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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>
244default_enable(struct irq_data *data)
245{
246	desc->irq_data.chip->irq_unmask(data);
247}
248
249default_disable(struct irq_data *data)
250{
251	if (!delay_disable(data))
252		desc->irq_data.chip->irq_mask(data);
253}
254
255default_ack(struct irq_data *data)
256{
257	chip->irq_ack(data);
258}
259
260default_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
270noop(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>
289desc->irq_data.chip->irq_mask_ack();
290handle_irq_event(desc->action);
291desc->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>
305handle_irq_event(desc->action);
306desc->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>
319if (desc->status &amp; running) {
320	desc->irq_data.chip->irq_mask_ack();
321	desc->status |= pending | masked;
322	return;
323}
324desc->irq_data.chip->irq_ack();
325desc->status |= running;
326do {
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);
332desc->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>
349handle_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>
367if (desc->irq_data.chip->irq_ack)
368	desc->irq_data.chip->irq_ack();
369handle_irq_event(desc->action);
370if (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>
521