irq_domain: add documentation and MAINTAINERS entry.

Documentation for irq_domain library which will be created in subsequent
patches.

v4: editorial changes

Signed-off-by: Grant Likely <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: Milton Miller <[e-mail]>
Tested-by: Olof Johansson <[e-mail]>

Documentation/IRQ-domain.txt

···
+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 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.
+
+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_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.

MAINTAINERS

···
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git irq/core
 F:	kernel/irq/
 
+IRQ DOMAINS (IRQ NUMBER MAPPING LIBRARY)
+M:	Benjamin Herrenschmidt <[e-mail]>
+M:	Grant Likely <[e-mail]>
+T:	git git://git.secretlab.ca/git/linux-2.6.git irqdomain/next
+S:	Maintained
+F:	Documentation/IRQ-domain.txt
+F:	include/linux/irqdomain.h
+F:	kernel/irq/irqdomain.c
+
 ISAPNP
 M:	Jaroslav Kysela <[e-mail]>
 S:	Maintained