irq_domain: Standardise legacy/linear domain selection

A large proportion of interrupt controllers that support legacy mappings
do so because non-DT systems need to use fixed IRQ numbers when registering
devices via buses but can otherwise use a linear mapping. The interrupt
controller itself typically is not affected by the mapping used and best
practice is to use a linear mapping where possible so drivers frequently
select at runtime depending on if a legacy range has been allocated to
them.

Standardise this behaviour by providing irq_domain_register_simple() which
will allocate a linear mapping unless a positive first_irq is provided in
which case it will fall back to a legacy mapping. This helps make best
practice for irq_domain adoption clearer.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

Documentation/IRQ-domain.txt

@@ -93,6 +93,7 @@ Linux IRQ number into the hardware.
 Most drivers cannot use this mapping.
 
 ==== Legacy ====
+irq_domain_add_simple()
 irq_domain_add_legacy()
 irq_domain_add_legacy_isa()
 
@@ -115,3 +116,7 @@ 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.
+
+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.

include/linux/irqdomain.h

@@ -112,6 +112,11 @@ struct irq_domain {
 };
 
 #ifdef CONFIG_IRQ_DOMAIN
+struct irq_domain *irq_domain_add_simple(struct device_node *of_node,
+					 unsigned int size,
+					 unsigned int first_irq,
+					 const struct irq_domain_ops *ops,
+					 void *host_data);
 struct irq_domain *irq_domain_add_legacy(struct device_node *of_node,
 					 unsigned int size,
 					 unsigned int first_irq,

kernel/irq/irqdomain.c

@@ -139,6 +139,36 @@ static unsigned int irq_domain_legacy_revmap(struct irq_domain *domain,
 	return hwirq - first_hwirq + domain->revmap_data.legacy.first_irq;
 }
 
+/**
+ * irq_domain_add_simple() - Allocate and register a simple irq_domain.
+ * @of_node: pointer to interrupt controller's device tree node.
+ * @size: total number of irqs in mapping
+ * @first_irq: first number of irq block assigned to the domain
+ * @ops: map/unmap domain callbacks
+ * @host_data: Controller private data pointer
+ *
+ * Allocates a legacy irq_domain if irq_base is positive or a linear
+ * domain otherwise.
+ *
+ * This is intended to implement the expected behaviour for most
+ * interrupt controllers which is that a linear mapping should
+ * normally be used unless the system requires a legacy mapping in
+ * order to support supplying interrupt numbers during non-DT
+ * registration of devices.
+ */
+struct irq_domain *irq_domain_add_simple(struct device_node *of_node,
+					 unsigned int size,
+					 unsigned int first_irq,
+					 const struct irq_domain_ops *ops,
+					 void *host_data)
+{
+	if (first_irq > 0)
+		return irq_domain_add_legacy(of_node, size, first_irq, 0,
+					     ops, host_data);
+	else
+		return irq_domain_add_linear(of_node, size, ops, host_data);
+}
+
 /**
  * irq_domain_add_legacy() - Allocate and register a legacy revmap irq_domain.
  * @of_node: pointer to interrupt controller's device tree node.