linux-kernel - Re: [PATCH] asm-generic: hard_irqs: handle NR

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Date:	Sat, 13 Jun 2009 23:18:18 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Mike Frysinger <vapier@...too.org>, linux-kernel@...r.kernel.org,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH] asm-generic: hard_irqs: handle NR_IRQS > 256 automatically

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, 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

	Arnd <><
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