linux-kernel - Re: [PATCH] asm-generic: drop HARDIRQ

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Date:	Mon, 15 Jun 2009 11:59:34 -0400
From:	Steven Rostedt <srostedt@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Mike Frysinger <vapier.adi@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic: drop HARDIRQ_BITS definition from
	hardirq.h


On Sun, 2009-06-14 at 22:43 +0200, Arnd Bergmann wrote:
> linux/hardirq.h contains a fallback for HARDIRQ_BITS to 10
> if it's not defined, so it is pointless to define a default
> of 8 in asm/hardirq.h. There does not seem to be a good
> reason why an architecture would want to limit the number
> of hardirqs this way.
> 
> Reported-by: Mike Frysinger <vapier@...too.org>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/asm-generic/hardirq.h |   13 -------------
>  1 files changed, 0 insertions(+), 13 deletions(-)
> On Sunday 14 June 2009, Mike Frysinger wrote:
> 
> Mike Frysinger wrote:
> > 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
> 
> None that I know of. I'm queuing this patch in my asm-generic tree now,
> unless Steven or someone else has a better idea.

ARGH!!! I guess the best patch would be to comment this better. That
"HARDIRQ_BITS" has nothing to do with the number of interrupts a machine
may have. It is the number of nested interrupts a machine may do.

If you plan on having more than 256 interrupts nesting, I suggest you
need to fix the stack problems first ;-)

Please, we only have a few bits in the preempt count (on 32 bit
machines) and this is the number of bits used to record the nesting of
interrupts.

-- Steve


> 
> 	Arnd <><
> 
> diff --git a/include/asm-generic/hardirq.h b/include/asm-generic/hardirq.h
> index 3d5d2c9..23bb4da 100644
> --- a/include/asm-generic/hardirq.h
> +++ b/include/asm-generic/hardirq.h
> @@ -11,19 +11,6 @@ typedef struct {
>  
>  #include <linux/irq_cpustat.h>	/* Standard mappings for irq_cpustat_t above */
>  
> -#ifndef HARDIRQ_BITS
> -#define HARDIRQ_BITS	8
> -#endif
> -
> -/*
> - * The hardirq mask has to be large enough to have
> - * space for potentially all IRQ sources in the system
> - * nesting on a single CPU:
> - */
> -#if (1 << HARDIRQ_BITS) < NR_IRQS
> -# error HARDIRQ_BITS is too low!
> -#endif
> -
>  #ifndef ack_bad_irq
>  static inline void ack_bad_irq(unsigned int irq)
>  {

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