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Grant Likely7ab3a832012-02-14 14:06:47 -07001irq_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
Linus Walleij023bba32012-12-01 19:05:16 +010010The 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
Grant Likely7ab3a832012-02-14 14:06:47 -070025The 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
Jiang Liua2579542014-05-27 16:07:37 +080044provide the allocator function with an irq_domain_ops structure.
Grant Likely7ab3a832012-02-14 14:06:47 -070045
46In most cases, the irq_domain will begin empty without any mappings
47between hwirq and IRQ numbers. Mappings are added to the irq_domain
48by calling irq_create_mapping() which accepts the irq_domain and a
49hwirq number as arguments. If a mapping for the hwirq doesn't already
50exist then it will allocate a new Linux irq_desc, associate it with
51the hwirq, and call the .map() callback so the driver can perform any
52required hardware setup.
53
54When an interrupt is received, irq_find_mapping() function should
55be used to find the Linux IRQ number from the hwirq number.
56
Linus Walleij023bba32012-12-01 19:05:16 +010057The irq_create_mapping() function must be called *atleast once*
58before any call to irq_find_mapping(), lest the descriptor will not
59be allocated.
60
Grant Likely7ab3a832012-02-14 14:06:47 -070061If the driver has the Linux IRQ number or the irq_data pointer, and
62needs to know the associated hwirq number (such as in the irq_chip
63callbacks) then it can be directly obtained from irq_data->hwirq.
64
65=== Types of irq_domain mappings ===
66There are several mechanisms available for reverse mapping from hwirq
67to Linux irq, and each mechanism uses a different allocation function.
68Which reverse map type should be used depends on the use case. Each
69of the reverse map types are described below:
70
71==== Linear ====
72irq_domain_add_linear()
73
74The linear reverse map maintains a fixed size table indexed by the
75hwirq number. When a hwirq is mapped, an irq_desc is allocated for
76the hwirq, and the IRQ number is stored in the table.
77
78The Linear map is a good choice when the maximum number of hwirqs is
79fixed and a relatively small number (~ < 256). The advantages of this
80map are fixed time lookup for IRQ numbers, and irq_descs are only
81allocated for in-use IRQs. The disadvantage is that the table must be
82as large as the largest possible hwirq number.
83
84The majority of drivers should use the linear map.
85
86==== Tree ====
87irq_domain_add_tree()
88
89The irq_domain maintains a radix tree map from hwirq numbers to Linux
90IRQs. When an hwirq is mapped, an irq_desc is allocated and the
91hwirq is used as the lookup key for the radix tree.
92
93The tree map is a good choice if the hwirq number can be very large
94since it doesn't need to allocate a table as large as the largest
95hwirq number. The disadvantage is that hwirq to IRQ number lookup is
96dependent on how many entries are in the table.
97
Kevin Cernekee7e229fa2014-12-25 09:49:01 -080098Very few drivers should need this mapping.
Grant Likely7ab3a832012-02-14 14:06:47 -070099
100==== No Map ===-
101irq_domain_add_nomap()
102
103The No Map mapping is to be used when the hwirq number is
104programmable in the hardware. In this case it is best to program the
105Linux IRQ number into the hardware itself so that no mapping is
106required. Calling irq_create_direct_mapping() will allocate a Linux
107IRQ number and call the .map() callback so that driver can program the
108Linux IRQ number into the hardware.
109
110Most drivers cannot use this mapping.
111
112==== Legacy ====
Mark Brown781d0f42012-07-05 12:19:19 +0100113irq_domain_add_simple()
Grant Likely7ab3a832012-02-14 14:06:47 -0700114irq_domain_add_legacy()
115irq_domain_add_legacy_isa()
116
117The Legacy mapping is a special case for drivers that already have a
118range of irq_descs allocated for the hwirqs. It is used when the
119driver cannot be immediately converted to use the linear mapping. For
120example, many embedded system board support files use a set of #defines
121for IRQ numbers that are passed to struct device registrations. In that
122case the Linux IRQ numbers cannot be dynamically assigned and the legacy
123mapping should be used.
124
125The legacy map assumes a contiguous range of IRQ numbers has already
126been allocated for the controller and that the IRQ number can be
127calculated by adding a fixed offset to the hwirq number, and
128visa-versa. The disadvantage is that it requires the interrupt
129controller to manage IRQ allocations and it requires an irq_desc to be
130allocated for every hwirq, even if it is unused.
131
132The legacy map should only be used if fixed IRQ mappings must be
133supported. For example, ISA controllers would use the legacy map for
134mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
135numbers.
Mark Brown781d0f42012-07-05 12:19:19 +0100136
137Most users of legacy mappings should use irq_domain_add_simple() which
138will use a legacy domain only if an IRQ range is supplied by the
Linus Walleij023bba32012-12-01 19:05:16 +0100139system and will otherwise use a linear domain mapping. The semantics
140of this call are such that if an IRQ range is specified then
141descriptors will be allocated on-the-fly for it, and if no range is
Xishi Qiud9a6ed12013-11-06 13:18:19 -0800142specified it will fall through to irq_domain_add_linear() which means
Linus Walleij023bba32012-12-01 19:05:16 +0100143*no* irq descriptors will be allocated.
144
145A typical use case for simple domains is where an irqchip provider
146is supporting both dynamic and static IRQ assignments.
147
148In order to avoid ending up in a situation where a linear domain is
149used and no descriptor gets allocated it is very important to make sure
150that the driver using the simple domain call irq_create_mapping()
151before any irq_find_mapping() since the latter will actually work
152for the static IRQ assignment case.
Jiang Liuf8264e32014-11-06 22:20:14 +0800153
154==== Hierarchy IRQ domain ====
155On some architectures, there may be multiple interrupt controllers
156involved in delivering an interrupt from the device to the target CPU.
157Let's look at a typical interrupt delivering path on x86 platforms:
158
159Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU
160
161There are three interrupt controllers involved:
1621) IOAPIC controller
1632) Interrupt remapping controller
1643) Local APIC controller
165
166To support such a hardware topology and make software architecture match
167hardware architecture, an irq_domain data structure is built for each
168interrupt controller and those irq_domains are organized into hierarchy.
169When building irq_domain hierarchy, the irq_domain near to the device is
170child and the irq_domain near to CPU is parent. So a hierarchy structure
171as below will be built for the example above.
172 CPU Vector irq_domain (root irq_domain to manage CPU vectors)
173 ^
174 |
175 Interrupt Remapping irq_domain (manage irq_remapping entries)
176 ^
177 |
178 IOAPIC irq_domain (manage IOAPIC delivery entries/pins)
179
180There are four major interfaces to use hierarchy irq_domain:
1811) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt
182 controller related resources to deliver these interrupts.
1832) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller
184 related resources associated with these interrupts.
1853) irq_domain_activate_irq(): activate interrupt controller hardware to
186 deliver the interrupt.
1873) irq_domain_deactivate_irq(): deactivate interrupt controller hardware
188 to stop delivering the interrupt.
189
190Following changes are needed to support hierarchy irq_domain.
1911) a new field 'parent' is added to struct irq_domain; it's used to
192 maintain irq_domain hierarchy information.
1932) a new field 'parent_data' is added to struct irq_data; it's used to
194 build hierarchy irq_data to match hierarchy irq_domains. The irq_data
195 is used to store irq_domain pointer and hardware irq number.
1963) new callbacks are added to struct irq_domain_ops to support hierarchy
197 irq_domain operations.
198
199With support of hierarchy irq_domain and hierarchy irq_data ready, an
200irq_domain structure is built for each interrupt controller, and an
201irq_data structure is allocated for each irq_domain associated with an
202IRQ. Now we could go one step further to support stacked(hierarchy)
203irq_chip. That is, an irq_chip is associated with each irq_data along
204the hierarchy. A child irq_chip may implement a required action by
205itself or by cooperating with its parent irq_chip.
206
207With stacked irq_chip, interrupt controller driver only needs to deal
208with the hardware managed by itself and may ask for services from its
209parent irq_chip when needed. So we could achieve a much cleaner
210software architecture.
211
212For an interrupt controller driver to support hierarchy irq_domain, it
213needs to:
2141) Implement irq_domain_ops.alloc and irq_domain_ops.free
2152) Optionally implement irq_domain_ops.activate and
216 irq_domain_ops.deactivate.
2173) Optionally implement an irq_chip to manage the interrupt controller
218 hardware.
2194) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
220 they are unused with hierarchy irq_domain.
221
222Hierarchy irq_domain may also be used to support other architectures,
223such as ARM, ARM64 etc.