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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Feb 28, 2012 at 09:32:28PM +0100, Cousson, Benoit wrote:
> Hi Russell,
>
> On 2/28/2012 3:36 PM, Russell King - ARM Linux wrote:
>> 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.
>
> You're right, in that case, it does not depend on any hard-coded number.
>
>> 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.
>
> Yes, I fully agree, and that's still the plan. That's why I started  
> sending last week a bunch of cleanup for SPARSE_IRQ support.  
> Unfortunately, they might not be ready for 3.4 either, but I'm still  
> working on it.

One thing I didn't consider is that the GIC has been converted to
sparse IRQ support, so enabling it on OMAP will make the irq_alloc_descs()
in there to fail if you try and keep it below NR_IRQS.

That rules out a piecemeal conversion, which rather sucks.