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Subject[PATCH] irqdomain: document the simple domain first_irq
From: Linus Walleij <>

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 <>
kernel/irq/irqdomain.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/irq/irqdomain.c b/kernel/irq/irqdomain.c
index 96f3a1d..059a280 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

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