Root/Documentation/IRQ-domain.txt

1irq_domain interrupt number mapping library
2
3The current design of the Linux kernel uses a single large number
4space where each separate IRQ source is assigned a different number.
5This is simple when there is only one interrupt controller, but in
6systems with multiple interrupt controllers the kernel must ensure
7that each one gets assigned non-overlapping allocations of Linux
8IRQ numbers.
9
10The number of interrupt controllers registered as unique irqchips
11show a rising tendency: for example subdrivers of different kinds
12such as GPIO controllers avoid reimplementing identical callback
13mechanisms as the IRQ core system by modelling their interrupt
14handlers as irqchips, i.e. in effect cascading interrupt controllers.
15
16Here the interrupt number loose all kind of correspondence to
17hardware interrupt numbers: whereas in the past, IRQ numbers could
18be chosen so they matched the hardware IRQ line into the root
19interrupt controller (i.e. the component actually fireing the
20interrupt line to the CPU) nowadays this number is just a number.
21
22For this reason we need a mechanism to separate controller-local
23interrupt numbers, called hardware irq's, from Linux IRQ numbers.
24
25The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
26irq numbers, but they don't provide any support for reverse mapping of
27the controller-local IRQ (hwirq) number into the Linux IRQ number
28space.
29
30The irq_domain library adds mapping between hwirq and IRQ numbers on
31top of the irq_alloc_desc*() API. An irq_domain to manage mapping is
32preferred over interrupt controller drivers open coding their own
33reverse mapping scheme.
34
35irq_domain also implements translation from Device Tree interrupt
36specifiers to hwirq numbers, and can be easily extended to support
37other IRQ topology data sources.
38
39=== irq_domain usage ===
40An interrupt controller driver creates and registers an irq_domain by
41calling one of the irq_domain_add_*() functions (each mapping method
42has a different allocator function, more on that later). The function
43will return a pointer to the irq_domain on success. The caller must
44provide the allocator function with an irq_domain_ops structure with
45the .map callback populated as a minimum.
46
47In most cases, the irq_domain will begin empty without any mappings
48between hwirq and IRQ numbers. Mappings are added to the irq_domain
49by calling irq_create_mapping() which accepts the irq_domain and a
50hwirq number as arguments. If a mapping for the hwirq doesn't already
51exist then it will allocate a new Linux irq_desc, associate it with
52the hwirq, and call the .map() callback so the driver can perform any
53required hardware setup.
54
55When an interrupt is received, irq_find_mapping() function should
56be used to find the Linux IRQ number from the hwirq number.
57
58The irq_create_mapping() function must be called *atleast once*
59before any call to irq_find_mapping(), lest the descriptor will not
60be allocated.
61
62If the driver has the Linux IRQ number or the irq_data pointer, and
63needs to know the associated hwirq number (such as in the irq_chip
64callbacks) then it can be directly obtained from irq_data->hwirq.
65
66=== Types of irq_domain mappings ===
67There are several mechanisms available for reverse mapping from hwirq
68to Linux irq, and each mechanism uses a different allocation function.
69Which reverse map type should be used depends on the use case. Each
70of the reverse map types are described below:
71
72==== Linear ====
73irq_domain_add_linear()
74
75The linear reverse map maintains a fixed size table indexed by the
76hwirq number. When a hwirq is mapped, an irq_desc is allocated for
77the hwirq, and the IRQ number is stored in the table.
78
79The Linear map is a good choice when the maximum number of hwirqs is
80fixed and a relatively small number (~ < 256). The advantages of this
81map are fixed time lookup for IRQ numbers, and irq_descs are only
82allocated for in-use IRQs. The disadvantage is that the table must be
83as large as the largest possible hwirq number.
84
85The majority of drivers should use the linear map.
86
87==== Tree ====
88irq_domain_add_tree()
89
90The irq_domain maintains a radix tree map from hwirq numbers to Linux
91IRQs. When an hwirq is mapped, an irq_desc is allocated and the
92hwirq is used as the lookup key for the radix tree.
93
94The tree map is a good choice if the hwirq number can be very large
95since it doesn't need to allocate a table as large as the largest
96hwirq number. The disadvantage is that hwirq to IRQ number lookup is
97dependent on how many entries are in the table.
98
99Very few drivers should need this mapping. At the moment, powerpc
100iseries is the only user.
101
102==== No Map ===-
103irq_domain_add_nomap()
104
105The No Map mapping is to be used when the hwirq number is
106programmable in the hardware. In this case it is best to program the
107Linux IRQ number into the hardware itself so that no mapping is
108required. Calling irq_create_direct_mapping() will allocate a Linux
109IRQ number and call the .map() callback so that driver can program the
110Linux IRQ number into the hardware.
111
112Most drivers cannot use this mapping.
113
114==== Legacy ====
115irq_domain_add_simple()
116irq_domain_add_legacy()
117irq_domain_add_legacy_isa()
118
119The Legacy mapping is a special case for drivers that already have a
120range of irq_descs allocated for the hwirqs. It is used when the
121driver cannot be immediately converted to use the linear mapping. For
122example, many embedded system board support files use a set of #defines
123for IRQ numbers that are passed to struct device registrations. In that
124case the Linux IRQ numbers cannot be dynamically assigned and the legacy
125mapping should be used.
126
127The legacy map assumes a contiguous range of IRQ numbers has already
128been allocated for the controller and that the IRQ number can be
129calculated by adding a fixed offset to the hwirq number, and
130visa-versa. The disadvantage is that it requires the interrupt
131controller to manage IRQ allocations and it requires an irq_desc to be
132allocated for every hwirq, even if it is unused.
133
134The legacy map should only be used if fixed IRQ mappings must be
135supported. For example, ISA controllers would use the legacy map for
136mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
137numbers.
138
139Most users of legacy mappings should use irq_domain_add_simple() which
140will use a legacy domain only if an IRQ range is supplied by the
141system and will otherwise use a linear domain mapping. The semantics
142of this call are such that if an IRQ range is specified then
143descriptors will be allocated on-the-fly for it, and if no range is
144specified it will fall through to irq_domain_add_linear() which meand
145*no* irq descriptors will be allocated.
146
147A typical use case for simple domains is where an irqchip provider
148is supporting both dynamic and static IRQ assignments.
149
150In order to avoid ending up in a situation where a linear domain is
151used and no descriptor gets allocated it is very important to make sure
152that the driver using the simple domain call irq_create_mapping()
153before any irq_find_mapping() since the latter will actually work
154for the static IRQ assignment case.
155

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