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Re: [PATCH] MIPS: Don't overflow cevt-r4k.c calculations at high clock r

To: David Daney <ddaney@caviumnetworks.com>
Subject: Re: [PATCH] MIPS: Don't overflow cevt-r4k.c calculations at high clock rates.
From: Wu Zhangjin <wuzhangjin@gmail.com>
Date: Wed, 19 May 2010 09:27:51 +0800
Cc: linux-mips@linux-mips.org, ralf@linux-mips.org, Thomas Gleixner <tglx@linutronix.de>
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In-reply-to: <1274220676-4562-1-git-send-email-ddaney@caviumnetworks.com>
Organization: DSLab, Lanzhou University, China
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <1274220676-4562-1-git-send-email-ddaney@caviumnetworks.com>
Reply-to: wuzhangjin@gmail.com
Sender: linux-mips-bounce@linux-mips.org
Hi,

Seems this is similar to the overflow problem in the high resolution
sched_clock(), If not ensure the 'mult' is in 32bit, we must use the
128bit calculation in the common function clocksource_cyc2ns() defined
in include/linux/clocksource.h.

Regards,
        Wu Zhangjin

On Tue, 2010-05-18 at 15:11 -0700, David Daney wrote:
> The 'mult' element of struct clock_event_device must never be wider
> than 32-bits.  If it were, it would get truncated when used by
> clockevent_delta2ns() when this calls do_div().
> 
> We meet the requirement by ensuring that the relationship:
> 
>  (mips_hpt_frequency >> (32 - shift)) < NSEC_PER_SEC
> 
> Always holds.
> 
> Signed-off-by: David Daney <ddaney@caviumnetworks.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/mips/kernel/cevt-r4k.c |   16 +++++++++++++---
>  1 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/mips/kernel/cevt-r4k.c b/arch/mips/kernel/cevt-r4k.c
> index 0b2450c..4495158 100644
> --- a/arch/mips/kernel/cevt-r4k.c
> +++ b/arch/mips/kernel/cevt-r4k.c
> @@ -163,10 +163,11 @@ int c0_compare_int_usable(void)
>  
>  int __cpuinit r4k_clockevent_init(void)
>  {
> -     uint64_t mips_freq = mips_hpt_frequency;
> +     uint64_t scaled_freq = mips_hpt_frequency;
>       unsigned int cpu = smp_processor_id();
>       struct clock_event_device *cd;
>       unsigned int irq;
> +     int shift;
>  
>       if (!cpu_has_counter || !mips_hpt_frequency)
>               return -ENXIO;
> @@ -189,8 +190,17 @@ int __cpuinit r4k_clockevent_init(void)
>       cd->features            = CLOCK_EVT_FEAT_ONESHOT;
>  
>       /* Calculate the min / max delta */
> -     cd->mult        = div_sc((unsigned long) mips_freq, NSEC_PER_SEC, 32);
> -     cd->shift               = 32;
> +     shift = 32;
> +     while (scaled_freq >= NSEC_PER_SEC && shift) {
> +             scaled_freq = scaled_freq >> 1;
> +             shift--;
> +     }
> +     BUG_ON(shift == 0);
> +
> +     cd->mult                = div_sc((unsigned long) mips_hpt_frequency,
> +                                      NSEC_PER_SEC, shift);
> +     cd->shift               = shift;
> +
>       cd->max_delta_ns        = clockevent_delta2ns(0x7fffffff, cd);
>       cd->min_delta_ns        = clockevent_delta2ns(0x300, cd);
>  



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