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

On Wed, 2009-10-14 at 12:59 -0600, Grant Likely wrote:

On Tue, Oct 13, 2009 at 11:44 PM, Michael Ellerman
[off-list ref] wrote:

quoted

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