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Date:   Tue, 26 Oct 2010 15:18:38 -0700
From:   David Daney <ddaney@caviumnetworks.com>
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To:     Grant Likely <grant.likely@secretlab.ca>
CC:     linux-mips@linux-mips.org, ralf@linux-mips.org
Subject: Re: [PATCH] MIPS: Add some irq definitins required by OF
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On 10/14/2010 06:27 PM, Grant Likely wrote:
> On Thu, Oct 14, 2010 at 3:02 PM, David Daney<ddaney@caviumnetworks.com>  wrote:
[...]
>>
>> +#define NO_IRQ UINT_MAX
>
> Really?  The verdict came down a long time ago that 0 is to be the
> value that means no irq, and only a few architectures still define
> NO_IRQ as -1.  It is assumed that the architectures which do not
> define NO_IRQ use 0 as the invalid value.  Mostly notably x86 does not
> define NO_IRQ, and Linus nack'd the patch to add it.
>

I was not part of that discussion.

I would however note, that all the irq functions return unsigned, so a 
value of -1 is meaningless.  Also my understanding is that 8259 based 
systems use the values of 0 - 15 as the interrupt numbers, making 0 
unavailable for use as NO_IRQ.

Given these constraints, UINT_MAX would seem to be a good value.  It has 
to be defined as something *and* have global visibility, because it is 
part of the OF irq mapping functions API.

David Daney