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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Sat, 27 Jan 2007 09:40:57 +1100

> 
> > What?!!! The whole point of the abstraction ("flat space") is
> > to be able to do reverse lookups for additional information.
> 
> You may want to look at the virtual irq scheme we implemented for
> powerpc, I think it could be useful for other architectures as well in
> fact... One mistake I did was to put the documentation in the .h instead
> of near the code though :-) asm-powerpc/irq.h is a good start to read.
> 
> The main reasons we did it in the first place are two fold:
> 
>  - On pSeries and to some extent with other hypervisors, IRQ numbers can
> be pretty big, from encoding the geographical informations about the
> slot/irq to just being an opaque 64 bits "token" from the hypervisor. So
> we need the ability to map that to/from linux smaller and flatter space.

We do the same thing on sparc64 btw, for the same reasons.  The
hypervisor on Niagara specifies IRQ numbers as opaque 64-bit
quantities.