linux-kernel - Re: [Patch] Fix the possibility of insane return value of hpet

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Date:	Wed, 18 Mar 2009 09:55:47 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Yasunori Goto <y-goto@...fujitsu.com>
CC:	Clemens Ladisch <clemens@...isch.de>,
	Linux Kernel ML <linux-kernel@...r.kernel.org>,
	robert.picco@...com, venkatesh.pallipadi@...el.com,
	vojtech@...e.cz, mingo@...hat.com
Subject: Re: [Patch] Fix the possibility of insane return value of hpet_calibrate()
 against SMI. (take 2)

Yasunori Goto wrote:
>>>>>> Each calibration of this has 1 milli second.
>>>>>> Do you think 5 msec is too long?
>>>>>>       
>>>>>>             
>>>>> This shouldn't matter when booting.  Anyway, I think it's possible to
>>>>> increase TICK_CALIBRATE without losing too much accuracy.
>>>>>     
>>>>>           
>>>> Hmm. If the person who is trying to reduce boot time for fastboot dislikes this impact,
>>>> then I'll try Vojtech-san's way.
>>>>   
>>>>         
>>> That probably makes sense -- if it is extremely rare, and if you get two
>>> values that are the same (within some small delta) then you probably
>>> are 99.99% confident that you have the right data.
>>>       
>> Ok. I'll make it.
>>
>> Thanks for your comment.
>>     
>
> Here is updated version.
>
> --------
>
> hpet_calibrate() has a possibility of miss-calibration due to SMI.
> If SMI interrupts in the while loop of calibration, then return value
> will be big. This change calibrates until stabilizing by the return
> value with a small value.
>
>
> Signed-off-by: Yasunori Goto <y-goto@...fujitsu.com>
>
>
> ---
>  drivers/char/hpet.c |   21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
>
> Index: hpet_test/drivers/char/hpet.c
> ===================================================================
> --- hpet_test.orig/drivers/char/hpet.c	2009-03-12 15:47:45.000000000 +0900
> +++ hpet_test/drivers/char/hpet.c	2009-03-18 11:12:42.000000000 +0900
> @@ -713,7 +713,7 @@
>   */
>  #define	TICK_CALIBRATE	(1000UL)
>  
> -static unsigned long hpet_calibrate(struct hpets *hpetp)
> +static unsigned long __hpet_calibrate(struct hpets *hpetp)
>  {
>  	struct hpet_timer __iomem *timer = NULL;
>  	unsigned long t, m, count, i, flags, start;
> @@ -750,6 +750,25 @@
>  	return (m - start) / i;
>  }
>  
> +static unsigned long hpet_calibrate(struct hpets *hpetp)
> +{
> +	unsigned long ret = ~0UL, tmp;
> +
> +	/*
> +	 * Try to calibrate until return value becomes stable small value.
> +	 * If SMI interruption occurs in calibration loop, the return value
> +	 * will be big. This avoids its impact.
> +	 */
> +	do {
> +		tmp = __hpet_calibrate(hpetp);
> +		if (ret <= tmp)
> +			break;
> +		ret = tmp;
> +	} while (1);
>   

For what it is worth, if a situation arose where continued calls to
hpet_calibrate() represented a monotonically decreasing function,
(perhaps some insane power management?) you could be stuck
in this function for an unbounded amount of time.   I don't expect
that should ever happen, but I figured I'd mention it.

Paul.

> +
> +	return ret;
> +}
> +
>  int hpet_alloc(struct hpet_data *hdp)
>  {
>  	u64 cap, mcfg;
>
>   

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