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Re: [MIPS]clocks_calc_mult_shift() may gen a too big mult value

To: Chen Jie <chenj@lemote.com>
Subject: Re: [MIPS]clocks_calc_mult_shift() may gen a too big mult value
From: John Stultz <johnstul@us.ibm.com>
Date: Mon, 31 Oct 2011 09:03:17 -0400
Cc: Yong Zhang <yong.zhang0@gmail.com>, linux-mips@linux-mips.org, LKML <linux-kernel@vger.kernel.org>, tglx@linutronix.de, yanhua <yanh@lemote.com>, 项宇 <xiangy@lemote.com>, zhangfx <zhangfx@lemote.com>, 孙海勇 <sunhy@lemote.com>
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On Mon, 2011-10-31 at 18:48 +0800, Chen Jie wrote:
> Hi,
> 
> 2011/10/31 Yong Zhang <yong.zhang0@gmail.com>:
> > On Mon, Oct 31, 2011 at 5:00 PM, Chen Jie <chenj@lemote.com> wrote:
> >> Hi all,
> >>
> >> On MIPS, with maxsec=4, clocks_calc_mult_shift() may generate a very
> >> big mult, which may easily cause timekeeper.mult overflow within
> >> timekeeping jobs.
> >
> > Hmmm, why not use clocksource_register_hz()/clocksource_register_khz()
> > instead? it's more convenient.
> 
> Thanks for the suggestion. And sorry for I didn't notice the upstream
> code has already hooked to clocksource_register_hz() in csrc-r4k.c
> (We're using r4000 clock source)
> 
> I'm afraid this still doesn't fix my case. Through
> clocksource_register_hz()->__clocksource_register_scale()->__clocksource_updatefreq_scale,
> I got a calculated maxsec = (0xffffffff - (0xffffffff>>5))/250000500 =
> 16        # assume mips_hpt_frequency=250000500
> 
> With this maxsec, I got a mult of 0xffffde72, still too big.

Hrmm. Yong Zang is right to suggest clocksource_register_hz(), as the
intention of that code is to try to avoid these sorts of issues.

What is the corresponding shift value you're getting for the value
above?

Could you annotate clocks_calc_mult_shift() a little bit to see where
things might be going wrong?

thanks
-john


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