genirq: Add kerneldoc for irq_disable.

Document the lazy disable functionality. comment based on changelog of
d209a699a0b975ad

Signed-off-by: Andreas Fenkart <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

kernel/irq/chip.c

···
 	irq_state_clr_masked(desc);
 }
 
+/**
+ * irq_disable - Mark interupt disabled
+ * @desc:	irq descriptor which should be disabled
+ *
+ * If the chip does not implement the irq_disable callback, we
+ * use a lazy disable approach. That means we mark the interrupt
+ * disabled, but leave the hardware unmasked. That's an
+ * optimization because we avoid the hardware access for the
+ * common case where no interrupt happens after we marked it
+ * disabled. If an interrupt happens, then the interrupt flow
+ * handler masks the line at the hardware level and marks it
+ * pending.
+ */
 void irq_disable(struct irq_desc *desc)
 {
 	irq_state_set_disabled(desc);