sh: intc: Support virtual mappings for IRQ subgroups.
Many interrupts that share a single mask source but are on different hardware vectors will have an associated register tied to an INTEVT that denotes the precise cause for the interrupt exception being triggered. This introduces the concept of IRQ subgroups in the intc core, where a virtual IRQ map is constructed for each of the pre-defined cause bits, and a higher level chained handler takes control of the parent INTEVT. This enables CPUs with heavily muxed IRQ vectors (especially across disjoint blocks) to break things out in to a series of managed chained handlers while being able to dynamically lookup and adopt the IRQs created for them. This is largely an opt-in interface, requiring CPUs to manually submit IRQs for subgroup splitting, in addition to providing identifiers in their enum maps that can be used for lazy lookup via the radix tree. Signed-off-by: Paul Mundt <firstname.lastname@example.org>
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