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1===============================================2The irq_domain interrupt number mapping library3===============================================4 5The current design of the Linux kernel uses a single large number6space where each separate IRQ source is assigned a different number.7This is simple when there is only one interrupt controller, but in8systems with multiple interrupt controllers the kernel must ensure9that each one gets assigned non-overlapping allocations of Linux10IRQ numbers.11 12The number of interrupt controllers registered as unique irqchips13show a rising tendency: for example subdrivers of different kinds14such as GPIO controllers avoid reimplementing identical callback15mechanisms as the IRQ core system by modelling their interrupt16handlers as irqchips, i.e. in effect cascading interrupt controllers.17 18Here the interrupt number loose all kind of correspondence to19hardware interrupt numbers: whereas in the past, IRQ numbers could20be chosen so they matched the hardware IRQ line into the root21interrupt controller (i.e. the component actually fireing the22interrupt line to the CPU) nowadays this number is just a number.23 24For this reason we need a mechanism to separate controller-local25interrupt numbers, called hardware irq's, from Linux IRQ numbers.26 27The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of28irq numbers, but they don't provide any support for reverse mapping of29the controller-local IRQ (hwirq) number into the Linux IRQ number30space.31 32The irq_domain library adds mapping between hwirq and IRQ numbers on33top of the irq_alloc_desc*() API. An irq_domain to manage mapping is34preferred over interrupt controller drivers open coding their own35reverse mapping scheme.36 37irq_domain also implements translation from an abstract irq_fwspec38structure to hwirq numbers (Device Tree and ACPI GSI so far), and can39be easily extended to support other IRQ topology data sources.40 41irq_domain usage42================43 44An interrupt controller driver creates and registers an irq_domain by45calling one of the irq_domain_add_*() or irq_domain_create_*() functions46(each mapping method has a different allocator function, more on that later).47The function will return a pointer to the irq_domain on success. The caller48must provide the allocator function with an irq_domain_ops structure.49 50In most cases, the irq_domain will begin empty without any mappings51between hwirq and IRQ numbers. Mappings are added to the irq_domain52by calling irq_create_mapping() which accepts the irq_domain and a53hwirq number as arguments. If a mapping for the hwirq doesn't already54exist then it will allocate a new Linux irq_desc, associate it with55the hwirq, and call the .map() callback so the driver can perform any56required hardware setup.57 58Once a mapping has been established, it can be retrieved or used via a59variety of methods:60 61- irq_resolve_mapping() returns a pointer to the irq_desc structure62 for a given domain and hwirq number, and NULL if there was no63 mapping.64- irq_find_mapping() returns a Linux IRQ number for a given domain and65 hwirq number, and 0 if there was no mapping66- irq_linear_revmap() is now identical to irq_find_mapping(), and is67 deprecated68- generic_handle_domain_irq() handles an interrupt described by a69 domain and a hwirq number70 71Note that irq domain lookups must happen in contexts that are72compatible with a RCU read-side critical section.73 74The irq_create_mapping() function must be called *at least once*75before any call to irq_find_mapping(), lest the descriptor will not76be allocated.77 78If the driver has the Linux IRQ number or the irq_data pointer, and79needs to know the associated hwirq number (such as in the irq_chip80callbacks) then it can be directly obtained from irq_data->hwirq.81 82Types of irq_domain mappings83============================84 85There are several mechanisms available for reverse mapping from hwirq86to Linux irq, and each mechanism uses a different allocation function.87Which reverse map type should be used depends on the use case. Each88of the reverse map types are described below:89 90Linear91------92 93::94 95 irq_domain_add_linear()96 irq_domain_create_linear()97 98The linear reverse map maintains a fixed size table indexed by the99hwirq number. When a hwirq is mapped, an irq_desc is allocated for100the hwirq, and the IRQ number is stored in the table.101 102The Linear map is a good choice when the maximum number of hwirqs is103fixed and a relatively small number (~ < 256). The advantages of this104map are fixed time lookup for IRQ numbers, and irq_descs are only105allocated for in-use IRQs. The disadvantage is that the table must be106as large as the largest possible hwirq number.107 108irq_domain_add_linear() and irq_domain_create_linear() are functionally109equivalent, except for the first argument is different - the former110accepts an Open Firmware specific 'struct device_node', while the latter111accepts a more general abstraction 'struct fwnode_handle'.112 113The majority of drivers should use the linear map.114 115Tree116----117 118::119 120 irq_domain_add_tree()121 irq_domain_create_tree()122 123The irq_domain maintains a radix tree map from hwirq numbers to Linux124IRQs. When an hwirq is mapped, an irq_desc is allocated and the125hwirq is used as the lookup key for the radix tree.126 127The tree map is a good choice if the hwirq number can be very large128since it doesn't need to allocate a table as large as the largest129hwirq number. The disadvantage is that hwirq to IRQ number lookup is130dependent on how many entries are in the table.131 132irq_domain_add_tree() and irq_domain_create_tree() are functionally133equivalent, except for the first argument is different - the former134accepts an Open Firmware specific 'struct device_node', while the latter135accepts a more general abstraction 'struct fwnode_handle'.136 137Very few drivers should need this mapping.138 139No Map140------141 142::143 144 irq_domain_add_nomap()145 146The No Map mapping is to be used when the hwirq number is147programmable in the hardware. In this case it is best to program the148Linux IRQ number into the hardware itself so that no mapping is149required. Calling irq_create_direct_mapping() will allocate a Linux150IRQ number and call the .map() callback so that driver can program the151Linux IRQ number into the hardware.152 153Most drivers cannot use this mapping, and it is now gated on the154CONFIG_IRQ_DOMAIN_NOMAP option. Please refrain from introducing new155users of this API.156 157Legacy158------159 160::161 162 irq_domain_add_simple()163 irq_domain_add_legacy()164 irq_domain_create_simple()165 irq_domain_create_legacy()166 167The Legacy mapping is a special case for drivers that already have a168range of irq_descs allocated for the hwirqs. It is used when the169driver cannot be immediately converted to use the linear mapping. For170example, many embedded system board support files use a set of #defines171for IRQ numbers that are passed to struct device registrations. In that172case the Linux IRQ numbers cannot be dynamically assigned and the legacy173mapping should be used.174 175As the name implies, the \*_legacy() functions are deprecated and only176exist to ease the support of ancient platforms. No new users should be177added. Same goes for the \*_simple() functions when their use results178in the legacy behaviour.179 180The legacy map assumes a contiguous range of IRQ numbers has already181been allocated for the controller and that the IRQ number can be182calculated by adding a fixed offset to the hwirq number, and183visa-versa. The disadvantage is that it requires the interrupt184controller to manage IRQ allocations and it requires an irq_desc to be185allocated for every hwirq, even if it is unused.186 187The legacy map should only be used if fixed IRQ mappings must be188supported. For example, ISA controllers would use the legacy map for189mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ190numbers.191 192Most users of legacy mappings should use irq_domain_add_simple() or193irq_domain_create_simple() which will use a legacy domain only if an IRQ range194is supplied by the system and will otherwise use a linear domain mapping.195The semantics of this call are such that if an IRQ range is specified then196descriptors will be allocated on-the-fly for it, and if no range is197specified it will fall through to irq_domain_add_linear() or198irq_domain_create_linear() which means *no* irq descriptors will be allocated.199 200A typical use case for simple domains is where an irqchip provider201is supporting both dynamic and static IRQ assignments.202 203In order to avoid ending up in a situation where a linear domain is204used and no descriptor gets allocated it is very important to make sure205that the driver using the simple domain call irq_create_mapping()206before any irq_find_mapping() since the latter will actually work207for the static IRQ assignment case.208 209irq_domain_add_simple() and irq_domain_create_simple() as well as210irq_domain_add_legacy() and irq_domain_create_legacy() are functionally211equivalent, except for the first argument is different - the former212accepts an Open Firmware specific 'struct device_node', while the latter213accepts a more general abstraction 'struct fwnode_handle'.214 215Hierarchy IRQ domain216--------------------217 218On some architectures, there may be multiple interrupt controllers219involved in delivering an interrupt from the device to the target CPU.220Let's look at a typical interrupt delivering path on x86 platforms::221 222 Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU223 224There are three interrupt controllers involved:225 2261) IOAPIC controller2272) Interrupt remapping controller2283) Local APIC controller229 230To support such a hardware topology and make software architecture match231hardware architecture, an irq_domain data structure is built for each232interrupt controller and those irq_domains are organized into hierarchy.233When building irq_domain hierarchy, the irq_domain near to the device is234child and the irq_domain near to CPU is parent. So a hierarchy structure235as below will be built for the example above::236 237 CPU Vector irq_domain (root irq_domain to manage CPU vectors)238 ^239 |240 Interrupt Remapping irq_domain (manage irq_remapping entries)241 ^242 |243 IOAPIC irq_domain (manage IOAPIC delivery entries/pins)244 245There are four major interfaces to use hierarchy irq_domain:246 2471) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt248 controller related resources to deliver these interrupts.2492) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller250 related resources associated with these interrupts.2513) irq_domain_activate_irq(): activate interrupt controller hardware to252 deliver the interrupt.2534) irq_domain_deactivate_irq(): deactivate interrupt controller hardware254 to stop delivering the interrupt.255 256Following changes are needed to support hierarchy irq_domain:257 2581) a new field 'parent' is added to struct irq_domain; it's used to259 maintain irq_domain hierarchy information.2602) a new field 'parent_data' is added to struct irq_data; it's used to261 build hierarchy irq_data to match hierarchy irq_domains. The irq_data262 is used to store irq_domain pointer and hardware irq number.2633) new callbacks are added to struct irq_domain_ops to support hierarchy264 irq_domain operations.265 266With support of hierarchy irq_domain and hierarchy irq_data ready, an267irq_domain structure is built for each interrupt controller, and an268irq_data structure is allocated for each irq_domain associated with an269IRQ. Now we could go one step further to support stacked(hierarchy)270irq_chip. That is, an irq_chip is associated with each irq_data along271the hierarchy. A child irq_chip may implement a required action by272itself or by cooperating with its parent irq_chip.273 274With stacked irq_chip, interrupt controller driver only needs to deal275with the hardware managed by itself and may ask for services from its276parent irq_chip when needed. So we could achieve a much cleaner277software architecture.278 279For an interrupt controller driver to support hierarchy irq_domain, it280needs to:281 2821) Implement irq_domain_ops.alloc and irq_domain_ops.free2832) Optionally implement irq_domain_ops.activate and284 irq_domain_ops.deactivate.2853) Optionally implement an irq_chip to manage the interrupt controller286 hardware.2874) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,288 they are unused with hierarchy irq_domain.289 290Hierarchy irq_domain is in no way x86 specific, and is heavily used to291support other architectures, such as ARM, ARM64 etc.292 293Debugging294=========295 296Most of the internals of the IRQ subsystem are exposed in debugfs by297turning CONFIG_GENERIC_IRQ_DEBUGFS on.298