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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:46 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 an abstract irq_fwspec
36	structure to hwirq numbers (Device Tree and ACPI GSI so far), and can
37	be easily extended to support 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	irq_domain_create_linear()
74	
75	The linear reverse map maintains a fixed size table indexed by the
76	hwirq number.  When a hwirq is mapped, an irq_desc is allocated for
77	the hwirq, and the IRQ number is stored in the table.
78	
79	The Linear map is a good choice when the maximum number of hwirqs is
80	fixed and a relatively small number (~ < 256).  The advantages of this
81	map are fixed time lookup for IRQ numbers, and irq_descs are only
82	allocated for in-use IRQs.  The disadvantage is that the table must be
83	as large as the largest possible hwirq number.
84	
85	irq_domain_add_linear() and irq_domain_create_linear() are functionally
86	equivalent, except for the first argument is different - the former
87	accepts an Open Firmware specific 'struct device_node', while the latter
88	accepts a more general abstraction 'struct fwnode_handle'.
89	
90	The majority of drivers should use the linear map.
91	
92	==== Tree ====
93	irq_domain_add_tree()
94	irq_domain_create_tree()
95	
96	The irq_domain maintains a radix tree map from hwirq numbers to Linux
97	IRQs.  When an hwirq is mapped, an irq_desc is allocated and the
98	hwirq is used as the lookup key for the radix tree.
99	
100	The tree map is a good choice if the hwirq number can be very large
101	since it doesn't need to allocate a table as large as the largest
102	hwirq number.  The disadvantage is that hwirq to IRQ number lookup is
103	dependent on how many entries are in the table.
104	
105	irq_domain_add_tree() and irq_domain_create_tree() are functionally
106	equivalent, except for the first argument is different - the former
107	accepts an Open Firmware specific 'struct device_node', while the latter
108	accepts a more general abstraction 'struct fwnode_handle'.
109	
110	Very few drivers should need this mapping.
111	
112	==== No Map ===-
113	irq_domain_add_nomap()
114	
115	The No Map mapping is to be used when the hwirq number is
116	programmable in the hardware.  In this case it is best to program the
117	Linux IRQ number into the hardware itself so that no mapping is
118	required.  Calling irq_create_direct_mapping() will allocate a Linux
119	IRQ number and call the .map() callback so that driver can program the
120	Linux IRQ number into the hardware.
121	
122	Most drivers cannot use this mapping.
123	
124	==== Legacy ====
125	irq_domain_add_simple()
126	irq_domain_add_legacy()
127	irq_domain_add_legacy_isa()
128	
129	The Legacy mapping is a special case for drivers that already have a
130	range of irq_descs allocated for the hwirqs.  It is used when the
131	driver cannot be immediately converted to use the linear mapping.  For
132	example, many embedded system board support files use a set of #defines
133	for IRQ numbers that are passed to struct device registrations.  In that
134	case the Linux IRQ numbers cannot be dynamically assigned and the legacy
135	mapping should be used.
136	
137	The legacy map assumes a contiguous range of IRQ numbers has already
138	been allocated for the controller and that the IRQ number can be
139	calculated by adding a fixed offset to the hwirq number, and
140	visa-versa.  The disadvantage is that it requires the interrupt
141	controller to manage IRQ allocations and it requires an irq_desc to be
142	allocated for every hwirq, even if it is unused.
143	
144	The legacy map should only be used if fixed IRQ mappings must be
145	supported.  For example, ISA controllers would use the legacy map for
146	mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
147	numbers.
148	
149	Most users of legacy mappings should use irq_domain_add_simple() which
150	will use a legacy domain only if an IRQ range is supplied by the
151	system and will otherwise use a linear domain mapping. The semantics
152	of this call are such that if an IRQ range is specified then
153	descriptors will be allocated on-the-fly for it, and if no range is
154	specified it will fall through to irq_domain_add_linear() which means
155	*no* irq descriptors will be allocated.
156	
157	A typical use case for simple domains is where an irqchip provider
158	is supporting both dynamic and static IRQ assignments.
159	
160	In order to avoid ending up in a situation where a linear domain is
161	used and no descriptor gets allocated it is very important to make sure
162	that the driver using the simple domain call irq_create_mapping()
163	before any irq_find_mapping() since the latter will actually work
164	for the static IRQ assignment case.
165	
166	==== Hierarchy IRQ domain ====
167	On some architectures, there may be multiple interrupt controllers
168	involved in delivering an interrupt from the device to the target CPU.
169	Let's look at a typical interrupt delivering path on x86 platforms:
170	
171	Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU
172	
173	There are three interrupt controllers involved:
174	1) IOAPIC controller
175	2) Interrupt remapping controller
176	3) Local APIC controller
177	
178	To support such a hardware topology and make software architecture match
179	hardware architecture, an irq_domain data structure is built for each
180	interrupt controller and those irq_domains are organized into hierarchy.
181	When building irq_domain hierarchy, the irq_domain near to the device is
182	child and the irq_domain near to CPU is parent. So a hierarchy structure
183	as below will be built for the example above.
184		CPU Vector irq_domain (root irq_domain to manage CPU vectors)
185			^
186			|
187		Interrupt Remapping irq_domain (manage irq_remapping entries)
188			^
189			|
190		IOAPIC irq_domain (manage IOAPIC delivery entries/pins)
191	
192	There are four major interfaces to use hierarchy irq_domain:
193	1) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt
194	   controller related resources to deliver these interrupts.
195	2) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller
196	   related resources associated with these interrupts.
197	3) irq_domain_activate_irq(): activate interrupt controller hardware to
198	   deliver the interrupt.
199	4) irq_domain_deactivate_irq(): deactivate interrupt controller hardware
200	   to stop delivering the interrupt.
201	
202	Following changes are needed to support hierarchy irq_domain.
203	1) a new field 'parent' is added to struct irq_domain; it's used to
204	   maintain irq_domain hierarchy information.
205	2) a new field 'parent_data' is added to struct irq_data; it's used to
206	   build hierarchy irq_data to match hierarchy irq_domains. The irq_data
207	   is used to store irq_domain pointer and hardware irq number.
208	3) new callbacks are added to struct irq_domain_ops to support hierarchy
209	   irq_domain operations.
210	
211	With support of hierarchy irq_domain and hierarchy irq_data ready, an
212	irq_domain structure is built for each interrupt controller, and an
213	irq_data structure is allocated for each irq_domain associated with an
214	IRQ. Now we could go one step further to support stacked(hierarchy)
215	irq_chip. That is, an irq_chip is associated with each irq_data along
216	the hierarchy. A child irq_chip may implement a required action by
217	itself or by cooperating with its parent irq_chip.
218	
219	With stacked irq_chip, interrupt controller driver only needs to deal
220	with the hardware managed by itself and may ask for services from its
221	parent irq_chip when needed. So we could achieve a much cleaner
222	software architecture.
223	
224	For an interrupt controller driver to support hierarchy irq_domain, it
225	needs to:
226	1) Implement irq_domain_ops.alloc and irq_domain_ops.free
227	2) Optionally implement irq_domain_ops.activate and
228	   irq_domain_ops.deactivate.
229	3) Optionally implement an irq_chip to manage the interrupt controller
230	   hardware.
231	4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
232	   they are unused with hierarchy irq_domain.
233	
234	Hierarchy irq_domain may also be used to support other architectures,
235	such as ARM, ARM64 etc.
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