Based on kernel version 4.8. Page generated on 2016-10-06 23:16 EST.
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.