers, the lowlevel arch code calls into
 188         the generic interrupt code by calling desc->handle_irq().
 189         This highlevel IRQ handling function only uses desc->chip primitives
 190         referenced by the assigned chip descriptor structure.
 191         </para>
 192     </sect1>
 193     <sect1 id="Highlevel_Driver_API">
 194         <title>Highlevel Driver API</title>
 195         <para>
 196           The highlevel Driver API consists of following functions:
 197           <itemizedlist>
 198           <listitem><para>request_irq()</para></listitem>
 199           <listitem><para>free_irq()</para></listitem>
 200           <listitem><para>disable_irq()</para></listitem>
 201           <listitem><para>enable_irq()</para></listitem>
 202           <listitem><para>disable_irq_nosync() (SMP only)</para></listitem>
 203           <listitem><para>synchronize_irq() (SMP only)</para></listitem>
 204           <listitem><para>set_irq_type()</para></listitem>
 205           <listitem><para>set_irq_wake()</para></listitem>
 206           <listitem><para>set_irq_data()</para></listitem>
 207           <listitem><para>set_irq_chip()</para></listitem>
 208           <listitem><para>set_irq_chip_data()</para></listitem>
 209           </itemizedlist>
 210           See the autogenerated function documentation for details.
 211         </para>
 212     </sect1>
 213     <sect1 id="Highlevel_IRQ_flow_handlers">
 214         <title>Highlevel IRQ flow handlers</title>
 215         <para>
 216           The generic layer provides a set of pre-defined irq-flow methods:
 217           <itemizedlist>
 218           <listitem><para>handle_level_irq</para></listitem>
 219           <listitem><para>handle_edge_irq</para></listitem>
 220           <listitem><para>handle_simple_irq</para></listitem>
 221           <listitem><para>handle_percpu_irq</para></listitem>
 222           </itemizedlist>
 223           The interrupt flow handlers (either predefined or architecture
 224           specific) are assigned to specific interrupts by the architecture
 225           either during bootup or during device initialization.
 226         </para>
 227         <sect2 id="Default_flow_implementations">
 228         <title>Default flow implementations</title>
 229             <sect3 id="Helper_functions">
 230                 <title>Helper functions</title>
 231                 <para>
 232                 The helper functions call the chip primitives and
 233                 are used by the default flow implementations.
 234                 The following helper functions are implemented (simplified excerpt):
 235                 <programlisting>
 236 default_enable(irq)
 237 {
 238         desc->chip->unmask(irq);
 239 }
 240
 241 default_disable(irq)
 242 {
 243         if (!delay_disable(irq))
 244                 desc->chip->mask(irq);
 245 }
 246
 247 default_ack(irq)
 248 {
 249         chip->ack(irq);
 250 }
 251
 252 default_mask_ack(irq)
 253 {
 254         if (chip->mask_ack) {
 255                 chip->mask_ack(irq);
 256         } else {
 257                 chip->mask(irq);
 258                 chip->ack(irq);
 259         }
 260 }
 261
 262 noop(irq)
 263 {
 264 }
 265
 266                 </programlisting>
 267                 </para>
 268             </sect3>
 269         </sect2>
 270         <sect2 id="Default_flow_handler_implementations">
 271         <title>Default flow handler implementations</title>
 272             <sect3 id="Default_Level_IRQ_flow_handler">
 273                 <title>Default Level IRQ flow handler</title>
 274                 <para>
 275                 handle_level_irq provides a generic implementation
 276                 for level-triggered interrupts.
 277                 </para>
 278                 <para>
 279                 The following control flow is implemented (simplified excerpt):
 280                 <programlisting>
 281 desc->chip->start();
 282 handle_IRQ_event(desc->action);
 283 desc->chip->end();
 284                 </programlisting>
 285                 </para>
 286             </sect3>
 287             <sect3 id="Default_Edge_IRQ_flow_handler">
 288                 <title>Default Edge IRQ flow handler</title>
 289                 <para>
 290                 handle_edge_irq provides a generic implementation
 291                 for edge-triggered interrupts.
 292                 </para>
 293                 <para>
 294                 The following control flow is implemented (simplified excerpt):
 295                 <programlisting>
 296 if (desc->status &amp; running) {
 297         desc->chip->hold();
 298         desc->status |= pending | masked;
 299         return;
 300 }
 301 desc->chip->start();
 302 desc->status |= running;
 303 do {
 304         if (desc->status &amp; masked)
 305                 desc->chip->enable();
 306         desc->status &amp;= ~pending;
 307         handle_IRQ_event(desc->action);
 308 } while (status &amp; pending);
 309 desc->status &amp;= ~running;
 310 desc->chip->end();
 311                 </programlisting>
 312                 </para>
 313             </sect3>
 314             <sect3 id="Default_simple_IRQ_flow_handler">
 315                 <title>Default simple IRQ flow handler</title>
 316                 <para>
 317                 handle_simple_irq provides a generic implementation
 318                 for simple interrupts.
 319                 </para>
 320                 <para>
 321                 Note: The simple flow handler does not call any
 322                 handler/chip primitives.
 323                 </para>
 324                 <para>
 325                 The following control flow is implemented (simplified excerpt):
 326                 <programlisting>
 327 handle_IRQ_event(desc->action);
 328                 </programlisting>
 329                 </para>
 330             </sect3>
 331             <sect3 id="Default_per_CPU_flow_handler">
 332                 <title>Default per CPU flow handler</title>
 333                 <para>
 334                 handle_percpu_irq provides a generic implementation
 335                 for per CPU interrupts.
 336                 </para>
 337                 <para>
 338                 Per CPU interrupts are only available on SMP and
 339                 the handler provides a simplified version without
 340                 locking.
 341                 </para>
 342                 <para>
 343                 The following control flow is implemented (simplified excerpt):
 344                 <programlisting>
 345 desc->chip->start();
 346 handle_IRQ_event(desc->action);
 347 desc->chip->end();
 348                 </programlisting>
 349                 </para>
 350             </sect3>
 351         </sect2>
 352         <sect2 id="Quirks_and_optimizations">
 353         <title>Quirks and optimizations</title>
 354         <para>
 355         The generic functions are intended for 'clean' architectures and chips,
 356         which have no platform-specific IRQ handling quirks. If an architecture
 357         needs to implement quirks on the 'flow' level then it can do so by
 358         overriding the highlevel irq-flow handler.
 359         </para>
 360         </sect2>
 361         <sect2 id="Delayed_interrupt_disable">
 362         <title>Delayed interrupt disable</title>
 363         <para>
 364         This per interrupt selectable feature, which was introduced by Russell
 365         King in the ARM interrupt implementation, does not mask an interrupt
 366         at the hardware level when disable_irq() is called. The interrupt is
 367         kept enabled and is masked in the flow handler when an interrupt event
 368         happens. This prevents losing edge interrupts on hardware which does
 369         not store an edge interrupt event while the interrupt is disabled at
 370         the hardware level. When an interrupt arrives while the IRQ_DISABLED
 371         flag is set, then the interrupt is masked at the hardware level and
 372         the IRQ_PENDING bit is set. When the interrupt is re-enabled by
 373         enable_irq() the pending bit is checked and if it is set, the
 374         interrupt is resent either via hardware or by a software resend
 375         mechanism. (It's necessary to enable CONFIG_HARDIRQS_SW_RESEND when
 376         you want to use the delayed interrupt disable feature and your
 377         hardware is not capable of retriggering an interrupt.)
 378         The delayed interrupt disable can be runtime enabled, per interrupt,
 379         by setting the IRQ_DELAYED_DISABLE flag in the irq_desc status field.
 380         </para>
 381         </sect2>
 382     </sect1>
 383     <sect1 id="Chiplevel_hardware_encapsulation">
 384         <title>Chiplevel hardware encapsulation</title>
 385         <para>
 386         The chip level hardware descriptor structure irq_chip
 387         contains all the direct chip relevant functions, which
 388         can be utilized by the irq flow implementations.
 389           <itemizedlist>
 390           <listitem><para>ack()</para></listitem>
 391           <listitem><para>mask_ack() - Optional, recommended for performance</para></listitem>
 392           <listitem><para>mask()</para></listitem>
 393           <listitem><para>unmask()</para></listitem>
 394           <listitem><para>retrigger() - Optional</para></listitem>
 395           <listitem><para>set_type() - Optional</para></listitem>
 396           <listitem><para>set_wake() - Optional</para></listitem>
 397           </itemizedlist>
 398         These primitives are strictly intended to mean what they say: ack means
 399         ACK, masking means masking of an IRQ line, etc. It is up to the flow
 400         handler(s) to use these basic units of lowlevel functionality.
 401         </para>
 402     </sect1>
 403   </chapter>
 404
 405   <chapter id="doirq">
 406      <title>__do_IRQ entry point</title>
 407      <para>
 408         The original implementation __do_IRQ() is an alternative entry
 409         point for all types of interrupts.
 410      </para>
 411      <para>
 412         This handler turned out to be not suitable for all
 413         interrupt hardware and was therefore reimplemented with split
 414         functionality for egde/level/simple/percpu interrupts. This is not
 415         only a functional optimization. It also shortens code paths for
 416         interrupts.
 417       </para>
 418       <para>
 419         To make use of the split implementation, replace the call to
 420         __do_IRQ by a call to desc->chip->handle_irq() and associate
 421         the appropriate handler function to desc->chip->handle_irq().
 422         In most cases the generic handler implementations should
 423         be sufficient.
 424      </para>
 425   </chapter>
 426
 427   <chapter id="locking">
 428      <title>Locking on SMP</title>
 429      <para>
 430         The locking of chip registers is up to the architecture that
 431         defines the chip primitives. There is a chip->lock field that can be used
 432         for serialization, but the generic layer does not touch it. The per-irq
 433         structure is protected via desc->lock, by the generic layer.
 434      </para>
 435   </chapter>
 436   <chapter id="structs">
 437      <title>Structures</title>
 438      <para>
 439      This chapter contains the autogenerated documentation of the structures which are
 440      used in the generic IRQ layer.
 441      </para>
 442 !Iinclude/linux/irq.h
 443   </chapter>
 444
 445   <chapter id="pubfunctions">
 446      <title>Public Functions Provided</title>
 447      <para>
 448      This chapter contains the autogenerated documentation of the kernel API functions
 449       which are exported.
 450      </para>
 451 !Ekernel/irq/manage.c
 452 !Ekernel/irq/chip.c
 453   </chapter>
 454
 455   <chapter id="intfunctions">
 456      <title>Internal Functions Provided</title>
 457      <para>
 458      This chapter contains the autogenerated documentation of the internal functions.
 459      </para>
 460 !Ikernel/irq/handle.c
 461 !Ikernel/irq/chip.c
 462   </chapter>
 463
 464   <chapter id="credits">
 465      <title>Credits</title>
 466         <para>
 467                 The following people have contributed to this document:
 468                 <orderedlist>
 469                         <listitem><para>Thomas Gleixner<email>tglx@linutronix.de</email></para></listitem>
 470                         <listitem><para>Ingo Molnar<email>mingo@elte.hu</email></para></listitem>
 471                 </orderedlist>
 472         </para>
 473   </chapter>
 474 </book>