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+irq_domain interrupt number mapping library
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+
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+The current design of the Linux kernel uses a single large number
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+space where each separate IRQ source is assigned a different number.
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+This is simple when there is only one interrupt controller, but in
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+systems with multiple interrupt controllers the kernel must ensure
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+that each one gets assigned non-overlapping allocations of Linux
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+IRQ numbers.
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+
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+The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
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+irq numbers, but they don't provide any support for reverse mapping of
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+the controller-local IRQ (hwirq) number into the Linux IRQ number
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+space.
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+
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+The irq_domain library adds mapping between hwirq and IRQ numbers on
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+top of the irq_alloc_desc*() API. An irq_domain to manage mapping is
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+preferred over interrupt controller drivers open coding their own
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+reverse mapping scheme.
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+
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+irq_domain also implements translation from Device Tree interrupt
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+specifiers to hwirq numbers, and can be easily extended to support
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+other IRQ topology data sources.
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+
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+=== irq_domain usage ===
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+An interrupt controller driver creates and registers an irq_domain by
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+calling one of the irq_domain_add_*() functions (each mapping method
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+has a different allocator function, more on that later). The function
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+will return a pointer to the irq_domain on success. The caller must
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+provide the allocator function with an irq_domain_ops structure with
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+the .map callback populated as a minimum.
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+
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+In most cases, the irq_domain will begin empty without any mappings
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+between hwirq and IRQ numbers. Mappings are added to the irq_domain
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+by calling irq_create_mapping() which accepts the irq_domain and a
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+hwirq number as arguments. If a mapping for the hwirq doesn't already
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+exist then it will allocate a new Linux irq_desc, associate it with
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+the hwirq, and call the .map() callback so the driver can perform any
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+required hardware setup.
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+
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+When an interrupt is received, irq_find_mapping() function should
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+be used to find the Linux IRQ number from the hwirq number.
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+
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+If the driver has the Linux IRQ number or the irq_data pointer, and
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+needs to know the associated hwirq number (such as in the irq_chip
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+callbacks) then it can be directly obtained from irq_data->hwirq.
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+
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+=== Types of irq_domain mappings ===
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+There are several mechanisms available for reverse mapping from hwirq
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+to Linux irq, and each mechanism uses a different allocation function.
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+Which reverse map type should be used depends on the use case. Each
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+of the reverse map types are described below:
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+
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+==== Linear ====
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+irq_domain_add_linear()
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+
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+The linear reverse map maintains a fixed size table indexed by the
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+hwirq number. When a hwirq is mapped, an irq_desc is allocated for
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+the hwirq, and the IRQ number is stored in the table.
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+
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+The Linear map is a good choice when the maximum number of hwirqs is
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+fixed and a relatively small number (~ < 256). The advantages of this
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+map are fixed time lookup for IRQ numbers, and irq_descs are only
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+allocated for in-use IRQs. The disadvantage is that the table must be
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+as large as the largest possible hwirq number.
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+
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+The majority of drivers should use the linear map.
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+
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+==== Tree ====
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+irq_domain_add_tree()
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+
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+The irq_domain maintains a radix tree map from hwirq numbers to Linux
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+IRQs. When an hwirq is mapped, an irq_desc is allocated and the
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+hwirq is used as the lookup key for the radix tree.
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+
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+The tree map is a good choice if the hwirq number can be very large
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+since it doesn't need to allocate a table as large as the largest
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+hwirq number. The disadvantage is that hwirq to IRQ number lookup is
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+dependent on how many entries are in the table.
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+
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+Very few drivers should need this mapping. At the moment, powerpc
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+iseries is the only user.
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+
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+==== No Map ===-
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+irq_domain_add_nomap()
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+
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+The No Map mapping is to be used when the hwirq number is
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+programmable in the hardware. In this case it is best to program the
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+Linux IRQ number into the hardware itself so that no mapping is
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+required. Calling irq_create_direct_mapping() will allocate a Linux
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+IRQ number and call the .map() callback so that driver can program the
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+Linux IRQ number into the hardware.
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+
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+Most drivers cannot use this mapping.
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+
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+==== Legacy ====
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+irq_domain_add_legacy()
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+irq_domain_add_legacy_isa()
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+
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+The Legacy mapping is a special case for drivers that already have a
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+range of irq_descs allocated for the hwirqs. It is used when the
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+driver cannot be immediately converted to use the linear mapping. For
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+example, many embedded system board support files use a set of #defines
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+for IRQ numbers that are passed to struct device registrations. In that
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+case the Linux IRQ numbers cannot be dynamically assigned and the legacy
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+mapping should be used.
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+
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+The legacy map assumes a contiguous range of IRQ numbers has already
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+been allocated for the controller and that the IRQ number can be
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+calculated by adding a fixed offset to the hwirq number, and
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+visa-versa. The disadvantage is that it requires the interrupt
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+controller to manage IRQ allocations and it requires an irq_desc to be
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+allocated for every hwirq, even if it is unused.
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+
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+The legacy map should only be used if fixed IRQ mappings must be
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+supported. For example, ISA controllers would use the legacy map for
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+mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
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+numbers.
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Grant Likely