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 ===============================================
 The 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
 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 an abstract irq_fwspec
 structure to hwirq numbers (Device Tree and ACPI GSI so far), 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_*() or irq_domain_create_*() 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.
 
 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.
 
 Once a mapping has been established, it can be retrieved or used via a
 variety of methods:
 
 - irq_resolve_mapping() returns a pointer to the irq_desc structure
   for a given domain and hwirq number, and NULL if there was no
   mapping.
 - irq_find_mapping() returns a Linux IRQ number for a given domain and
   hwirq number, and 0 if there was no mapping
 - irq_linear_revmap() is now identical to irq_find_mapping(), and is
   deprecated
 - generic_handle_domain_irq() handles an interrupt described by a
   domain and a hwirq number
 
 Note that irq domain lookups must happen in contexts that are
 compatible with a RCU read-side critical section.
 
 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
 ------
 
 ::
 
         irq_domain_add_linear()
         irq_domain_create_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.
 
 irq_domain_add_linear() and irq_domain_create_linear() are functionally
 equivalent, except for the first argument is different - the former
 accepts an Open Firmware specific 'struct device_node', while the latter
 accepts a more general abstraction 'struct fwnode_handle'.
 
 The majority of drivers should use the linear map.
 
 Tree
 ----
 
 ::
 
         irq_domain_add_tree()
         irq_domain_create_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.
 
 irq_domain_add_tree() and irq_domain_create_tree() are functionally
 equivalent, except for the first argument is different - the former
 accepts an Open Firmware specific 'struct device_node', while the latter
 accepts a more general abstraction 'struct fwnode_handle'.
 
 Very few drivers should need this mapping.
 
 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, and it is now gated on the
 CONFIG_IRQ_DOMAIN_NOMAP option. Please refrain from introducing new
 users of this API.
 
 Legacy
 ------
 
 ::
 
         irq_domain_add_simple()
         irq_domain_add_legacy()
         irq_domain_create_simple()
         irq_domain_create_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.
 
 As the name implies, the \*_legacy() functions are deprecated and only
 exist to ease the support of ancient platforms. No new users should be
 added. Same goes for the \*_simple() functions when their use results
 in the legacy behaviour.
 
 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.
 
 Most users of legacy mappings should use irq_domain_add_simple() or
 irq_domain_create_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() or
 irq_domain_create_linear() which means *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.
 
 irq_domain_add_simple() and irq_domain_create_simple() as well as
 irq_domain_add_legacy() and irq_domain_create_legacy() are functionally
 equivalent, except for the first argument is different - the former
 accepts an Open Firmware specific 'struct device_node', while the latter
 accepts a more general abstraction 'struct fwnode_handle'.
 
 Hierarchy IRQ domain
 --------------------
 
 On some architectures, there may be multiple interrupt controllers
 involved in delivering an interrupt from the device to the target CPU.
 Let's look at a typical interrupt delivering path on x86 platforms::
 
   Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU
 
 There are three interrupt controllers involved:
 
 1) IOAPIC controller
 2) Interrupt remapping controller
 3) Local APIC controller
 
 To support such a hardware topology and make software architecture match
 hardware architecture, an irq_domain data structure is built for each
 interrupt controller and those irq_domains are organized into hierarchy.
 When building irq_domain hierarchy, the irq_domain near to the device is
 child and the irq_domain near to CPU is parent. So a hierarchy structure
 as below will be built for the example above::
 
         CPU Vector irq_domain (root irq_domain to manage CPU vectors)
                 ^
                 |
         Interrupt Remapping irq_domain (manage irq_remapping entries)
                 ^
                 |
         IOAPIC irq_domain (manage IOAPIC delivery entries/pins)
 
 There are four major interfaces to use hierarchy irq_domain:
 
 1) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt
    controller related resources to deliver these interrupts.
 2) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller
    related resources associated with these interrupts.
 3) irq_domain_activate_irq(): activate interrupt controller hardware to
    deliver the interrupt.
 4) irq_domain_deactivate_irq(): deactivate interrupt controller hardware
    to stop delivering the interrupt.
 
 Following changes are needed to support hierarchy irq_domain:
 
 1) a new field 'parent' is added to struct irq_domain; it's used to
    maintain irq_domain hierarchy information.
 2) a new field 'parent_data' is added to struct irq_data; it's used to
    build hierarchy irq_data to match hierarchy irq_domains. The irq_data
    is used to store irq_domain pointer and hardware irq number.
 3) new callbacks are added to struct irq_domain_ops to support hierarchy
    irq_domain operations.
 
 With support of hierarchy irq_domain and hierarchy irq_data ready, an
 irq_domain structure is built for each interrupt controller, and an
 irq_data structure is allocated for each irq_domain associated with an
 IRQ. Now we could go one step further to support stacked(hierarchy)
 irq_chip. That is, an irq_chip is associated with each irq_data along
 the hierarchy. A child irq_chip may implement a required action by
 itself or by cooperating with its parent irq_chip.
 
 With stacked irq_chip, interrupt controller driver only needs to deal
 with the hardware managed by itself and may ask for services from its
 parent irq_chip when needed. So we could achieve a much cleaner
 software architecture.
 
 For an interrupt controller driver to support hierarchy irq_domain, it
 needs to:
 
 1) Implement irq_domain_ops.alloc and irq_domain_ops.free
 2) Optionally implement irq_domain_ops.activate and
    irq_domain_ops.deactivate.
 3) Optionally implement an irq_chip to manage the interrupt controller
    hardware.
 4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
    they are unused with hierarchy irq_domain.
 
 Hierarchy irq_domain is in no way x86 specific, and is heavily used to
 support other architectures, such as ARM, ARM64 etc.
 
 Debugging
 =========
 
 Most of the internals of the IRQ subsystem are exposed in debugfs by
 turning CONFIG_GENERIC_IRQ_DEBUGFS on.

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