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Linux/Documentation/IRQ-domain.txt

  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 an abstract irq_fwspec
 36 structure to hwirq numbers (Device Tree and ACPI GSI so far), and can
 37 be easily extended to support 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.
 45 
 46 In most cases, the irq_domain will begin empty without any mappings
 47 between hwirq and IRQ numbers.  Mappings are added to the irq_domain
 48 by calling irq_create_mapping() which accepts the irq_domain and a
 49 hwirq number as arguments.  If a mapping for the hwirq doesn't already
 50 exist then it will allocate a new Linux irq_desc, associate it with
 51 the hwirq, and call the .map() callback so the driver can perform any
 52 required hardware setup.
 53 
 54 When an interrupt is received, irq_find_mapping() function should
 55 be used to find the Linux IRQ number from the hwirq number.
 56 
 57 The irq_create_mapping() function must be called *atleast once*
 58 before any call to irq_find_mapping(), lest the descriptor will not
 59 be allocated.
 60 
 61 If the driver has the Linux IRQ number or the irq_data pointer, and
 62 needs to know the associated hwirq number (such as in the irq_chip
 63 callbacks) then it can be directly obtained from irq_data->hwirq.
 64 
 65 === Types of irq_domain mappings ===
 66 There are several mechanisms available for reverse mapping from hwirq
 67 to Linux irq, and each mechanism uses a different allocation function.
 68 Which reverse map type should be used depends on the use case.  Each
 69 of the reverse map types are described below:
 70 
 71 ==== Linear ====
 72 irq_domain_add_linear()
 73 irq_domain_create_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 irq_domain_add_linear() and irq_domain_create_linear() are functionally
 86 equivalent, except for the first argument is different - the former
 87 accepts an Open Firmware specific 'struct device_node', while the latter
 88 accepts a more general abstraction 'struct fwnode_handle'.
 89 
 90 The majority of drivers should use the linear map.
 91 
 92 ==== Tree ====
 93 irq_domain_add_tree()
 94 irq_domain_create_tree()
 95 
 96 The irq_domain maintains a radix tree map from hwirq numbers to Linux
 97 IRQs.  When an hwirq is mapped, an irq_desc is allocated and the
 98 hwirq is used as the lookup key for the radix tree.
 99 
100 The tree map is a good choice if the hwirq number can be very large
101 since it doesn't need to allocate a table as large as the largest
102 hwirq number.  The disadvantage is that hwirq to IRQ number lookup is
103 dependent on how many entries are in the table.
104 
105 irq_domain_add_tree() and irq_domain_create_tree() are functionally
106 equivalent, except for the first argument is different - the former
107 accepts an Open Firmware specific 'struct device_node', while the latter
108 accepts a more general abstraction 'struct fwnode_handle'.
109 
110 Very few drivers should need this mapping.
111 
112 ==== No Map ===-
113 irq_domain_add_nomap()
114 
115 The No Map mapping is to be used when the hwirq number is
116 programmable in the hardware.  In this case it is best to program the
117 Linux IRQ number into the hardware itself so that no mapping is
118 required.  Calling irq_create_direct_mapping() will allocate a Linux
119 IRQ number and call the .map() callback so that driver can program the
120 Linux IRQ number into the hardware.
121 
122 Most drivers cannot use this mapping.
123 
124 ==== Legacy ====
125 irq_domain_add_simple()
126 irq_domain_add_legacy()
127 irq_domain_add_legacy_isa()
128 
129 The Legacy mapping is a special case for drivers that already have a
130 range of irq_descs allocated for the hwirqs.  It is used when the
131 driver cannot be immediately converted to use the linear mapping.  For
132 example, many embedded system board support files use a set of #defines
133 for IRQ numbers that are passed to struct device registrations.  In that
134 case the Linux IRQ numbers cannot be dynamically assigned and the legacy
135 mapping should be used.
136 
137 The legacy map assumes a contiguous range of IRQ numbers has already
138 been allocated for the controller and that the IRQ number can be
139 calculated by adding a fixed offset to the hwirq number, and
140 visa-versa.  The disadvantage is that it requires the interrupt
141 controller to manage IRQ allocations and it requires an irq_desc to be
142 allocated for every hwirq, even if it is unused.
143 
144 The legacy map should only be used if fixed IRQ mappings must be
145 supported.  For example, ISA controllers would use the legacy map for
146 mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
147 numbers.
148 
149 Most users of legacy mappings should use irq_domain_add_simple() which
150 will use a legacy domain only if an IRQ range is supplied by the
151 system and will otherwise use a linear domain mapping. The semantics
152 of this call are such that if an IRQ range is specified then
153 descriptors will be allocated on-the-fly for it, and if no range is
154 specified it will fall through to irq_domain_add_linear() which means
155 *no* irq descriptors will be allocated.
156 
157 A typical use case for simple domains is where an irqchip provider
158 is supporting both dynamic and static IRQ assignments.
159 
160 In order to avoid ending up in a situation where a linear domain is
161 used and no descriptor gets allocated it is very important to make sure
162 that the driver using the simple domain call irq_create_mapping()
163 before any irq_find_mapping() since the latter will actually work
164 for the static IRQ assignment case.
165 
166 ==== Hierarchy IRQ domain ====
167 On some architectures, there may be multiple interrupt controllers
168 involved in delivering an interrupt from the device to the target CPU.
169 Let's look at a typical interrupt delivering path on x86 platforms:
170 
171 Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU
172 
173 There are three interrupt controllers involved:
174 1) IOAPIC controller
175 2) Interrupt remapping controller
176 3) Local APIC controller
177 
178 To support such a hardware topology and make software architecture match
179 hardware architecture, an irq_domain data structure is built for each
180 interrupt controller and those irq_domains are organized into hierarchy.
181 When building irq_domain hierarchy, the irq_domain near to the device is
182 child and the irq_domain near to CPU is parent. So a hierarchy structure
183 as below will be built for the example above.
184         CPU Vector irq_domain (root irq_domain to manage CPU vectors)
185                 ^
186                 |
187         Interrupt Remapping irq_domain (manage irq_remapping entries)
188                 ^
189                 |
190         IOAPIC irq_domain (manage IOAPIC delivery entries/pins)
191 
192 There are four major interfaces to use hierarchy irq_domain:
193 1) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt
194    controller related resources to deliver these interrupts.
195 2) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller
196    related resources associated with these interrupts.
197 3) irq_domain_activate_irq(): activate interrupt controller hardware to
198    deliver the interrupt.
199 4) irq_domain_deactivate_irq(): deactivate interrupt controller hardware
200    to stop delivering the interrupt.
201 
202 Following changes are needed to support hierarchy irq_domain.
203 1) a new field 'parent' is added to struct irq_domain; it's used to
204    maintain irq_domain hierarchy information.
205 2) a new field 'parent_data' is added to struct irq_data; it's used to
206    build hierarchy irq_data to match hierarchy irq_domains. The irq_data
207    is used to store irq_domain pointer and hardware irq number.
208 3) new callbacks are added to struct irq_domain_ops to support hierarchy
209    irq_domain operations.
210 
211 With support of hierarchy irq_domain and hierarchy irq_data ready, an
212 irq_domain structure is built for each interrupt controller, and an
213 irq_data structure is allocated for each irq_domain associated with an
214 IRQ. Now we could go one step further to support stacked(hierarchy)
215 irq_chip. That is, an irq_chip is associated with each irq_data along
216 the hierarchy. A child irq_chip may implement a required action by
217 itself or by cooperating with its parent irq_chip.
218 
219 With stacked irq_chip, interrupt controller driver only needs to deal
220 with the hardware managed by itself and may ask for services from its
221 parent irq_chip when needed. So we could achieve a much cleaner
222 software architecture.
223 
224 For an interrupt controller driver to support hierarchy irq_domain, it
225 needs to:
226 1) Implement irq_domain_ops.alloc and irq_domain_ops.free
227 2) Optionally implement irq_domain_ops.activate and
228    irq_domain_ops.deactivate.
229 3) Optionally implement an irq_chip to manage the interrupt controller
230    hardware.
231 4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
232    they are unused with hierarchy irq_domain.
233 
234 Hierarchy irq_domain may also be used to support other architectures,
235 such as ARM, ARM64 etc.

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