=============================================== 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 unique 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 shows 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. So in the past, IRQ numbers could be chosen so that they match the hardware IRQ line into the root interrupt controller (i.e. the component actually firing the interrupt line to the CPU). Nowadays, this number is just a number and the number loose all kind of correspondence to hardware interrupt numbers. For this reason, we need a mechanism to separate controller-local interrupt numbers, called hardware IRQs, 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 a mapping between hwirq and IRQ numbers on top of the irq_alloc_desc*() API. An irq_domain to manage the mapping is preferred over interrupt controller drivers open coding their own reverse mapping scheme. irq_domain also implements a translation from an abstract struct irq_fwspec to hwirq numbers (Device Tree, non-DT firmware node, ACPI GSI, and software node so far), and can be easily extended to support other IRQ topology data sources. The implementation is performed without any extra platform support code. irq_domain Usage ================ struct irq_domain could be defined as an irq domain controller. That is, it handles the mapping between hardware and virtual interrupt numbers for a given interrupt domain. The domain structure is generally created by the PIC code for a given PIC instance (though a domain can cover more than one PIC if they have a flat number model). It is the domain callbacks that are responsible for setting the irq_chip on a given irq_desc after it has been mapped. The host code and data structures use a fwnode_handle pointer to identify the domain. In some cases, and in order to preserve source code compatibility, this fwnode pointer is "upgraded" to a DT device_node. For those firmware infrastructures that do not provide a unique identifier for an interrupt controller, the irq_domain code offers a fwnode allocator. An interrupt controller driver creates and registers a struct irq_domain by calling one of the irq_domain_create_*() functions (each mapping method has a different allocator function, more on that later). The function will return a pointer to the struct irq_domain on success. The caller must provide the allocator function with a struct irq_domain_ops pointer. 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, irq_create_mapping() allocates a new Linux irq_desc, associates it with the hwirq, and calls the :c:member:`irq_domain_ops.map()` callback. In there, 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 - 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 :c:member:`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_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. The majority of drivers should use the Linear map. 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. Very few drivers should need this mapping. No Map ------ :: irq_domain_create_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_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_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_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. 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. The following is needed to support hierarchy irq_domain: 1) The :c:member:`parent` field in struct irq_domain is used to maintain irq_domain hierarchy information. 2) The :c:member:`parent_data` field in struct irq_data is 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) The :c:member:`alloc()`, :c:member:`free()`, and other callbacks in struct irq_domain_ops to support hierarchy irq_domain operations. With the 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. 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) There is no need to implement irq_domain_ops.map() and irq_domain_ops.unmap(). They are unused with hierarchy irq_domain. Note the hierarchy irq_domain is in no way x86-specific, and is heavily used to support other architectures, such as ARM, ARM64 etc. Stacked irq_chip ~~~~~~~~~~~~~~~~ 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. Debugging ========= Most of the internals of the IRQ subsystem are exposed in debugfs by turning CONFIG_GENERIC_IRQ_DEBUGFS on. Structures and Public Functions Provided ======================================== This chapter contains the autogenerated documentation of the structures and exported kernel API functions which are used for IRQ domains. .. kernel-doc:: include/linux/irqdomain.h .. kernel-doc:: kernel/irq/irqdomain.c :export: Internal Functions Provided =========================== This chapter contains the autogenerated documentation of the internal functions. .. kernel-doc:: kernel/irq/irqdomain.c :internal: