1 ===============================================
2 The irq_domain interrupt number mapping library
3 ===============================================
5 The current design of the Linux kernel uses a single large number
6 space where each separate IRQ source is assigned a different number.
7 This is simple when there is only one interrupt controller, but in
8 systems with multiple interrupt controllers the kernel must ensure
9 that each one gets assigned non-overlapping allocations of Linux
12 The number of interrupt controllers registered as unique irqchips
13 show a rising tendency: for example subdrivers of different kinds
14 such as GPIO controllers avoid reimplementing identical callback
15 mechanisms as the IRQ core system by modelling their interrupt
16 handlers as irqchips, i.e. in effect cascading interrupt controllers.
18 Here the interrupt number loose all kind of correspondence to
19 hardware interrupt numbers: whereas in the past, IRQ numbers could
20 be chosen so they matched the hardware IRQ line into the root
21 interrupt controller (i.e. the component actually fireing the
22 interrupt line to the CPU) nowadays this number is just a number.
24 For this reason we need a mechanism to separate controller-local
25 interrupt numbers, called hardware irq's, from Linux IRQ numbers.
27 The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
28 irq numbers, but they don't provide any support for reverse mapping of
29 the controller-local IRQ (hwirq) number into the Linux IRQ number
32 The irq_domain library adds mapping between hwirq and IRQ numbers on
33 top of the irq_alloc_desc*() API. An irq_domain to manage mapping is
34 preferred over interrupt controller drivers open coding their own
35 reverse mapping scheme.
37 irq_domain also implements translation from an abstract irq_fwspec
38 structure to hwirq numbers (Device Tree and ACPI GSI so far), and can
39 be easily extended to support other IRQ topology data sources.
44 An interrupt controller driver creates and registers an irq_domain by
45 calling one of the irq_domain_add_*() functions (each mapping method
46 has a different allocator function, more on that later). The function
47 will return a pointer to the irq_domain on success. The caller must
48 provide the allocator function with an irq_domain_ops structure.
50 In most cases, the irq_domain will begin empty without any mappings
51 between hwirq and IRQ numbers. Mappings are added to the irq_domain
52 by calling irq_create_mapping() which accepts the irq_domain and a
53 hwirq number as arguments. If a mapping for the hwirq doesn't already
54 exist then it will allocate a new Linux irq_desc, associate it with
55 the hwirq, and call the .map() callback so the driver can perform any
56 required hardware setup.
58 When an interrupt is received, irq_find_mapping() function should
59 be used to find the Linux IRQ number from the hwirq number.
61 The irq_create_mapping() function must be called *atleast once*
62 before any call to irq_find_mapping(), lest the descriptor will not
65 If the driver has the Linux IRQ number or the irq_data pointer, and
66 needs to know the associated hwirq number (such as in the irq_chip
67 callbacks) then it can be directly obtained from irq_data->hwirq.
69 Types of irq_domain mappings
70 ============================
72 There are several mechanisms available for reverse mapping from hwirq
73 to Linux irq, and each mechanism uses a different allocation function.
74 Which reverse map type should be used depends on the use case. Each
75 of the reverse map types are described below:
82 irq_domain_add_linear()
83 irq_domain_create_linear()
85 The linear reverse map maintains a fixed size table indexed by the
86 hwirq number. When a hwirq is mapped, an irq_desc is allocated for
87 the hwirq, and the IRQ number is stored in the table.
89 The Linear map is a good choice when the maximum number of hwirqs is
90 fixed and a relatively small number (~ < 256). The advantages of this
91 map are fixed time lookup for IRQ numbers, and irq_descs are only
92 allocated for in-use IRQs. The disadvantage is that the table must be
93 as large as the largest possible hwirq number.
95 irq_domain_add_linear() and irq_domain_create_linear() are functionally
96 equivalent, except for the first argument is different - the former
97 accepts an Open Firmware specific 'struct device_node', while the latter
98 accepts a more general abstraction 'struct fwnode_handle'.
100 The majority of drivers should use the linear map.
107 irq_domain_add_tree()
108 irq_domain_create_tree()
110 The irq_domain maintains a radix tree map from hwirq numbers to Linux
111 IRQs. When an hwirq is mapped, an irq_desc is allocated and the
112 hwirq is used as the lookup key for the radix tree.
114 The tree map is a good choice if the hwirq number can be very large
115 since it doesn't need to allocate a table as large as the largest
116 hwirq number. The disadvantage is that hwirq to IRQ number lookup is
117 dependent on how many entries are in the table.
119 irq_domain_add_tree() and irq_domain_create_tree() are functionally
120 equivalent, except for the first argument is different - the former
121 accepts an Open Firmware specific 'struct device_node', while the latter
122 accepts a more general abstraction 'struct fwnode_handle'.
124 Very few drivers should need this mapping.
131 irq_domain_add_nomap()
133 The No Map mapping is to be used when the hwirq number is
134 programmable in the hardware. In this case it is best to program the
135 Linux IRQ number into the hardware itself so that no mapping is
136 required. Calling irq_create_direct_mapping() will allocate a Linux
137 IRQ number and call the .map() callback so that driver can program the
138 Linux IRQ number into the hardware.
140 Most drivers cannot use this mapping.
147 irq_domain_add_simple()
148 irq_domain_add_legacy()
149 irq_domain_add_legacy_isa()
151 The Legacy mapping is a special case for drivers that already have a
152 range of irq_descs allocated for the hwirqs. It is used when the
153 driver cannot be immediately converted to use the linear mapping. For
154 example, many embedded system board support files use a set of #defines
155 for IRQ numbers that are passed to struct device registrations. In that
156 case the Linux IRQ numbers cannot be dynamically assigned and the legacy
157 mapping should be used.
159 The legacy map assumes a contiguous range of IRQ numbers has already
160 been allocated for the controller and that the IRQ number can be
161 calculated by adding a fixed offset to the hwirq number, and
162 visa-versa. The disadvantage is that it requires the interrupt
163 controller to manage IRQ allocations and it requires an irq_desc to be
164 allocated for every hwirq, even if it is unused.
166 The legacy map should only be used if fixed IRQ mappings must be
167 supported. For example, ISA controllers would use the legacy map for
168 mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
171 Most users of legacy mappings should use irq_domain_add_simple() which
172 will use a legacy domain only if an IRQ range is supplied by the
173 system and will otherwise use a linear domain mapping. The semantics
174 of this call are such that if an IRQ range is specified then
175 descriptors will be allocated on-the-fly for it, and if no range is
176 specified it will fall through to irq_domain_add_linear() which means
177 *no* irq descriptors will be allocated.
179 A typical use case for simple domains is where an irqchip provider
180 is supporting both dynamic and static IRQ assignments.
182 In order to avoid ending up in a situation where a linear domain is
183 used and no descriptor gets allocated it is very important to make sure
184 that the driver using the simple domain call irq_create_mapping()
185 before any irq_find_mapping() since the latter will actually work
186 for the static IRQ assignment case.
191 On some architectures, there may be multiple interrupt controllers
192 involved in delivering an interrupt from the device to the target CPU.
193 Let's look at a typical interrupt delivering path on x86 platforms::
195 Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU
197 There are three interrupt controllers involved:
200 2) Interrupt remapping controller
201 3) Local APIC controller
203 To support such a hardware topology and make software architecture match
204 hardware architecture, an irq_domain data structure is built for each
205 interrupt controller and those irq_domains are organized into hierarchy.
206 When building irq_domain hierarchy, the irq_domain near to the device is
207 child and the irq_domain near to CPU is parent. So a hierarchy structure
208 as below will be built for the example above::
210 CPU Vector irq_domain (root irq_domain to manage CPU vectors)
213 Interrupt Remapping irq_domain (manage irq_remapping entries)
216 IOAPIC irq_domain (manage IOAPIC delivery entries/pins)
218 There are four major interfaces to use hierarchy irq_domain:
220 1) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt
221 controller related resources to deliver these interrupts.
222 2) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller
223 related resources associated with these interrupts.
224 3) irq_domain_activate_irq(): activate interrupt controller hardware to
225 deliver the interrupt.
226 4) irq_domain_deactivate_irq(): deactivate interrupt controller hardware
227 to stop delivering the interrupt.
229 Following changes are needed to support hierarchy irq_domain:
231 1) a new field 'parent' is added to struct irq_domain; it's used to
232 maintain irq_domain hierarchy information.
233 2) a new field 'parent_data' is added to struct irq_data; it's used to
234 build hierarchy irq_data to match hierarchy irq_domains. The irq_data
235 is used to store irq_domain pointer and hardware irq number.
236 3) new callbacks are added to struct irq_domain_ops to support hierarchy
237 irq_domain operations.
239 With support of hierarchy irq_domain and hierarchy irq_data ready, an
240 irq_domain structure is built for each interrupt controller, and an
241 irq_data structure is allocated for each irq_domain associated with an
242 IRQ. Now we could go one step further to support stacked(hierarchy)
243 irq_chip. That is, an irq_chip is associated with each irq_data along
244 the hierarchy. A child irq_chip may implement a required action by
245 itself or by cooperating with its parent irq_chip.
247 With stacked irq_chip, interrupt controller driver only needs to deal
248 with the hardware managed by itself and may ask for services from its
249 parent irq_chip when needed. So we could achieve a much cleaner
250 software architecture.
252 For an interrupt controller driver to support hierarchy irq_domain, it
255 1) Implement irq_domain_ops.alloc and irq_domain_ops.free
256 2) Optionally implement irq_domain_ops.activate and
257 irq_domain_ops.deactivate.
258 3) Optionally implement an irq_chip to manage the interrupt controller
260 4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
261 they are unused with hierarchy irq_domain.
263 Hierarchy irq_domain is in no way x86 specific, and is heavily used to
264 support other architectures, such as ARM, ARM64 etc.
268 Most of the internals of the IRQ subsystem are exposed in debugfs by
269 turning CONFIG_GENERIC_IRQ_DEBUGFS on.