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Documentation / IRQ-domain.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.

1	irq_domain interrupt number mapping library
2	
3	The current design of the Linux kernel uses a single large number
4	space where each separate IRQ source is assigned a different number.
5	This is simple when there is only one interrupt controller, but in
6	systems with multiple interrupt controllers the kernel must ensure
7	that each one gets assigned non-overlapping allocations of Linux
8	IRQ numbers.
9	
10	The number of interrupt controllers registered as unique irqchips
11	show a rising tendency: for example subdrivers of different kinds
12	such as GPIO controllers avoid reimplementing identical callback
13	mechanisms as the IRQ core system by modelling their interrupt
14	handlers as irqchips, i.e. in effect cascading interrupt controllers.
15	
16	Here the interrupt number loose all kind of correspondence to
17	hardware interrupt numbers: whereas in the past, IRQ numbers could
18	be chosen so they matched the hardware IRQ line into the root
19	interrupt controller (i.e. the component actually fireing the
20	interrupt line to the CPU) nowadays this number is just a number.
21	
22	For this reason we need a mechanism to separate controller-local
23	interrupt numbers, called hardware irq's, from Linux IRQ numbers.
24	
25	The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
26	irq numbers, but they don't provide any support for reverse mapping of
27	the controller-local IRQ (hwirq) number into the Linux IRQ number
28	space.
29	
30	The irq_domain library adds mapping between hwirq and IRQ numbers on
31	top of the irq_alloc_desc*() API.  An irq_domain to manage mapping is
32	preferred over interrupt controller drivers open coding their own
33	reverse mapping scheme.
34	
35	irq_domain also implements translation from Device Tree interrupt
36	specifiers to hwirq numbers, and can be easily extended to support
37	other IRQ topology data sources.
38	
39	=== irq_domain usage ===
40	An interrupt controller driver creates and registers an irq_domain by
41	calling one of the irq_domain_add_*() functions (each mapping method
42	has a different allocator function, more on that later).  The function
43	will return a pointer to the irq_domain on success.  The caller must
44	provide the allocator function with an irq_domain_ops structure with
45	the .map callback populated as a minimum.
46	
47	In most cases, the irq_domain will begin empty without any mappings
48	between hwirq and IRQ numbers.  Mappings are added to the irq_domain
49	by calling irq_create_mapping() which accepts the irq_domain and a
50	hwirq number as arguments.  If a mapping for the hwirq doesn't already
51	exist then it will allocate a new Linux irq_desc, associate it with
52	the hwirq, and call the .map() callback so the driver can perform any
53	required hardware setup.
54	
55	When an interrupt is received, irq_find_mapping() function should
56	be used to find the Linux IRQ number from the hwirq number.
57	
58	The irq_create_mapping() function must be called *atleast once*
59	before any call to irq_find_mapping(), lest the descriptor will not
60	be allocated.
61	
62	If the driver has the Linux IRQ number or the irq_data pointer, and
63	needs to know the associated hwirq number (such as in the irq_chip
64	callbacks) then it can be directly obtained from irq_data->hwirq.
65	
66	=== Types of irq_domain mappings ===
67	There are several mechanisms available for reverse mapping from hwirq
68	to Linux irq, and each mechanism uses a different allocation function.
69	Which reverse map type should be used depends on the use case.  Each
70	of the reverse map types are described below:
71	
72	==== Linear ====
73	irq_domain_add_linear()
74	
75	The linear reverse map maintains a fixed size table indexed by the
76	hwirq number.  When a hwirq is mapped, an irq_desc is allocated for
77	the hwirq, and the IRQ number is stored in the table.
78	
79	The Linear map is a good choice when the maximum number of hwirqs is
80	fixed and a relatively small number (~ < 256).  The advantages of this
81	map are fixed time lookup for IRQ numbers, and irq_descs are only
82	allocated for in-use IRQs.  The disadvantage is that the table must be
83	as large as the largest possible hwirq number.
84	
85	The majority of drivers should use the linear map.
86	
87	==== Tree ====
88	irq_domain_add_tree()
89	
90	The irq_domain maintains a radix tree map from hwirq numbers to Linux
91	IRQs.  When an hwirq is mapped, an irq_desc is allocated and the
92	hwirq is used as the lookup key for the radix tree.
93	
94	The tree map is a good choice if the hwirq number can be very large
95	since it doesn't need to allocate a table as large as the largest
96	hwirq number.  The disadvantage is that hwirq to IRQ number lookup is
97	dependent on how many entries are in the table.
98	
99	Very few drivers should need this mapping.  At the moment, powerpc
100	iseries is the only user.
101	
102	==== No Map ===-
103	irq_domain_add_nomap()
104	
105	The No Map mapping is to be used when the hwirq number is
106	programmable in the hardware.  In this case it is best to program the
107	Linux IRQ number into the hardware itself so that no mapping is
108	required.  Calling irq_create_direct_mapping() will allocate a Linux
109	IRQ number and call the .map() callback so that driver can program the
110	Linux IRQ number into the hardware.
111	
112	Most drivers cannot use this mapping.
113	
114	==== Legacy ====
115	irq_domain_add_simple()
116	irq_domain_add_legacy()
117	irq_domain_add_legacy_isa()
118	
119	The Legacy mapping is a special case for drivers that already have a
120	range of irq_descs allocated for the hwirqs.  It is used when the
121	driver cannot be immediately converted to use the linear mapping.  For
122	example, many embedded system board support files use a set of #defines
123	for IRQ numbers that are passed to struct device registrations.  In that
124	case the Linux IRQ numbers cannot be dynamically assigned and the legacy
125	mapping should be used.
126	
127	The legacy map assumes a contiguous range of IRQ numbers has already
128	been allocated for the controller and that the IRQ number can be
129	calculated by adding a fixed offset to the hwirq number, and
130	visa-versa.  The disadvantage is that it requires the interrupt
131	controller to manage IRQ allocations and it requires an irq_desc to be
132	allocated for every hwirq, even if it is unused.
133	
134	The legacy map should only be used if fixed IRQ mappings must be
135	supported.  For example, ISA controllers would use the legacy map for
136	mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
137	numbers.
138	
139	Most users of legacy mappings should use irq_domain_add_simple() which
140	will use a legacy domain only if an IRQ range is supplied by the
141	system and will otherwise use a linear domain mapping. The semantics
142	of this call are such that if an IRQ range is specified then
143	descriptors will be allocated on-the-fly for it, and if no range is
144	specified it will fall through to irq_domain_add_linear() which meand
145	*no* irq descriptors will be allocated.
146	
147	A typical use case for simple domains is where an irqchip provider
148	is supporting both dynamic and static IRQ assignments.
149	
150	In order to avoid ending up in a situation where a linear domain is
151	used and no descriptor gets allocated it is very important to make sure
152	that the driver using the simple domain call irq_create_mapping()
153	before any irq_find_mapping() since the latter will actually work
154	for the static IRQ assignment case.
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