What does irq_canonicalize mean?

["jidong xiao" <jidong.xiao@gmail.com>]

I found this function, inside include/asm-i386/irq.h or
include/asm-x86_64/irq.h,

static __inline__ int irq_canonicalize(int irq)
{
	return ((irq == 2) ? 9 : irq);
}

there is no comments,so I am totally confused what does this function
mean, why there is a "2", and why there is a "9"?

Thanks
Jason Xiao
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