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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 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 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.
-
-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()
-
-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_add_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 ===-
-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.
-
-==== Legacy ====
-irq_domain_add_legacy()
-irq_domain_add_legacy_isa()
-
-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
-numbers.