IIO irq allocation fails on AT91SAM9G45

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

On Wed, Feb 29, 2012 at 08:35:27PM +0000, Jonathan Cameron wrote:
> On 02/29/2012 02:32 PM, Maxime Ripard wrote:
> > Hi everyone,
> > 
> > I'm working on adding the support for the AT91SAM9M10G45-EK board from
> > Atmel for the at91_adc driver I previously posted, and I encounter some
> > weird issue here.
> > 
> > When calling the iio_allocate_trigger
> > (http://lxr.free-electrons.com/source/drivers/staging/iio/industrialio-trigger.c?a=arm#L421)
> > from my driver on the G45, it returns ENOMEM, while on the
> > AT91SAM9G20-EK board, it works perfectly.
> > 
> > Digging a bit into it, it seems that the call to irq_alloc_descs is
> > returning the error (the value of CONFIG_IIO_CONSUMERS_PER_TRIGGER is 2
> > in my configuration, which seems pretty reasonable and is the default
> > value anyway), which is itself getting that return value from
> > irq_expand_nr_irqs.
> > 
> > Here, I'm left confused, I don't know this part of the kernel anymore,
> > and most importantly, it seems to be pretty-much arch-independant, while
> > the nature of my issue seems really platform-dependant.
> > 
> > Do you have any clue of what's going on here ?
> We ran into this originally on the pxa as well.   My guess is that
> nr_irqs is not set high enough for that particular board.
> 
> Looking back I can find some mention of a nasty bit of code that
> just adds a bit of padding but I can't find it now.
> 
> Anyhow, you probably have a line somewhere in the kernel log
> saying something like:
> 
> [    0.000000] NR_IRQS:288 nr_irqs:296 296
> 
> NR_IRQS is typically the number of the SoC
> nr_irqs should be large enough to accomodate those provided by
> other peripherals.
> 
> I also have a vague recollection that the problem goes away entirely
> with sparse irqs?

Yes, because IRQs will be allocated above the last figure on that
line, up to IRQ_BITMAP_BITS which happens to be 8192 above NR_IRQS.

There's an issue though: if your on-SoC IRQ controller is already
using irq_alloc_descs(), it will fail if you want it to grab IRQs
below the last figure on that line, because those will have already
been allocated for you.