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Diffstat (limited to 'Documentation/IRQ-domain.txt')
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diff --git a/Documentation/IRQ-domain.txt b/Documentation/IRQ-domain.txt deleted file mode 100644 index 9bc95942ec22..000000000000 --- a/Documentation/IRQ-domain.txt +++ /dev/null @@ -1,154 +0,0 @@ -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 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. - -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() - -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_simple() -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. - -Most users of legacy mappings should use irq_domain_add_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() which meand -*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. |