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>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/IRQ-domain.txt b/Documentation/IRQ-domain.txt
index 27dcaabfb4db..1401cece745a 100644
--- a/Documentation/IRQ-domain.txt
+++ b/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.