linux-kernel - Re: [PATCH] x86/hpet: Use the FSEC_PER_SEC constant for femto-second periods

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Date:	Mon, 09 Aug 2010 12:04:24 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/hpet: Use the FSEC_PER_SEC constant for
 femto-second periods

On Sun, 2010-08-08 at 13:58 +0100, Chris Wilson wrote:
> The current computation, introduced with f12a15be63, of FSEC_PER_SEC using
> the multiplication of (FSEC_PER_NSEC * NSEC_PER_SEC) is performed only
> with 32bit integers on small machines, resulting in an overflow and a
> *very* short intervals being programmed.  An interrupt storm follows.
> 
> Note that we also have to specify FSEC_PER_SEC as being long long to
> overcome the same limitations.

Thanks so much for catching and debugging this!

Acked-by: John Stultz <johnstul@...ibm.com>

> Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: John Stultz <johnstul@...ibm.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> ---
>  arch/x86/kernel/hpet.c |    4 ++--
>  include/linux/time.h   |    2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
> index 33dbcc4..351f9c0 100644
> --- a/arch/x86/kernel/hpet.c
> +++ b/arch/x86/kernel/hpet.c
> @@ -582,7 +582,7 @@ static void init_one_hpet_msi_clockevent(struct hpet_dev *hdev, int cpu)
>  	 * scaled math multiplication factor for nanosecond to hpet tick
>  	 * conversion.
>  	 */
> -	hpet_freq = 1000000000000000ULL;
> +	hpet_freq = FSEC_PER_SEC;
>  	do_div(hpet_freq, hpet_period);
>  	evt->mult = div_sc((unsigned long) hpet_freq,
>  				      NSEC_PER_SEC, evt->shift);
> @@ -837,7 +837,7 @@ static int hpet_clocksource_register(void)
>  	 * cyc/sec = FSEC_PER_SEC/hpet_period(fsec/cyc)
>  	 * cyc/sec = (FSEC_PER_NSEC * NSEC_PER_SEC)/hpet_period
>  	 */
> -	hpet_freq = FSEC_PER_NSEC * NSEC_PER_SEC;
> +	hpet_freq = FSEC_PER_SEC;
>  	do_div(hpet_freq, hpet_period);
>  	clocksource_register_hz(&clocksource_hpet, (u32)hpet_freq);
> 
> diff --git a/include/linux/time.h b/include/linux/time.h
> index cb34e35..1261270 100644
> --- a/include/linux/time.h
> +++ b/include/linux/time.h
> @@ -38,7 +38,7 @@ extern struct timezone sys_tz;
>  #define NSEC_PER_MSEC	1000000L
>  #define USEC_PER_SEC	1000000L
>  #define NSEC_PER_SEC	1000000000L
> -#define FSEC_PER_SEC	1000000000000000L
> +#define FSEC_PER_SEC	1000000000000000LL
> 
>  #define TIME_T_MAX	(time_t)((1UL << ((sizeof(time_t) << 3) - 1)) - 1)
> 


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