On Sat, 5 May 2012, Linus Torvalds wrote:

> On Sat, May 5, 2012 at 11:53 AM, Mikulas Patocka <mpatocka@redhat.com> wrote:
> >
> > What NO_IRQ problem do you mean? There is
> > #define NO_IRQ          (-1)
> > in arch/parisc/include/asm/irq.h.
> 
> That's the one he means.
> 
> Switching it to zero and testing that things still work would be appreciated.

It works, but there is plenty of interrupt controllers on PC-RISC and I 
can only test it on C8000 with IO-SAPIC. I don't know if irq 0 is used on 
some PA-RISC interrupt controller. It would be best if James Bottomley 
tests it on his set of machines.

> Much code already knows that NO_IRQ is supposed to be zero, and
> there's tons of drivers that just do the (correct!) "if (!dev->irq)"
> kind of thing.
> 
> Any architecture that has a non-zero NO_IRQ is basically broken.
> Always has been.
> 
>                     Linus

And what about x86? --- irq 0 is used for timer and there is
void __init setup_default_timer_irq(void)
{
        setup_irq(0, &irq0);
}
in arch/x86/kernel/time.c.

Mikulas