linux-kernel - Re: [patch]: fixing of pmtimer overflow that make Cx states time incorrect

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Message-Id: <20090202232438.d91d19f7.akpm@linux-foundation.org>
Date:	Mon, 2 Feb 2009 23:24:38 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	alex.shi@...el.com
Cc:	Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	linux-acpi@...r.kernel.org
Subject: Re: [patch]: fixing of pmtimer overflow that make Cx states time
 incorrect


(Cc: line rewritten.  What on earth is your email client doing??)

On Mon, 02 Feb 2009 09:46:57 +0800 "alex.shi" <alex.shi@...el.com> wrote:

> Yagui want to give a clear explanation to be used for commitment. So I resend
> this again. 
> 
>     On most boxes the ACPI PM timer is 24-bit counter that runs on 3.579545MHz
> clock. In such case the max C-state sleep time should be less than 4687ms when
> it is used to record C2/C3 duration time. 
>     But on some boxes the max C-state sleep time is more than 4687ms. In such
> case the overflow happens and the C-state duration time can't be counted
> accurately.
> 
>     Use clocksource to get the C-state time instead of ACPI PM timer. and use
> div64_u64 to convert US_TO_PM_TIME_TICKS in i386 mode.
> 

A minor thing..

> --- linux-2.6.29-rc3.orig/drivers/acpi/processor_idle.c
> +++ linux-2.6.29-rc3/drivers/acpi/processor_idle.c
> @@ -64,7 +64,8 @@
>  #define _COMPONENT              ACPI_PROCESSOR_COMPONENT
>  ACPI_MODULE_NAME("processor_idle");
>  #define ACPI_PROCESSOR_FILE_POWER	"power"
> -#define US_TO_PM_TIMER_TICKS(t)		((t * (PM_TIMER_FREQUENCY/1000)) / 1000)
> +#define US_TO_PM_TIMER_TICKS(t)		div64_u64(\
> +	(t * (PM_TIMER_FREQUENCY/1000)), 1000ULL)

I suppose it doesn't matter much, but we could make this function more
accurate via

	(t * PM_TIMER_FREQUENCY) / 1000

Also, it would be nicer to implement this is a regular C function. 
There's no need at all for it to be an ugly all-caps macro.


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