1=============================================== 2The irq_domain interrupt number mapping library 3=============================================== 4 5The current design of the Linux kernel uses a single large number 6space where each separate IRQ source is assigned a different number. 7This is simple when there is only one interrupt controller, but in 8systems with multiple interrupt controllers the kernel must ensure 9that each one gets assigned non-overlapping allocations of Linux 10IRQ numbers. 11 12The number of interrupt controllers registered as unique irqchips 13show a rising tendency: for example subdrivers of different kinds 14such as GPIO controllers avoid reimplementing identical callback 15mechanisms as the IRQ core system by modelling their interrupt 16handlers as irqchips, i.e. in effect cascading interrupt controllers. 17 18Here the interrupt number loose all kind of correspondence to 19hardware interrupt numbers: whereas in the past, IRQ numbers could 20be chosen so they matched the hardware IRQ line into the root 21interrupt controller (i.e. the component actually fireing the 22interrupt line to the CPU) nowadays this number is just a number. 23 24For this reason we need a mechanism to separate controller-local 25interrupt numbers, called hardware irq's, from Linux IRQ numbers. 26 27The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of 28irq numbers, but they don't provide any support for reverse mapping of 29the controller-local IRQ (hwirq) number into the Linux IRQ number 30space. 31 32The irq_domain library adds mapping between hwirq and IRQ numbers on 33top of the irq_alloc_desc*() API. An irq_domain to manage mapping is 34preferred over interrupt controller drivers open coding their own 35reverse mapping scheme. 36 37irq_domain also implements translation from an abstract irq_fwspec 38structure to hwirq numbers (Device Tree and ACPI GSI so far), and can 39be easily extended to support other IRQ topology data sources. 40 41irq_domain usage 42================ 43 44An interrupt controller driver creates and registers an irq_domain by 45calling one of the irq_domain_add_*() functions (each mapping method 46has a different allocator function, more on that later). The function 47will return a pointer to the irq_domain on success. The caller must 48provide the allocator function with an irq_domain_ops structure. 49 50In most cases, the irq_domain will begin empty without any mappings 51between hwirq and IRQ numbers. Mappings are added to the irq_domain 52by calling irq_create_mapping() which accepts the irq_domain and a 53hwirq number as arguments. If a mapping for the hwirq doesn't already 54exist then it will allocate a new Linux irq_desc, associate it with 55the hwirq, and call the .map() callback so the driver can perform any 56required hardware setup. 57 58When an interrupt is received, irq_find_mapping() function should 59be used to find the Linux IRQ number from the hwirq number. 60 61The irq_create_mapping() function must be called *atleast once* 62before any call to irq_find_mapping(), lest the descriptor will not 63be allocated. 64 65If the driver has the Linux IRQ number or the irq_data pointer, and 66needs to know the associated hwirq number (such as in the irq_chip 67callbacks) then it can be directly obtained from irq_data->hwirq. 68 69Types of irq_domain mappings 70============================ 71 72There are several mechanisms available for reverse mapping from hwirq 73to Linux irq, and each mechanism uses a different allocation function. 74Which reverse map type should be used depends on the use case. Each 75of the reverse map types are described below: 76 77Linear 78------ 79 80:: 81 82 irq_domain_add_linear() 83 irq_domain_create_linear() 84 85The linear reverse map maintains a fixed size table indexed by the 86hwirq number. When a hwirq is mapped, an irq_desc is allocated for 87the hwirq, and the IRQ number is stored in the table. 88 89The Linear map is a good choice when the maximum number of hwirqs is 90fixed and a relatively small number (~ < 256). The advantages of this 91map are fixed time lookup for IRQ numbers, and irq_descs are only 92allocated for in-use IRQs. The disadvantage is that the table must be 93as large as the largest possible hwirq number. 94 95irq_domain_add_linear() and irq_domain_create_linear() are functionally 96equivalent, except for the first argument is different - the former 97accepts an Open Firmware specific 'struct device_node', while the latter 98accepts a more general abstraction 'struct fwnode_handle'. 99 100The majority of drivers should use the linear map. 101 102Tree 103---- 104 105:: 106 107 irq_domain_add_tree() 108 irq_domain_create_tree() 109 110The irq_domain maintains a radix tree map from hwirq numbers to Linux 111IRQs. When an hwirq is mapped, an irq_desc is allocated and the 112hwirq is used as the lookup key for the radix tree. 113 114The tree map is a good choice if the hwirq number can be very large 115since it doesn't need to allocate a table as large as the largest 116hwirq number. The disadvantage is that hwirq to IRQ number lookup is 117dependent on how many entries are in the table. 118 119irq_domain_add_tree() and irq_domain_create_tree() are functionally 120equivalent, except for the first argument is different - the former 121accepts an Open Firmware specific 'struct device_node', while the latter 122accepts a more general abstraction 'struct fwnode_handle'. 123 124Very few drivers should need this mapping. 125 126No Map 127------ 128 129:: 130 131 irq_domain_add_nomap() 132 133The No Map mapping is to be used when the hwirq number is 134programmable in the hardware. In this case it is best to program the 135Linux IRQ number into the hardware itself so that no mapping is 136required. Calling irq_create_direct_mapping() will allocate a Linux 137IRQ number and call the .map() callback so that driver can program the 138Linux IRQ number into the hardware. 139 140Most drivers cannot use this mapping. 141 142Legacy 143------ 144 145:: 146 147 irq_domain_add_simple() 148 irq_domain_add_legacy() 149 irq_domain_add_legacy_isa() 150 151The Legacy mapping is a special case for drivers that already have a 152range of irq_descs allocated for the hwirqs. It is used when the 153driver cannot be immediately converted to use the linear mapping. For 154example, many embedded system board support files use a set of #defines 155for IRQ numbers that are passed to struct device registrations. In that 156case the Linux IRQ numbers cannot be dynamically assigned and the legacy 157mapping should be used. 158 159The legacy map assumes a contiguous range of IRQ numbers has already 160been allocated for the controller and that the IRQ number can be 161calculated by adding a fixed offset to the hwirq number, and 162visa-versa. The disadvantage is that it requires the interrupt 163controller to manage IRQ allocations and it requires an irq_desc to be 164allocated for every hwirq, even if it is unused. 165 166The legacy map should only be used if fixed IRQ mappings must be 167supported. For example, ISA controllers would use the legacy map for 168mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ 169numbers. 170 171Most users of legacy mappings should use irq_domain_add_simple() which 172will use a legacy domain only if an IRQ range is supplied by the 173system and will otherwise use a linear domain mapping. The semantics 174of this call are such that if an IRQ range is specified then 175descriptors will be allocated on-the-fly for it, and if no range is 176specified it will fall through to irq_domain_add_linear() which means 177*no* irq descriptors will be allocated. 178 179A typical use case for simple domains is where an irqchip provider 180is supporting both dynamic and static IRQ assignments. 181 182In order to avoid ending up in a situation where a linear domain is 183used and no descriptor gets allocated it is very important to make sure 184that the driver using the simple domain call irq_create_mapping() 185before any irq_find_mapping() since the latter will actually work 186for the static IRQ assignment case. 187 188Hierarchy IRQ domain 189-------------------- 190 191On some architectures, there may be multiple interrupt controllers 192involved in delivering an interrupt from the device to the target CPU. 193Let's look at a typical interrupt delivering path on x86 platforms:: 194 195 Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU 196 197There are three interrupt controllers involved: 198 1991) IOAPIC controller 2002) Interrupt remapping controller 2013) Local APIC controller 202 203To support such a hardware topology and make software architecture match 204hardware architecture, an irq_domain data structure is built for each 205interrupt controller and those irq_domains are organized into hierarchy. 206When building irq_domain hierarchy, the irq_domain near to the device is 207child and the irq_domain near to CPU is parent. So a hierarchy structure 208as below will be built for the example above:: 209 210 CPU Vector irq_domain (root irq_domain to manage CPU vectors) 211 ^ 212 | 213 Interrupt Remapping irq_domain (manage irq_remapping entries) 214 ^ 215 | 216 IOAPIC irq_domain (manage IOAPIC delivery entries/pins) 217 218There are four major interfaces to use hierarchy irq_domain: 219 2201) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt 221 controller related resources to deliver these interrupts. 2222) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller 223 related resources associated with these interrupts. 2243) irq_domain_activate_irq(): activate interrupt controller hardware to 225 deliver the interrupt. 2264) irq_domain_deactivate_irq(): deactivate interrupt controller hardware 227 to stop delivering the interrupt. 228 229Following changes are needed to support hierarchy irq_domain: 230 2311) a new field 'parent' is added to struct irq_domain; it's used to 232 maintain irq_domain hierarchy information. 2332) a new field 'parent_data' is added to struct irq_data; it's used to 234 build hierarchy irq_data to match hierarchy irq_domains. The irq_data 235 is used to store irq_domain pointer and hardware irq number. 2363) new callbacks are added to struct irq_domain_ops to support hierarchy 237 irq_domain operations. 238 239With support of hierarchy irq_domain and hierarchy irq_data ready, an 240irq_domain structure is built for each interrupt controller, and an 241irq_data structure is allocated for each irq_domain associated with an 242IRQ. Now we could go one step further to support stacked(hierarchy) 243irq_chip. That is, an irq_chip is associated with each irq_data along 244the hierarchy. A child irq_chip may implement a required action by 245itself or by cooperating with its parent irq_chip. 246 247With stacked irq_chip, interrupt controller driver only needs to deal 248with the hardware managed by itself and may ask for services from its 249parent irq_chip when needed. So we could achieve a much cleaner 250software architecture. 251 252For an interrupt controller driver to support hierarchy irq_domain, it 253needs to: 254 2551) Implement irq_domain_ops.alloc and irq_domain_ops.free 2562) Optionally implement irq_domain_ops.activate and 257 irq_domain_ops.deactivate. 2583) Optionally implement an irq_chip to manage the interrupt controller 259 hardware. 2604) No need to implement irq_domain_ops.map and irq_domain_ops.unmap, 261 they are unused with hierarchy irq_domain. 262 263Hierarchy irq_domain is in no way x86 specific, and is heavily used to 264support other architectures, such as ARM, ARM64 etc. 265 266=== Debugging === 267 268Most of the internals of the IRQ subsystem are exposed in debugfs by 269turning CONFIG_GENERIC_IRQ_DEBUGFS on. 270
