Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.
1 irq_domain interrupt number mapping library 2 3 The current design of the Linux kernel uses a single large number 4 space where each separate IRQ source is assigned a different number. 5 This is simple when there is only one interrupt controller, but in 6 systems with multiple interrupt controllers the kernel must ensure 7 that each one gets assigned non-overlapping allocations of Linux 8 IRQ numbers. 9 10 The number of interrupt controllers registered as unique irqchips 11 show a rising tendency: for example subdrivers of different kinds 12 such as GPIO controllers avoid reimplementing identical callback 13 mechanisms as the IRQ core system by modelling their interrupt 14 handlers as irqchips, i.e. in effect cascading interrupt controllers. 15 16 Here the interrupt number loose all kind of correspondence to 17 hardware interrupt numbers: whereas in the past, IRQ numbers could 18 be chosen so they matched the hardware IRQ line into the root 19 interrupt controller (i.e. the component actually fireing the 20 interrupt line to the CPU) nowadays this number is just a number. 21 22 For this reason we need a mechanism to separate controller-local 23 interrupt numbers, called hardware irq's, from Linux IRQ numbers. 24 25 The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of 26 irq numbers, but they don't provide any support for reverse mapping of 27 the controller-local IRQ (hwirq) number into the Linux IRQ number 28 space. 29 30 The irq_domain library adds mapping between hwirq and IRQ numbers on 31 top of the irq_alloc_desc*() API. An irq_domain to manage mapping is 32 preferred over interrupt controller drivers open coding their own 33 reverse mapping scheme. 34 35 irq_domain also implements translation from Device Tree interrupt 36 specifiers to hwirq numbers, and can be easily extended to support 37 other IRQ topology data sources. 38 39 === irq_domain usage === 40 An interrupt controller driver creates and registers an irq_domain by 41 calling one of the irq_domain_add_*() functions (each mapping method 42 has a different allocator function, more on that later). The function 43 will return a pointer to the irq_domain on success. The caller must 44 provide the allocator function with an irq_domain_ops structure. 45 46 In most cases, the irq_domain will begin empty without any mappings 47 between hwirq and IRQ numbers. Mappings are added to the irq_domain 48 by calling irq_create_mapping() which accepts the irq_domain and a 49 hwirq number as arguments. If a mapping for the hwirq doesn't already 50 exist then it will allocate a new Linux irq_desc, associate it with 51 the hwirq, and call the .map() callback so the driver can perform any 52 required hardware setup. 53 54 When an interrupt is received, irq_find_mapping() function should 55 be used to find the Linux IRQ number from the hwirq number. 56 57 The irq_create_mapping() function must be called *atleast once* 58 before any call to irq_find_mapping(), lest the descriptor will not 59 be allocated. 60 61 If the driver has the Linux IRQ number or the irq_data pointer, and 62 needs to know the associated hwirq number (such as in the irq_chip 63 callbacks) then it can be directly obtained from irq_data->hwirq. 64 65 === Types of irq_domain mappings === 66 There are several mechanisms available for reverse mapping from hwirq 67 to Linux irq, and each mechanism uses a different allocation function. 68 Which reverse map type should be used depends on the use case. Each 69 of the reverse map types are described below: 70 71 ==== Linear ==== 72 irq_domain_add_linear() 73 74 The linear reverse map maintains a fixed size table indexed by the 75 hwirq number. When a hwirq is mapped, an irq_desc is allocated for 76 the hwirq, and the IRQ number is stored in the table. 77 78 The Linear map is a good choice when the maximum number of hwirqs is 79 fixed and a relatively small number (~ < 256). The advantages of this 80 map are fixed time lookup for IRQ numbers, and irq_descs are only 81 allocated for in-use IRQs. The disadvantage is that the table must be 82 as large as the largest possible hwirq number. 83 84 The majority of drivers should use the linear map. 85 86 ==== Tree ==== 87 irq_domain_add_tree() 88 89 The irq_domain maintains a radix tree map from hwirq numbers to Linux 90 IRQs. When an hwirq is mapped, an irq_desc is allocated and the 91 hwirq is used as the lookup key for the radix tree. 92 93 The tree map is a good choice if the hwirq number can be very large 94 since it doesn't need to allocate a table as large as the largest 95 hwirq number. The disadvantage is that hwirq to IRQ number lookup is 96 dependent on how many entries are in the table. 97 98 Very few drivers should need this mapping. At the moment, powerpc 99 iseries is the only user. 100 101 ==== No Map ===- 102 irq_domain_add_nomap() 103 104 The No Map mapping is to be used when the hwirq number is 105 programmable in the hardware. In this case it is best to program the 106 Linux IRQ number into the hardware itself so that no mapping is 107 required. Calling irq_create_direct_mapping() will allocate a Linux 108 IRQ number and call the .map() callback so that driver can program the 109 Linux IRQ number into the hardware. 110 111 Most drivers cannot use this mapping. 112 113 ==== Legacy ==== 114 irq_domain_add_simple() 115 irq_domain_add_legacy() 116 irq_domain_add_legacy_isa() 117 118 The Legacy mapping is a special case for drivers that already have a 119 range of irq_descs allocated for the hwirqs. It is used when the 120 driver cannot be immediately converted to use the linear mapping. For 121 example, many embedded system board support files use a set of #defines 122 for IRQ numbers that are passed to struct device registrations. In that 123 case the Linux IRQ numbers cannot be dynamically assigned and the legacy 124 mapping should be used. 125 126 The legacy map assumes a contiguous range of IRQ numbers has already 127 been allocated for the controller and that the IRQ number can be 128 calculated by adding a fixed offset to the hwirq number, and 129 visa-versa. The disadvantage is that it requires the interrupt 130 controller to manage IRQ allocations and it requires an irq_desc to be 131 allocated for every hwirq, even if it is unused. 132 133 The legacy map should only be used if fixed IRQ mappings must be 134 supported. For example, ISA controllers would use the legacy map for 135 mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ 136 numbers. 137 138 Most users of legacy mappings should use irq_domain_add_simple() which 139 will use a legacy domain only if an IRQ range is supplied by the 140 system and will otherwise use a linear domain mapping. The semantics 141 of this call are such that if an IRQ range is specified then 142 descriptors will be allocated on-the-fly for it, and if no range is 143 specified it will fall through to irq_domain_add_linear() which means 144 *no* irq descriptors will be allocated. 145 146 A typical use case for simple domains is where an irqchip provider 147 is supporting both dynamic and static IRQ assignments. 148 149 In order to avoid ending up in a situation where a linear domain is 150 used and no descriptor gets allocated it is very important to make sure 151 that the driver using the simple domain call irq_create_mapping() 152 before any irq_find_mapping() since the latter will actually work 153 for the static IRQ assignment case.