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+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
+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 Device Tree interrupt
+specifiers to hwirq numbers, 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_*() 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 with
+the .map callback populated as a minimum.
+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.
+When an interrupt is received, irq_find_mapping() function should
+be used to find the Linux IRQ number from the hwirq number.
+The irq_create_mapping() function must be called *atleast 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 ====
+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 ====
+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. At the moment, powerpc
+iseries is the only user.
+==== No Map ===-
+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.
+==== 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.
+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
+Most users of legacy mappings should use irq_domain_add_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() which meand
+*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.