linux/Documentation/IRQ-domain.txt
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   1irq_domain interrupt number mapping library
   2
   3The current design of the Linux kernel uses a single large number
   4space where each separate IRQ source is assigned a different number.
   5This is simple when there is only one interrupt controller, but in
   6systems with multiple interrupt controllers the kernel must ensure
   7that each one gets assigned non-overlapping allocations of Linux
   8IRQ numbers.
   9
  10The number of interrupt controllers registered as unique irqchips
  11show a rising tendency: for example subdrivers of different kinds
  12such as GPIO controllers avoid reimplementing identical callback
  13mechanisms as the IRQ core system by modelling their interrupt
  14handlers as irqchips, i.e. in effect cascading interrupt controllers.
  15
  16Here the interrupt number loose all kind of correspondence to
  17hardware interrupt numbers: whereas in the past, IRQ numbers could
  18be chosen so they matched the hardware IRQ line into the root
  19interrupt controller (i.e. the component actually fireing the
  20interrupt line to the CPU) nowadays this number is just a number.
  21
  22For this reason we need a mechanism to separate controller-local
  23interrupt numbers, called hardware irq's, from Linux IRQ numbers.
  24
  25The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
  26irq numbers, but they don't provide any support for reverse mapping of
  27the controller-local IRQ (hwirq) number into the Linux IRQ number
  28space.
  29
  30The irq_domain library adds mapping between hwirq and IRQ numbers on
  31top of the irq_alloc_desc*() API.  An irq_domain to manage mapping is
  32preferred over interrupt controller drivers open coding their own
  33reverse mapping scheme.
  34
  35irq_domain also implements translation from Device Tree interrupt
  36specifiers to hwirq numbers, and can be easily extended to support
  37other IRQ topology data sources.
  38
  39=== irq_domain usage ===
  40An interrupt controller driver creates and registers an irq_domain by
  41calling one of the irq_domain_add_*() functions (each mapping method
  42has a different allocator function, more on that later).  The function
  43will return a pointer to the irq_domain on success.  The caller must
  44provide the allocator function with an irq_domain_ops structure with
  45the .map callback populated as a minimum.
  46
  47In most cases, the irq_domain will begin empty without any mappings
  48between hwirq and IRQ numbers.  Mappings are added to the irq_domain
  49by calling irq_create_mapping() which accepts the irq_domain and a
  50hwirq number as arguments.  If a mapping for the hwirq doesn't already
  51exist then it will allocate a new Linux irq_desc, associate it with
  52the hwirq, and call the .map() callback so the driver can perform any
  53required hardware setup.
  54
  55When an interrupt is received, irq_find_mapping() function should
  56be used to find the Linux IRQ number from the hwirq number.
  57
  58The irq_create_mapping() function must be called *atleast once*
  59before any call to irq_find_mapping(), lest the descriptor will not
  60be allocated.
  61
  62If the driver has the Linux IRQ number or the irq_data pointer, and
  63needs to know the associated hwirq number (such as in the irq_chip
  64callbacks) then it can be directly obtained from irq_data->hwirq.
  65
  66=== Types of irq_domain mappings ===
  67There are several mechanisms available for reverse mapping from hwirq
  68to Linux irq, and each mechanism uses a different allocation function.
  69Which reverse map type should be used depends on the use case.  Each
  70of the reverse map types are described below:
  71
  72==== Linear ====
  73irq_domain_add_linear()
  74
  75The linear reverse map maintains a fixed size table indexed by the
  76hwirq number.  When a hwirq is mapped, an irq_desc is allocated for
  77the hwirq, and the IRQ number is stored in the table.
  78
  79The Linear map is a good choice when the maximum number of hwirqs is
  80fixed and a relatively small number (~ < 256).  The advantages of this
  81map are fixed time lookup for IRQ numbers, and irq_descs are only
  82allocated for in-use IRQs.  The disadvantage is that the table must be
  83as large as the largest possible hwirq number.
  84
  85The majority of drivers should use the linear map.
  86
  87==== Tree ====
  88irq_domain_add_tree()
  89
  90The irq_domain maintains a radix tree map from hwirq numbers to Linux
  91IRQs.  When an hwirq is mapped, an irq_desc is allocated and the
  92hwirq is used as the lookup key for the radix tree.
  93
  94The tree map is a good choice if the hwirq number can be very large
  95since it doesn't need to allocate a table as large as the largest
  96hwirq number.  The disadvantage is that hwirq to IRQ number lookup is
  97dependent on how many entries are in the table.
  98
  99Very few drivers should need this mapping.  At the moment, powerpc
 100iseries is the only user.
 101
 102==== No Map ===-
 103irq_domain_add_nomap()
 104
 105The No Map mapping is to be used when the hwirq number is
 106programmable in the hardware.  In this case it is best to program the
 107Linux IRQ number into the hardware itself so that no mapping is
 108required.  Calling irq_create_direct_mapping() will allocate a Linux
 109IRQ number and call the .map() callback so that driver can program the
 110Linux IRQ number into the hardware.
 111
 112Most drivers cannot use this mapping.
 113
 114==== Legacy ====
 115irq_domain_add_simple()
 116irq_domain_add_legacy()
 117irq_domain_add_legacy_isa()
 118
 119The Legacy mapping is a special case for drivers that already have a
 120range of irq_descs allocated for the hwirqs.  It is used when the
 121driver cannot be immediately converted to use the linear mapping.  For
 122example, many embedded system board support files use a set of #defines
 123for IRQ numbers that are passed to struct device registrations.  In that
 124case the Linux IRQ numbers cannot be dynamically assigned and the legacy
 125mapping should be used.
 126
 127The legacy map assumes a contiguous range of IRQ numbers has already
 128been allocated for the controller and that the IRQ number can be
 129calculated by adding a fixed offset to the hwirq number, and
 130visa-versa.  The disadvantage is that it requires the interrupt
 131controller to manage IRQ allocations and it requires an irq_desc to be
 132allocated for every hwirq, even if it is unused.
 133
 134The legacy map should only be used if fixed IRQ mappings must be
 135supported.  For example, ISA controllers would use the legacy map for
 136mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
 137numbers.
 138
 139Most users of legacy mappings should use irq_domain_add_simple() which
 140will use a legacy domain only if an IRQ range is supplied by the
 141system and will otherwise use a linear domain mapping. The semantics
 142of this call are such that if an IRQ range is specified then
 143descriptors will be allocated on-the-fly for it, and if no range is
 144specified it will fall through to irq_domain_add_linear() which meand
 145*no* irq descriptors will be allocated.
 146
 147A typical use case for simple domains is where an irqchip provider
 148is supporting both dynamic and static IRQ assignments.
 149
 150In order to avoid ending up in a situation where a linear domain is
 151used and no descriptor gets allocated it is very important to make sure
 152that the driver using the simple domain call irq_create_mapping()
 153before any irq_find_mapping() since the latter will actually work
 154for the static IRQ assignment case.
 155
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