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

To: Yong Zhang <yong.zhang0@gmail.com>
Subject: Re: [MIPS]clocks_calc_mult_shift() may gen a too big mult value
From: Chen Jie <chenj@lemote.com>
Date: Mon, 31 Oct 2011 18:48:45 +0800
Cc: linux-mips@linux-mips.org, LKML <linux-kernel@vger.kernel.org>, johnstul@us.ibm.com, 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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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.



Regards,
-- Chen Jie

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