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    SubjectRe: [PATCH] asm-generic: hard_irqs: handle NR_IRQS > 256 automatically
    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
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