Most drivers cannot use this mapping.
==== Legacy ====
+irq_domain_add_simple()
irq_domain_add_legacy()
irq_domain_add_legacy_isa()
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.
+
+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.