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Date: Wed, 07 Dec 2011 10:39:46 -0600
From: Rob Herring <robherring2@gmail.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Michal Simek <monstr@monstr.eu>, John Linn <John.Linn@xilinx.com>,
        Grant Likely <grant.likely@secretlab.ca>,
        Rob Herring <rob.herring@calxeda.com>, Ingo Molnar <mingo@elte.hu>,
        Jeff Garzik <jgarzik@redhat.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        devicetree-discuss@lists.ozlabs.org,
        LKML <linux-kernel@vger.kernel.org>,
        Randy Dunlap <rdunlap@xenotime.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Anton Vorontsov <anton.vorontsov@linaro.org>
Subject: Re: [RFC PATCH] microblaze/irq: Change NO_IRQ to 0
References: <4EDEE2B2.7080606@gmail.com> <1323268911-6104-1-git-send-email-robherring2@gmail.com> <CA+55aFzz2SMvKJcRzOU0+Rd1MHMtygTmT9rDcyy7qGjuVFJKHg@mail.gmail.com>
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Linus,

On 12/07/2011 10:05 AM, Linus Torvalds wrote:
> On Wed, Dec 7, 2011 at 6:41 AM, Rob Herring <robherring2@gmail.com> wrote:
>>
>> This patch changes the Microblaze NO_IRQ setting from -1 to 0 to bring
>> it in line with most of the rest of the kernel.  It also prepares for
>> Microblaze eventually supporting multiple interrupt controllers by
>> breaking the assumption that hwirq# == Linux IRQ#.  The Linux IRQ
>> number is just a cookie with no guarantee of a direct relationship
>> with the hardware irq arrangement.
> 
> This looks really nice and would probably be a great example for other
> architectures with the same issue.
> 
> My only (small) nit is that just from a debuggability standpoint I'd
> actually suggest keeping the "translated" and "hardware" interrupt
> numbers totally disjoint, which would seem to be trivial - make the
> offset be 32 instead of 1. Sure, some NR_IRQ arrays would end up being
> a bit bigger that way, but it would allow for trivially seeing whether
> anybody reports the raw or translated irq numbers by just looking at
> the number. That could reduce lots of confusion if somebody ends up
> having the old non-translated number hardcoded etc.
> 
> It would also potentially allow for actual debug checks - having code
> like "WARN_ON_ONCE(irq < 32)" in some arch-specific irq controller
> registration path etc, which is much harder to do with the overlapping
> numbers.

Agreed. I debated making it 16 just to skip the legacy interrupts as
we're doing on ARM. So there's 2 benefits to skipping to 32.

Anyway, I'll leave it to someone that can actually build and test this.

> But it's nice to see how small the patch *can* be.

If only ARM was this easy...

Rob