irqdomain changes for Linux v3.8

Trivial changes to irqdomain. An update to the documentation and make
 one of the error paths not quite so obnoxious.
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Merge tag 'irqdomain-for-linus' of git://git.secretlab.ca/git/linux-2.6

Pull irqdomain changes from Grant Likely:
 "Trivial changes to irqdomain.  An update to the documentation and make
  one of the error paths not quite so obnoxious."

* tag 'irqdomain-for-linus' of git://git.secretlab.ca/git/linux-2.6:
  irqdomain: update documentation
  irqdomain: stop screaming about preallocated irqdescs

Documentation/IRQ-domain.txt

@@ -7,6 +7,21 @@ 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
@@ -40,6 +55,10 @@ 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.
@@ -119,4 +138,17 @@ 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.
+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.

kernel/irq/irqdomain.c

@@ -177,8 +177,8 @@ struct irq_domain *irq_domain_add_simple(struct device_node *of_node,
 			irq_base = irq_alloc_descs(first_irq, first_irq, size,
 						   of_node_to_nid(of_node));
 			if (irq_base < 0) {
-				WARN(1, "Cannot allocate irq_descs @ IRQ%d, assuming pre-allocated\n",
+				pr_info("Cannot allocate irq_descs @ IRQ%d, assuming pre-allocated\n",
-				     first_irq);
+					first_irq);
 				irq_base = first_irq;
 			}
 		} else