1irq_domain interrupt number mapping library
   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.
  10The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
  11irq numbers, but they don't provide any support for reverse mapping of
  12the controller-local IRQ (hwirq) number into the Linux IRQ number
  15The irq_domain library adds mapping between hwirq and IRQ numbers on
  16top of the irq_alloc_desc*() API.  An irq_domain to manage mapping is
  17preferred over interrupt controller drivers open coding their own
  18reverse mapping scheme.
  20irq_domain also implements translation from Device Tree interrupt
  21specifiers to hwirq numbers, and can be easily extended to support
  22other IRQ topology data sources.
  24=== irq_domain usage ===
  25An interrupt controller driver creates and registers an irq_domain by
  26calling one of the irq_domain_add_*() functions (each mapping method
  27has a different allocator function, more on that later).  The function
  28will return a pointer to the irq_domain on success.  The caller must
  29provide the allocator function with an irq_domain_ops structure with
  30the .map callback populated as a minimum.
  32In most cases, the irq_domain will begin empty without any mappings
  33between hwirq and IRQ numbers.  Mappings are added to the irq_domain
  34by calling irq_create_mapping() which accepts the irq_domain and a
  35hwirq number as arguments.  If a mapping for the hwirq doesn't already
  36exist then it will allocate a new Linux irq_desc, associate it with
  37the hwirq, and call the .map() callback so the driver can perform any
  38required hardware setup.
  40When an interrupt is received, irq_find_mapping() function should
  41be used to find the Linux IRQ number from the hwirq number.
  43If the driver has the Linux IRQ number or the irq_data pointer, and
  44needs to know the associated hwirq number (such as in the irq_chip
  45callbacks) then it can be directly obtained from irq_data->hwirq.
  47=== Types of irq_domain mappings ===
  48There are several mechanisms available for reverse mapping from hwirq
  49to Linux irq, and each mechanism uses a different allocation function.
  50Which reverse map type should be used depends on the use case.  Each
  51of the reverse map types are described below:
  53==== Linear ====
  56The linear reverse map maintains a fixed size table indexed by the
  57hwirq number.  When a hwirq is mapped, an irq_desc is allocated for
  58the hwirq, and the IRQ number is stored in the table.
  60The Linear map is a good choice when the maximum number of hwirqs is
  61fixed and a relatively small number (~ < 256).  The advantages of this
  62map are fixed time lookup for IRQ numbers, and irq_descs are only
  63allocated for in-use IRQs.  The disadvantage is that the table must be
  64as large as the largest possible hwirq number.
  66The majority of drivers should use the linear map.
  68==== Tree ====
  71The irq_domain maintains a radix tree map from hwirq numbers to Linux
  72IRQs.  When an hwirq is mapped, an irq_desc is allocated and the
  73hwirq is used as the lookup key for the radix tree.
  75The tree map is a good choice if the hwirq number can be very large
  76since it doesn't need to allocate a table as large as the largest
  77hwirq number.  The disadvantage is that hwirq to IRQ number lookup is
  78dependent on how many entries are in the table.
  80Very few drivers should need this mapping.  At the moment, powerpc
  81iseries is the only user.
  83==== No Map ===-
  86The No Map mapping is to be used when the hwirq number is
  87programmable in the hardware.  In this case it is best to program the
  88Linux IRQ number into the hardware itself so that no mapping is
  89required.  Calling irq_create_direct_mapping() will allocate a Linux
  90IRQ number and call the .map() callback so that driver can program the
  91Linux IRQ number into the hardware.
  93Most drivers cannot use this mapping.
  95==== Legacy ====
  99The Legacy mapping is a special case for drivers that already have a
 100range of irq_descs allocated for the hwirqs.  It is used when the
 101driver cannot be immediately converted to use the linear mapping.  For
 102example, many embedded system board support files use a set of #defines
 103for IRQ numbers that are passed to struct device registrations.  In that
 104case the Linux IRQ numbers cannot be dynamically assigned and the legacy
 105mapping should be used.
 107The legacy map assumes a contiguous range of IRQ numbers has already
 108been allocated for the controller and that the IRQ number can be
 109calculated by adding a fixed offset to the hwirq number, and
 110visa-versa.  The disadvantage is that it requires the interrupt
 111controller to manage IRQ allocations and it requires an irq_desc to be
 112allocated for every hwirq, even if it is unused.
 114The legacy map should only be used if fixed IRQ mappings must be
 115supported.  For example, ISA controllers would use the legacy map for
 116mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
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