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diff --git a/Documentation/core-api/irq/irq-domain.rst b/Documentation/core-api/irq/irq-domain.rst new file mode 100644 index 000000000..f88a6ee67 --- /dev/null +++ b/Documentation/core-api/irq/irq-domain.rst @@ -0,0 +1,297 @@ +=============================================== +The irq_domain interrupt number mapping library +=============================================== + +The current design of the Linux kernel uses a single large number +space where each separate IRQ source is assigned a different number. +This is simple when there is only one interrupt controller, but in +systems with multiple interrupt controllers the kernel must ensure +that each one gets assigned non-overlapping allocations of Linux +IRQ numbers. + +The number of interrupt controllers registered as unique irqchips +show a rising tendency: for example subdrivers of different kinds +such as GPIO controllers avoid reimplementing identical callback +mechanisms as the IRQ core system by modelling their interrupt +handlers as irqchips, i.e. in effect cascading interrupt controllers. + +Here the interrupt number loose all kind of correspondence to +hardware interrupt numbers: whereas in the past, IRQ numbers could +be chosen so they matched the hardware IRQ line into the root +interrupt controller (i.e. the component actually fireing the +interrupt line to the CPU) nowadays this number is just a number. + +For this reason we need a mechanism to separate controller-local +interrupt numbers, called hardware irq's, from Linux IRQ numbers. + +The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of +irq numbers, but they don't provide any support for reverse mapping of +the controller-local IRQ (hwirq) number into the Linux IRQ number +space. + +The irq_domain library adds mapping between hwirq and IRQ numbers on +top of the irq_alloc_desc*() API. An irq_domain to manage mapping is +preferred over interrupt controller drivers open coding their own +reverse mapping scheme. + +irq_domain also implements translation from an abstract irq_fwspec +structure to hwirq numbers (Device Tree and ACPI GSI so far), and can +be easily extended to support other IRQ topology data sources. + +irq_domain usage +================ + +An interrupt controller driver creates and registers an irq_domain by +calling one of the irq_domain_add_*() or irq_domain_create_*() functions +(each mapping method has a different allocator function, more on that later). +The function will return a pointer to the irq_domain on success. The caller +must provide the allocator function with an irq_domain_ops structure. + +In most cases, the irq_domain will begin empty without any mappings +between hwirq and IRQ numbers. Mappings are added to the irq_domain +by calling irq_create_mapping() which accepts the irq_domain and a +hwirq number as arguments. If a mapping for the hwirq doesn't already +exist then it will allocate a new Linux irq_desc, associate it with +the hwirq, and call the .map() callback so the driver can perform any +required hardware setup. + +Once a mapping has been established, it can be retrieved or used via a +variety of methods: + +- irq_resolve_mapping() returns a pointer to the irq_desc structure + for a given domain and hwirq number, and NULL if there was no + mapping. +- irq_find_mapping() returns a Linux IRQ number for a given domain and + hwirq number, and 0 if there was no mapping +- irq_linear_revmap() is now identical to irq_find_mapping(), and is + deprecated +- generic_handle_domain_irq() handles an interrupt described by a + domain and a hwirq number + +Note that irq domain lookups must happen in contexts that are +compatible with a RCU read-side critical section. + +The irq_create_mapping() function must be called *at least once* +before any call to irq_find_mapping(), lest the descriptor will not +be allocated. + +If the driver has the Linux IRQ number or the irq_data pointer, and +needs to know the associated hwirq number (such as in the irq_chip +callbacks) then it can be directly obtained from irq_data->hwirq. + +Types of irq_domain mappings +============================ + +There are several mechanisms available for reverse mapping from hwirq +to Linux irq, and each mechanism uses a different allocation function. +Which reverse map type should be used depends on the use case. Each +of the reverse map types are described below: + +Linear +------ + +:: + + irq_domain_add_linear() + irq_domain_create_linear() + +The linear reverse map maintains a fixed size table indexed by the +hwirq number. When a hwirq is mapped, an irq_desc is allocated for +the hwirq, and the IRQ number is stored in the table. + +The Linear map is a good choice when the maximum number of hwirqs is +fixed and a relatively small number (~ < 256). The advantages of this +map are fixed time lookup for IRQ numbers, and irq_descs are only +allocated for in-use IRQs. The disadvantage is that the table must be +as large as the largest possible hwirq number. + +irq_domain_add_linear() and irq_domain_create_linear() are functionally +equivalent, except for the first argument is different - the former +accepts an Open Firmware specific 'struct device_node', while the latter +accepts a more general abstraction 'struct fwnode_handle'. + +The majority of drivers should use the linear map. + +Tree +---- + +:: + + irq_domain_add_tree() + irq_domain_create_tree() + +The irq_domain maintains a radix tree map from hwirq numbers to Linux +IRQs. When an hwirq is mapped, an irq_desc is allocated and the +hwirq is used as the lookup key for the radix tree. + +The tree map is a good choice if the hwirq number can be very large +since it doesn't need to allocate a table as large as the largest +hwirq number. The disadvantage is that hwirq to IRQ number lookup is +dependent on how many entries are in the table. + +irq_domain_add_tree() and irq_domain_create_tree() are functionally +equivalent, except for the first argument is different - the former +accepts an Open Firmware specific 'struct device_node', while the latter +accepts a more general abstraction 'struct fwnode_handle'. + +Very few drivers should need this mapping. + +No Map +------ + +:: + + irq_domain_add_nomap() + +The No Map mapping is to be used when the hwirq number is +programmable in the hardware. In this case it is best to program the +Linux IRQ number into the hardware itself so that no mapping is +required. Calling irq_create_direct_mapping() will allocate a Linux +IRQ number and call the .map() callback so that driver can program the +Linux IRQ number into the hardware. + +Most drivers cannot use this mapping, and it is now gated on the +CONFIG_IRQ_DOMAIN_NOMAP option. Please refrain from introducing new +users of this API. + +Legacy +------ + +:: + + irq_domain_add_simple() + irq_domain_add_legacy() + irq_domain_create_simple() + irq_domain_create_legacy() + +The Legacy mapping is a special case for drivers that already have a +range of irq_descs allocated for the hwirqs. It is used when the +driver cannot be immediately converted to use the linear mapping. For +example, many embedded system board support files use a set of #defines +for IRQ numbers that are passed to struct device registrations. In that +case the Linux IRQ numbers cannot be dynamically assigned and the legacy +mapping should be used. + +As the name implies, the \*_legacy() functions are deprecated and only +exist to ease the support of ancient platforms. No new users should be +added. Same goes for the \*_simple() functions when their use results +in the legacy behaviour. + +The legacy map assumes a contiguous range of IRQ numbers has already +been allocated for the controller and that the IRQ number can be +calculated by adding a fixed offset to the hwirq number, and +visa-versa. The disadvantage is that it requires the interrupt +controller to manage IRQ allocations and it requires an irq_desc to be +allocated for every hwirq, even if it is unused. + +The legacy map should only be used if fixed IRQ mappings must be +supported. For example, ISA controllers would use the legacy map for +mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ +numbers. + +Most users of legacy mappings should use irq_domain_add_simple() or +irq_domain_create_simple() which will use a legacy domain only if an IRQ range +is supplied by the system and will otherwise use a linear domain mapping. +The semantics of this call are such that if an IRQ range is specified then +descriptors will be allocated on-the-fly for it, and if no range is +specified it will fall through to irq_domain_add_linear() or +irq_domain_create_linear() which means *no* irq descriptors will be allocated. + +A typical use case for simple domains is where an irqchip provider +is supporting both dynamic and static IRQ assignments. + +In order to avoid ending up in a situation where a linear domain is +used and no descriptor gets allocated it is very important to make sure +that the driver using the simple domain call irq_create_mapping() +before any irq_find_mapping() since the latter will actually work +for the static IRQ assignment case. + +irq_domain_add_simple() and irq_domain_create_simple() as well as +irq_domain_add_legacy() and irq_domain_create_legacy() are functionally +equivalent, except for the first argument is different - the former +accepts an Open Firmware specific 'struct device_node', while the latter +accepts a more general abstraction 'struct fwnode_handle'. + +Hierarchy IRQ domain +-------------------- + +On some architectures, there may be multiple interrupt controllers +involved in delivering an interrupt from the device to the target CPU. +Let's look at a typical interrupt delivering path on x86 platforms:: + + Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU + +There are three interrupt controllers involved: + +1) IOAPIC controller +2) Interrupt remapping controller +3) Local APIC controller + +To support such a hardware topology and make software architecture match +hardware architecture, an irq_domain data structure is built for each +interrupt controller and those irq_domains are organized into hierarchy. +When building irq_domain hierarchy, the irq_domain near to the device is +child and the irq_domain near to CPU is parent. So a hierarchy structure +as below will be built for the example above:: + + CPU Vector irq_domain (root irq_domain to manage CPU vectors) + ^ + | + Interrupt Remapping irq_domain (manage irq_remapping entries) + ^ + | + IOAPIC irq_domain (manage IOAPIC delivery entries/pins) + +There are four major interfaces to use hierarchy irq_domain: + +1) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt + controller related resources to deliver these interrupts. +2) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller + related resources associated with these interrupts. +3) irq_domain_activate_irq(): activate interrupt controller hardware to + deliver the interrupt. +4) irq_domain_deactivate_irq(): deactivate interrupt controller hardware + to stop delivering the interrupt. + +Following changes are needed to support hierarchy irq_domain: + +1) a new field 'parent' is added to struct irq_domain; it's used to + maintain irq_domain hierarchy information. +2) a new field 'parent_data' is added to struct irq_data; it's used to + build hierarchy irq_data to match hierarchy irq_domains. The irq_data + is used to store irq_domain pointer and hardware irq number. +3) new callbacks are added to struct irq_domain_ops to support hierarchy + irq_domain operations. + +With support of hierarchy irq_domain and hierarchy irq_data ready, an +irq_domain structure is built for each interrupt controller, and an +irq_data structure is allocated for each irq_domain associated with an +IRQ. Now we could go one step further to support stacked(hierarchy) +irq_chip. That is, an irq_chip is associated with each irq_data along +the hierarchy. A child irq_chip may implement a required action by +itself or by cooperating with its parent irq_chip. + +With stacked irq_chip, interrupt controller driver only needs to deal +with the hardware managed by itself and may ask for services from its +parent irq_chip when needed. So we could achieve a much cleaner +software architecture. + +For an interrupt controller driver to support hierarchy irq_domain, it +needs to: + +1) Implement irq_domain_ops.alloc and irq_domain_ops.free +2) Optionally implement irq_domain_ops.activate and + irq_domain_ops.deactivate. +3) Optionally implement an irq_chip to manage the interrupt controller + hardware. +4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap, + they are unused with hierarchy irq_domain. + +Hierarchy irq_domain is in no way x86 specific, and is heavily used to +support other architectures, such as ARM, ARM64 etc. + +Debugging +========= + +Most of the internals of the IRQ subsystem are exposed in debugfs by +turning CONFIG_GENERIC_IRQ_DEBUGFS on. |