irqdomain: document the simple domain first_irq

The first_irq needs to be zero to get a linear domain and that
comes with special semantics. We want to simplify this going
forward but some documentation never hurts.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/irq/irqdomain.c b/kernel/irq/irqdomain.c
index 8c4c8ea6a205..54a4d5223238 100644
--- a/kernel/irq/irqdomain.c
+++ b/kernel/irq/irqdomain.c
@@ -143,7 +143,10 @@ static unsigned int irq_domain_legacy_revmap(struct irq_domain *domain,
  * 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
+ * @first_irq: first number of irq block assigned to the domain,
+ *      pass zero to assign irqs on-the-fly. This will result in a
+ *      linear IRQ domain so it is important to use irq_create_mapping()
+ *      for each used IRQ, especially when SPARSE_IRQ is enabled.
  * @ops: map/unmap domain callbacks
  * @host_data: Controller private data pointer
  *