Re: [PATCH 1/6] powerpc: Make NR_IRQS a CONFIG option

[Michael Ellerman <michael@ellerman.id.au>]

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On Wed, 2009-10-14 at 12:59 -0600, Grant Likely wrote:
> On Tue, Oct 13, 2009 at 11:44 PM, Michael Ellerman
> <michael@ellerman.id.au> wrote:
> > The irq_desc array consumes quite a lot of space, and for systems
> > that don't need or can't have 512 irqs it's just wasted space.
> >
> > The first 16 are reserved for ISA, so the minimum of 32 is really
> > 16 - and no one has asked for more than 512 so leave that as the
> > maximum.
> 
> Does it really make sense to have this as a user twiddlable value?
> Especially when many users just don't have the background to know what
> an appropriate value is here and will get it wrong?  I believe your
> sparse IRQ patch has a bigger impact anyway on systems where memory is
> tight.

We have users? But yes I think it's reasonable, there's a million other
options people can fiddle with and break their kernel, I don't see that
this is much different.

The sparse IRQ patch has a bigger difference on the size of the irq_desc
array, but there are still other things that are statically sized based
on NR_IRQs. So if you're building an machine-specific kernel and you
know you're only going to have N interrupts, then this will give you a
bigger saving.

But I'm not super fussed, if other people think it's too dangerous we
can drop it.

cheers



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