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Documentation / IRQ-domain.txt

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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	===============================================
2	The irq_domain interrupt number mapping library
3	===============================================
5	The current design of the Linux kernel uses a single large number
6	space where each separate IRQ source is assigned a different number.
7	This is simple when there is only one interrupt controller, but in
8	systems with multiple interrupt controllers the kernel must ensure
9	that each one gets assigned non-overlapping allocations of Linux
10	IRQ numbers.
12	The number of interrupt controllers registered as unique irqchips
13	show a rising tendency: for example subdrivers of different kinds
14	such as GPIO controllers avoid reimplementing identical callback
15	mechanisms as the IRQ core system by modelling their interrupt
16	handlers as irqchips, i.e. in effect cascading interrupt controllers.
18	Here the interrupt number loose all kind of correspondence to
19	hardware interrupt numbers: whereas in the past, IRQ numbers could
20	be chosen so they matched the hardware IRQ line into the root
21	interrupt controller (i.e. the component actually fireing the
22	interrupt line to the CPU) nowadays this number is just a number.
24	For this reason we need a mechanism to separate controller-local
25	interrupt numbers, called hardware irq's, from Linux IRQ numbers.
27	The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
28	irq numbers, but they don't provide any support for reverse mapping of
29	the controller-local IRQ (hwirq) number into the Linux IRQ number
30	space.
32	The irq_domain library adds mapping between hwirq and IRQ numbers on
33	top of the irq_alloc_desc*() API.  An irq_domain to manage mapping is
34	preferred over interrupt controller drivers open coding their own
35	reverse mapping scheme.
37	irq_domain also implements translation from an abstract irq_fwspec
38	structure to hwirq numbers (Device Tree and ACPI GSI so far), and can
39	be easily extended to support other IRQ topology data sources.
41	irq_domain usage
42	================
44	An interrupt controller driver creates and registers an irq_domain by
45	calling one of the irq_domain_add_*() functions (each mapping method
46	has a different allocator function, more on that later).  The function
47	will return a pointer to the irq_domain on success.  The caller must
48	provide the allocator function with an irq_domain_ops structure.
50	In most cases, the irq_domain will begin empty without any mappings
51	between hwirq and IRQ numbers.  Mappings are added to the irq_domain
52	by calling irq_create_mapping() which accepts the irq_domain and a
53	hwirq number as arguments.  If a mapping for the hwirq doesn't already
54	exist then it will allocate a new Linux irq_desc, associate it with
55	the hwirq, and call the .map() callback so the driver can perform any
56	required hardware setup.
58	When an interrupt is received, irq_find_mapping() function should
59	be used to find the Linux IRQ number from the hwirq number.
61	The irq_create_mapping() function must be called *atleast once*
62	before any call to irq_find_mapping(), lest the descriptor will not
63	be allocated.
65	If the driver has the Linux IRQ number or the irq_data pointer, and
66	needs to know the associated hwirq number (such as in the irq_chip
67	callbacks) then it can be directly obtained from irq_data->hwirq.
69	Types of irq_domain mappings
70	============================
72	There are several mechanisms available for reverse mapping from hwirq
73	to Linux irq, and each mechanism uses a different allocation function.
74	Which reverse map type should be used depends on the use case.  Each
75	of the reverse map types are described below:
77	Linear
78	------
80	::
82		irq_domain_add_linear()
83		irq_domain_create_linear()
85	The linear reverse map maintains a fixed size table indexed by the
86	hwirq number.  When a hwirq is mapped, an irq_desc is allocated for
87	the hwirq, and the IRQ number is stored in the table.
89	The Linear map is a good choice when the maximum number of hwirqs is
90	fixed and a relatively small number (~ < 256).  The advantages of this
91	map are fixed time lookup for IRQ numbers, and irq_descs are only
92	allocated for in-use IRQs.  The disadvantage is that the table must be
93	as large as the largest possible hwirq number.
95	irq_domain_add_linear() and irq_domain_create_linear() are functionally
96	equivalent, except for the first argument is different - the former
97	accepts an Open Firmware specific 'struct device_node', while the latter
98	accepts a more general abstraction 'struct fwnode_handle'.
100	The majority of drivers should use the linear map.
102	Tree
103	----
105	::
107		irq_domain_add_tree()
108		irq_domain_create_tree()
110	The irq_domain maintains a radix tree map from hwirq numbers to Linux
111	IRQs.  When an hwirq is mapped, an irq_desc is allocated and the
112	hwirq is used as the lookup key for the radix tree.
114	The tree map is a good choice if the hwirq number can be very large
115	since it doesn't need to allocate a table as large as the largest
116	hwirq number.  The disadvantage is that hwirq to IRQ number lookup is
117	dependent on how many entries are in the table.
119	irq_domain_add_tree() and irq_domain_create_tree() are functionally
120	equivalent, except for the first argument is different - the former
121	accepts an Open Firmware specific 'struct device_node', while the latter
122	accepts a more general abstraction 'struct fwnode_handle'.
124	Very few drivers should need this mapping.
126	No Map
127	------
129	::
131		irq_domain_add_nomap()
133	The No Map mapping is to be used when the hwirq number is
134	programmable in the hardware.  In this case it is best to program the
135	Linux IRQ number into the hardware itself so that no mapping is
136	required.  Calling irq_create_direct_mapping() will allocate a Linux
137	IRQ number and call the .map() callback so that driver can program the
138	Linux IRQ number into the hardware.
140	Most drivers cannot use this mapping.
142	Legacy
143	------
145	::
147		irq_domain_add_simple()
148		irq_domain_add_legacy()
149		irq_domain_add_legacy_isa()
151	The Legacy mapping is a special case for drivers that already have a
152	range of irq_descs allocated for the hwirqs.  It is used when the
153	driver cannot be immediately converted to use the linear mapping.  For
154	example, many embedded system board support files use a set of #defines
155	for IRQ numbers that are passed to struct device registrations.  In that
156	case the Linux IRQ numbers cannot be dynamically assigned and the legacy
157	mapping should be used.
159	The legacy map assumes a contiguous range of IRQ numbers has already
160	been allocated for the controller and that the IRQ number can be
161	calculated by adding a fixed offset to the hwirq number, and
162	visa-versa.  The disadvantage is that it requires the interrupt
163	controller to manage IRQ allocations and it requires an irq_desc to be
164	allocated for every hwirq, even if it is unused.
166	The legacy map should only be used if fixed IRQ mappings must be
167	supported.  For example, ISA controllers would use the legacy map for
168	mapping Linux IRQs 0-15 so that existing ISA drivers get the correct IRQ
169	numbers.
171	Most users of legacy mappings should use irq_domain_add_simple() which
172	will use a legacy domain only if an IRQ range is supplied by the
173	system and will otherwise use a linear domain mapping. The semantics
174	of this call are such that if an IRQ range is specified then
175	descriptors will be allocated on-the-fly for it, and if no range is
176	specified it will fall through to irq_domain_add_linear() which means
177	*no* irq descriptors will be allocated.
179	A typical use case for simple domains is where an irqchip provider
180	is supporting both dynamic and static IRQ assignments.
182	In order to avoid ending up in a situation where a linear domain is
183	used and no descriptor gets allocated it is very important to make sure
184	that the driver using the simple domain call irq_create_mapping()
185	before any irq_find_mapping() since the latter will actually work
186	for the static IRQ assignment case.
188	Hierarchy IRQ domain
189	--------------------
191	On some architectures, there may be multiple interrupt controllers
192	involved in delivering an interrupt from the device to the target CPU.
193	Let's look at a typical interrupt delivering path on x86 platforms::
195	  Device --> IOAPIC -> Interrupt remapping Controller -> Local APIC -> CPU
197	There are three interrupt controllers involved:
199	1) IOAPIC controller
200	2) Interrupt remapping controller
201	3) Local APIC controller
203	To support such a hardware topology and make software architecture match
204	hardware architecture, an irq_domain data structure is built for each
205	interrupt controller and those irq_domains are organized into hierarchy.
206	When building irq_domain hierarchy, the irq_domain near to the device is
207	child and the irq_domain near to CPU is parent. So a hierarchy structure
208	as below will be built for the example above::
210		CPU Vector irq_domain (root irq_domain to manage CPU vectors)
211			^
212			|
213		Interrupt Remapping irq_domain (manage irq_remapping entries)
214			^
215			|
216		IOAPIC irq_domain (manage IOAPIC delivery entries/pins)
218	There are four major interfaces to use hierarchy irq_domain:
220	1) irq_domain_alloc_irqs(): allocate IRQ descriptors and interrupt
221	   controller related resources to deliver these interrupts.
222	2) irq_domain_free_irqs(): free IRQ descriptors and interrupt controller
223	   related resources associated with these interrupts.
224	3) irq_domain_activate_irq(): activate interrupt controller hardware to
225	   deliver the interrupt.
226	4) irq_domain_deactivate_irq(): deactivate interrupt controller hardware
227	   to stop delivering the interrupt.
229	Following changes are needed to support hierarchy irq_domain:
231	1) a new field 'parent' is added to struct irq_domain; it's used to
232	   maintain irq_domain hierarchy information.
233	2) a new field 'parent_data' is added to struct irq_data; it's used to
234	   build hierarchy irq_data to match hierarchy irq_domains. The irq_data
235	   is used to store irq_domain pointer and hardware irq number.
236	3) new callbacks are added to struct irq_domain_ops to support hierarchy
237	   irq_domain operations.
239	With support of hierarchy irq_domain and hierarchy irq_data ready, an
240	irq_domain structure is built for each interrupt controller, and an
241	irq_data structure is allocated for each irq_domain associated with an
242	IRQ. Now we could go one step further to support stacked(hierarchy)
243	irq_chip. That is, an irq_chip is associated with each irq_data along
244	the hierarchy. A child irq_chip may implement a required action by
245	itself or by cooperating with its parent irq_chip.
247	With stacked irq_chip, interrupt controller driver only needs to deal
248	with the hardware managed by itself and may ask for services from its
249	parent irq_chip when needed. So we could achieve a much cleaner
250	software architecture.
252	For an interrupt controller driver to support hierarchy irq_domain, it
253	needs to:
255	1) Implement irq_domain_ops.alloc and irq_domain_ops.free
256	2) Optionally implement irq_domain_ops.activate and
257	   irq_domain_ops.deactivate.
258	3) Optionally implement an irq_chip to manage the interrupt controller
259	   hardware.
260	4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
261	   they are unused with hierarchy irq_domain.
263	Hierarchy irq_domain is in no way x86 specific, and is heavily used to
264	support other architectures, such as ARM, ARM64 etc.
266	=== Debugging ===
268	Most of the internals of the IRQ subsystem are exposed in debugfs by
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