linux-kernel - Re: [PATCH] arm: irq: Allow for specification of no preallocated irqs

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Message-ID: <20120120161557.GT4223@ponder.secretlab.ca>
Date:	Fri, 20 Jan 2012 09:15:57 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Michael Bohan <mbohan@...eaurora.org>
Cc:	Rob Herring <robherring2@...il.com>, tglx@...utronix.de,
	Russell King <linux@....linux.org.uk>,
	linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm: irq: Allow for specification of no preallocated irqs

On Thu, Jan 19, 2012 at 04:29:43PM -0800, Michael Bohan wrote:
> On 1/19/2012 3:04 PM, Rob Herring wrote:
> >No doubt that arch_probe_nr_irqs is doing the wrong thing on ARM, but no
> >pre-allocation is not what we want either. We ultimately want
> >arch_probe_nr_irqs to return NR_IRQS_LEGACY (16) to reserve IRQ0 (aka
> >NO_IRQ) and legacy ISA IRQs. With my series, NR_IRQS is set to
> >NR_IRQS_LEGACY for SPARSE_IRQ. You can accomplish the same thing without
> >that series by setting .nr_irqs to NR_IRQS for non-DT and to
> >NR_IRQS_LEGACY for DT. For platforms to work in single kernel builds,
> >they will need to select SPARSE_IRQ.
> 
> One issue here is that IRQ_BITMAP_BITS is defined as a function of
> NR_IRQS. Currently, there's a hack in place that arbitrarily tacks
> on 8196 bits to the end, giving the max virq supported as 8212 with
> your patches. Unfortunately, the system I'm running on will require
> higher values than that, so this actually breaks me.

Are that many irqs actually going to be in-use at one time?  If not, then the driver should use the irq_domain radix tree reverse map anyway.

g.

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