linux-kernel - Re: [RFC PATCH] microblaze/irq: Change NO

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Date:	Wed, 07 Dec 2011 10:39:46 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Michal Simek <monstr@...str.eu>, John Linn <John.Linn@...inx.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Ingo Molnar <mingo@...e.hu>, Jeff Garzik <jgarzik@...hat.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	devicetree-discuss@...ts.ozlabs.org,
	LKML <linux-kernel@...r.kernel.org>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Anton Vorontsov <anton.vorontsov@...aro.org>
Subject: Re: [RFC PATCH] microblaze/irq: Change NO_IRQ to 0

Linus,

On 12/07/2011 10:05 AM, Linus Torvalds wrote:
> On Wed, Dec 7, 2011 at 6:41 AM, Rob Herring <robherring2@...il.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
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