[PATCH] ARM: OMAP: irqs: Fix NR_IRQS value to handle PRCM interrupts

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Feb 28, 2012 at 02:10:09PM +0100, Cousson, Benoit wrote:
> The following commit: 2f31b51659c2d8315ea2888ba5b93076febe672b
> Author: Tero Kristo <t-kristo@ti.com>
> Date:   Fri Dec 16 14:37:00 2011 -0700
> 
>     ARM: OMAP4: PRM: use PRCM interrupt handler
> 
> introduced the PRCM interrupt handler and thus the need
> for 64 more interrupts. Since SPARSE_IRQ is still not fully
> functional on OMAP, the NR_IRQS needs to be updated to avoid
> the failure that happen during irq_alloc_descs call inside
> the PRCM driver:
> 
> [    0.208221] PRCM: failed to allocate irq descs: -12
> 
> Later the mux framework is then unable to request an IRQ from
> the PRCM interrupt handler.
> 
> [    1.802795] mux: Failed to setup hwmod io irq -22

This is fine for rc, but longer term...

Do any of these have hard-coded interrupt numbers associated with them?
If not, just enabling sparse IRQ will sort this out.

As I tried to explain yesterday, there are two modes for IRQ allocation:

1. Without sparse IRQ enabled, irq_alloc_descs(-1, from, num, -1) will
   allocate IRQs _within_ the existing from..NR_IRQS range, and will fail
   if there is insufficient IRQs available.

2. With sparse IRQs enabled, irq_alloc_descs(-1, from, num, -1) will
   allocate IRQs starting at max(from, NR_IRQS) and working upwards.

In either case, irq_alloc_descs(start, 0, num, -1) will allocate 'num'
IRQs at 'start' or fail if the range is already in use (and 0..NR_IRQS
is defined as 'being in use' when sparse IRQs are enabled.)

So, if the PRCM interrupts aren't statically assigned (the code suggests
that they aren't) then it's already sparse-IRQ compliant, and enabling
sparse IRQ support will mean that they will be allocated above NR_IRQS.

Therefore, I suggest rather than raising NR_IRQS, you instead enable
SPARSE_IRQ now so that anyone using the dynamic IRQ allocation can
benefit from sparse IRQ support without having to have a large NR_IRQS.

So, you don't have to wait until everything is converted to use
sparse IRQ.  You just need to make sure that nothing uses
irq_alloc_descs(start, from, num, ...) where start < NR_IRQS, and
nothing using that requires statically defined IRQ numbering.