Re: [PATCH 4/5] Centralise NO_IRQ definition

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@osdl.org> wrote:

> At which point you might as well just do something like
> 
> 	struct interrupt_descriptor {
> 		unsigned int nr:31;
> 		unsigned int valid:1;
> 	};
> 
> and then people can just say
> 
> 	if (!dev->irq.valid)
> 		return;
> 
> instead, which is also readable, and where you simply cannot do the old 
> "if (!dev->irq)" at all.
> 
> The fact is, 0 _is_ special. Not just for hardware, but because 0 has 
> a magical meaning as "false" in the C language.

yeah, i wanted to suggest this originally, but got distracted by the x86 
quirk that 'IRQ#0' is often the i8253 timer interrupt.

is there any architecture where irq 0 is a legitimate setting that could 
occur in drivers, and which would make NO_IRQ define of 0 non-practical?  
If not (which i think is the case) then we should indeed standardize on 
0. (in one way or another) It's not like any real driver will ever have 
IRQ#0 even on a PC: the timer IRQ is 'known' to be routed to 0, and we 
do a platform-specific setup_irq() on it. Not a good reason to abstract 
the notion of 'no irq' away into a define.

in fact we dont even have to do the irq.valid thing, !dev->irq is 
obviously readable.

	Ingo