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Subject: Re: IRQs in i2c-mpc.c
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jon Smirl <jonsmirl@gmail.com>
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Date: Sun, 11 Nov 2007 17:27:51 +1100
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> irq_of_parse_and_map() returns NO_IRQ when the irq parameter is missing.
> 
> This API appears to be broken. In asm-powerpc/irq.h NO_IRQ is defined
> as (0). There is no way to tell an error in the attribute from a valid
> attribute selecting interrupt zero.

No, it is not broken. Interrupt numbers in arch/powerpc are virtually
mapped and 0 is never a legal value, thus NO_IRQ is 0. This was done in
part because Linus wanted it that way.

The hardware number can perfectly be 0 though, that's a different thing,
and has nothing to do with what you pass to request_irq().

Ben.