Based on kernel version 4.2. Page generated on 2015-09-09 12:13 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 Device Tree interrupt 36 specifiers to hwirq numbers, and can be easily extended to support 37 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 74 The linear reverse map maintains a fixed size table indexed by the 75 hwirq number. When a hwirq is mapped, an irq_desc is allocated for 76 the hwirq, and the IRQ number is stored in the table. 77 78 The Linear map is a good choice when the maximum number of hwirqs is 79 fixed and a relatively small number (~ < 256). The advantages of this 80 map are fixed time lookup for IRQ numbers, and irq_descs are only 81 allocated for in-use IRQs. The disadvantage is that the table must be 82 as large as the largest possible hwirq number. 83 84 The majority of drivers should use the linear map. 85 86 ==== Tree ==== 87 irq_domain_add_tree() 88 89 The irq_domain maintains a radix tree map from hwirq numbers to Linux 90 IRQs. When an hwirq is mapped, an irq_desc is allocated and the 91 hwirq is used as the lookup key for the radix tree. 92 93 The tree map is a good choice if the hwirq number can be very large 94 since it doesn't need to allocate a table as large as the largest 95 hwirq number. The disadvantage is that hwirq to IRQ number lookup is 96 dependent on how many entries are in the table. 97 98 Very few drivers should need this mapping. 99 100 ==== No Map ===- 101 irq_domain_add_nomap() 102 103 The No Map mapping is to be used when the hwirq number is 104 programmable in the hardware. In this case it is best to program the 105 Linux IRQ number into the hardware itself so that no mapping is 106 required. Calling irq_create_direct_mapping() will allocate a Linux 107 IRQ number and call the .map() callback so that driver can program the 108 Linux IRQ number into the hardware. 109 110 Most drivers cannot use this mapping. 111 112 ==== Legacy ==== 113 irq_domain_add_simple() 114 irq_domain_add_legacy() 115 irq_domain_add_legacy_isa() 116 117 The Legacy mapping is a special case for drivers that already have a 118 range of irq_descs allocated for the hwirqs. It is used when the 119 driver cannot be immediately converted to use the linear mapping. For 120 example, many embedded system board support files use a set of #defines 121 for IRQ numbers that are passed to struct device registrations. In that 122 case the Linux IRQ numbers cannot be dynamically assigned and the legacy 123 mapping should be used. 124 125 The legacy map assumes a contiguous range of IRQ numbers has already 126 been allocated for the controller and that the IRQ number can be 127 calculated by adding a fixed offset to the hwirq number, and 128 visa-versa. The disadvantage is that it requires the interrupt 129 controller to manage IRQ allocations and it requires an irq_desc to be 130 allocated for every hwirq, even if it is unused. 131 132 The legacy map should only be used if fixed IRQ mappings must be 133 supported. For example, ISA controllers would use the legacy map for 134 mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ 135 numbers. 136 137 Most users of legacy mappings should use irq_domain_add_simple() which 138 will use a legacy domain only if an IRQ range is supplied by the 139 system and will otherwise use a linear domain mapping. The semantics 140 of this call are such that if an IRQ range is specified then 141 descriptors will be allocated on-the-fly for it, and if no range is 142 specified it will fall through to irq_domain_add_linear() which means 143 *no* irq descriptors will be allocated. 144 145 A typical use case for simple domains is where an irqchip provider 146 is supporting both dynamic and static IRQ assignments. 147 148 In order to avoid ending up in a situation where a linear domain is 149 used and no descriptor gets allocated it is very important to make sure 150 that the driver using the simple domain call irq_create_mapping() 151 before any irq_find_mapping() since the latter will actually work 152 for the static IRQ assignment case. 153 154 ==== Hierarchy IRQ domain ==== 155 On some architectures, there may be multiple interrupt controllers 156 involved in delivering an interrupt from the device to the target CPU. 157 Let's look at a typical interrupt delivering path on x86 platforms: 158 159 Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU 160 161 There are three interrupt controllers involved: 162 1) IOAPIC controller 163 2) Interrupt remapping controller 164 3) Local APIC controller 165 166 To support such a hardware topology and make software architecture match 167 hardware architecture, an irq_domain data structure is built for each 168 interrupt controller and those irq_domains are organized into hierarchy. 169 When building irq_domain hierarchy, the irq_domain near to the device is 170 child and the irq_domain near to CPU is parent. So a hierarchy structure 171 as 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) 179 180 There are four major interfaces to use hierarchy irq_domain: 181 1) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt 182 controller related resources to deliver these interrupts. 183 2) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller 184 related resources associated with these interrupts. 185 3) irq_domain_activate_irq(): activate interrupt controller hardware to 186 deliver the interrupt. 187 3) irq_domain_deactivate_irq(): deactivate interrupt controller hardware 188 to stop delivering the interrupt. 189 190 Following changes are needed to support hierarchy irq_domain. 191 1) a new field 'parent' is added to struct irq_domain; it's used to 192 maintain irq_domain hierarchy information. 193 2) 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. 196 3) new callbacks are added to struct irq_domain_ops to support hierarchy 197 irq_domain operations. 198 199 With support of hierarchy irq_domain and hierarchy irq_data ready, an 200 irq_domain structure is built for each interrupt controller, and an 201 irq_data structure is allocated for each irq_domain associated with an 202 IRQ. Now we could go one step further to support stacked(hierarchy) 203 irq_chip. That is, an irq_chip is associated with each irq_data along 204 the hierarchy. A child irq_chip may implement a required action by 205 itself or by cooperating with its parent irq_chip. 206 207 With stacked irq_chip, interrupt controller driver only needs to deal 208 with the hardware managed by itself and may ask for services from its 209 parent irq_chip when needed. So we could achieve a much cleaner 210 software architecture. 211 212 For an interrupt controller driver to support hierarchy irq_domain, it 213 needs to: 214 1) Implement irq_domain_ops.alloc and irq_domain_ops.free 215 2) Optionally implement irq_domain_ops.activate and 216 irq_domain_ops.deactivate. 217 3) Optionally implement an irq_chip to manage the interrupt controller 218 hardware. 219 4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap, 220 they are unused with hierarchy irq_domain. 221 222 Hierarchy irq_domain may also be used to support other architectures, 223 such as ARM, ARM64 etc.