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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
98Very few drivers should need this mapping. At the moment, powerpc
99iseries is the only user.
100
101==== No Map ===-
102irq_domain_add_nomap()
103
104The No Map mapping is to be used when the hwirq number is
105programmable in the hardware. In this case it is best to program the
106Linux IRQ number into the hardware itself so that no mapping is
107required. Calling irq_create_direct_mapping() will allocate a Linux
108IRQ number and call the .map() callback so that driver can program the
109Linux IRQ number into the hardware.
110
111Most drivers cannot use this mapping.
112
113==== Legacy ====
Mark Brown781d0f42012-07-05 12:19:19 +0100114irq_domain_add_simple()
Grant Likely7ab3a832012-02-14 14:06:47 -0700115irq_domain_add_legacy()
116irq_domain_add_legacy_isa()
117
118The Legacy mapping is a special case for drivers that already have a
119range of irq_descs allocated for the hwirqs. It is used when the
120driver cannot be immediately converted to use the linear mapping. For
121example, many embedded system board support files use a set of #defines
122for IRQ numbers that are passed to struct device registrations. In that
123case the Linux IRQ numbers cannot be dynamically assigned and the legacy
124mapping should be used.
125
126The legacy map assumes a contiguous range of IRQ numbers has already
127been allocated for the controller and that the IRQ number can be
128calculated by adding a fixed offset to the hwirq number, and
129visa-versa. The disadvantage is that it requires the interrupt
130controller to manage IRQ allocations and it requires an irq_desc to be
131allocated for every hwirq, even if it is unused.
132
133The legacy map should only be used if fixed IRQ mappings must be
134supported. For example, ISA controllers would use the legacy map for
135mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
136numbers.
Mark Brown781d0f42012-07-05 12:19:19 +0100137
138Most users of legacy mappings should use irq_domain_add_simple() which
139will use a legacy domain only if an IRQ range is supplied by the
Linus Walleij023bba32012-12-01 19:05:16 +0100140system and will otherwise use a linear domain mapping. The semantics
141of this call are such that if an IRQ range is specified then
142descriptors will be allocated on-the-fly for it, and if no range is
Xishi Qiud9a6ed12013-11-06 13:18:19 -0800143specified it will fall through to irq_domain_add_linear() which means
Linus Walleij023bba32012-12-01 19:05:16 +0100144*no* irq descriptors will be allocated.
145
146A typical use case for simple domains is where an irqchip provider
147is supporting both dynamic and static IRQ assignments.
148
149In order to avoid ending up in a situation where a linear domain is
150used and no descriptor gets allocated it is very important to make sure
151that the driver using the simple domain call irq_create_mapping()
152before any irq_find_mapping() since the latter will actually work
153for the static IRQ assignment case.
Jiang Liuf8264e32014-11-06 22:20:14 +0800154
155==== Hierarchy IRQ domain ====
156On some architectures, there may be multiple interrupt controllers
157involved in delivering an interrupt from the device to the target CPU.
158Let's look at a typical interrupt delivering path on x86 platforms:
159
160Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU
161
162There are three interrupt controllers involved:
1631) IOAPIC controller
1642) Interrupt remapping controller
1653) Local APIC controller
166
167To support such a hardware topology and make software architecture match
168hardware architecture, an irq_domain data structure is built for each
169interrupt controller and those irq_domains are organized into hierarchy.
170When building irq_domain hierarchy, the irq_domain near to the device is
171child and the irq_domain near to CPU is parent. So a hierarchy structure
172as below will be built for the example above.
173 CPU Vector irq_domain (root irq_domain to manage CPU vectors)
174 ^
175 |
176 Interrupt Remapping irq_domain (manage irq_remapping entries)
177 ^
178 |
179 IOAPIC irq_domain (manage IOAPIC delivery entries/pins)
180
181There are four major interfaces to use hierarchy irq_domain:
1821) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt
183 controller related resources to deliver these interrupts.
1842) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller
185 related resources associated with these interrupts.
1863) irq_domain_activate_irq(): activate interrupt controller hardware to
187 deliver the interrupt.
1883) irq_domain_deactivate_irq(): deactivate interrupt controller hardware
189 to stop delivering the interrupt.
190
191Following changes are needed to support hierarchy irq_domain.
1921) a new field 'parent' is added to struct irq_domain; it's used to
193 maintain irq_domain hierarchy information.
1942) a new field 'parent_data' is added to struct irq_data; it's used to
195 build hierarchy irq_data to match hierarchy irq_domains. The irq_data
196 is used to store irq_domain pointer and hardware irq number.
1973) new callbacks are added to struct irq_domain_ops to support hierarchy
198 irq_domain operations.
199
200With support of hierarchy irq_domain and hierarchy irq_data ready, an
201irq_domain structure is built for each interrupt controller, and an
202irq_data structure is allocated for each irq_domain associated with an
203IRQ. Now we could go one step further to support stacked(hierarchy)
204irq_chip. That is, an irq_chip is associated with each irq_data along
205the hierarchy. A child irq_chip may implement a required action by
206itself or by cooperating with its parent irq_chip.
207
208With stacked irq_chip, interrupt controller driver only needs to deal
209with the hardware managed by itself and may ask for services from its
210parent irq_chip when needed. So we could achieve a much cleaner
211software architecture.
212
213For an interrupt controller driver to support hierarchy irq_domain, it
214needs to:
2151) Implement irq_domain_ops.alloc and irq_domain_ops.free
2162) Optionally implement irq_domain_ops.activate and
217 irq_domain_ops.deactivate.
2183) Optionally implement an irq_chip to manage the interrupt controller
219 hardware.
2204) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
221 they are unused with hierarchy irq_domain.
222
223Hierarchy irq_domain may also be used to support other architectures,
224such as ARM, ARM64 etc.