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Message-ID: <4BF3B4ED.7000807@mvista.com>
Date:   Wed, 19 May 2010 13:52:45 +0400
From:   Sergei Shtylyov <sshtylyov@mvista.com>
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To:     David Daney <ddaney@caviumnetworks.com>
CC:     linux-mips@linux-mips.org, ralf@linux-mips.org,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] MIPS: Don't overflow cevt-r4k.c calculations at high
 clock rates.
References: <1274220676-4562-1-git-send-email-ddaney@caviumnetworks.com>
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Hello.

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>
[...]
> 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
[...]
> @@ -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;

    Why not:

		scaled_freq >>= 1;

    It's C after all. :-)

WBR, Sergei