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

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

[Chris Wilson]

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.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.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)
 
-- 
1.7.1

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

[john stultz]

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@us.ibm.com>

> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: John Stultz <johnstul@us.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.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)
>