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| -rw-r--r-- | Documentation/IRQ-domain.txt | 34 |
1 files changed, 33 insertions, 1 deletions
diff --git a/Documentation/IRQ-domain.txt b/Documentation/IRQ-domain.txt index 1401cece745a..9bc95942ec22 100644 --- a/Documentation/IRQ-domain.txt +++ b/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. |