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From: Mike Frysinger <vapier.adi@gmail.com>
Date: Sat, 13 Jun 2009 20:25:53 -0400
Message-ID: <8bd0f97a0906131725l214007fcpfa90e72b03cad2ac@mail.gmail.com>
Subject: Re: [PATCH] asm-generic: hard_irqs: handle NR_IRQS > 256 
	automatically
To: Arnd Bergmann <arnd@arndb.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
       Steven Rostedt <srostedt@redhat.com>
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On Sat, Jun 13, 2009 at 17:18, Arnd Bergmann wrote:
> On Saturday 13 June 2009, H. Peter Anvin wrote:
>> Mike Frysinger wrote:
>> > If we're going to automatically set HARDIRQ_BITS for the arch, might as
>> > well be a little bit smart about it and set it to 9 automatically if
>> > NR_IRQS is larger than 8 bits.
>> >
>>
>> Why would the only possible values be 8 or 9?
>
> All architectures that define this either set it to 8 or 9, I chose
> 8 because it is the more common constant

right, i was going by what was in use -- there are no requirements
here, just defaults

> but I now realized that
> we also have (in include/linux/hardirq.h, last touched by Steven):
>
> #define MAX_HARDIRQ_BITS 10
> #ifndef HARDIRQ_BITS
> # define HARDIRQ_BITS   MAX_HARDIRQ_BITS
> #endif
> #if HARDIRQ_BITS > MAX_HARDIRQ_BITS
> #error HARDIRQ_BITS too high!
> #endif
>
> Not sure why we even need to make this overridable from the architecture,
> 10 still seems like a reasonable default that should always work.
>
> I'd suggest we either drop the definition for HARDIRQ_BITS from
> asm-generic/hardirq.h, or we use
>
>  #ifndef HARDIRQ_BITS
> -#define HARDIRQ_BITS   8
> +# if NR_IRQS > 255
> +#  define HARDIRQ_BITS 9
> +# elif NR_IRQS > 511
> +#  define HARDIRQ_BITS 10
> +# elif NR_IRQS > 1023
> +#  warning too many interrupts for HARDIRQ_BITS
> +# endif
>  #endif

is there any downsides to using a "too large" value ?  i.e. if my
system has less than 256, does it make any difference at all if it's
set to 10 ?
-mike