linux-kernel - Re: [PATCH] x86: clean up hpet timer reinit

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Date:	Fri, 6 Feb 2009 09:42:01 -0600
From:	Daniel Forrest <dan.forrest@...c.wisc.edu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Pavel Emelyanov <xemul@...nvz.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kirill Korotaev <dev@...nvz.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: clean up hpet timer reinit

On Fri, Feb 06, 2009 at 03:09:43PM +0100, Ingo Molnar wrote:
> 
> * Pavel Emelyanov <xemul@...nvz.org> wrote:
> 
> > Sorry for late response - took some time to re-check this...
> 
> Applied to tip:timers/urgent, thanks Pavel!
> 
> Note, since i've already queued up the minimal fix i've created a delta 
> cleanup patch from your v2 patch - see it below. (It is the exact same end 
> result in terms of code, just a nicer splitup.)
> 
> 	Ingo
> 
> ------------------->
> >From ff08f76d738d0ec0f334b187f61e160caa321d54 Mon Sep 17 00:00:00 2001
> From: Pavel Emelyanov <xemul@...nvz.org>
> Date: Wed, 4 Feb 2009 13:40:31 +0300
> Subject: [PATCH] x86: clean up hpet timer reinit
> 
> Implement Linus's suggestion: introduce the hpet_cnt_ahead()
> helper function to compare hpet time values - like other
> wrapping counter comparisons are abstracted away elsewhere.
> (jiffies, ktime_t, etc.)
> 
> Reported-by: Kirill Korotaev <dev@...nvz.org>
> Signed-off-by: Pavel Emelyanov <xemul@...nvz.org>
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
>  arch/x86/kernel/hpet.c |   12 ++++++++++--
>  1 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
> index c761f91..388254f 100644
> --- a/arch/x86/kernel/hpet.c
> +++ b/arch/x86/kernel/hpet.c
> @@ -897,7 +897,7 @@ static unsigned long hpet_rtc_flags;
>  static int hpet_prev_update_sec;
>  static struct rtc_time hpet_alarm_time;
>  static unsigned long hpet_pie_count;
> -static unsigned long hpet_t1_cmp;
> +static u32 hpet_t1_cmp;
>  static unsigned long hpet_default_delta;
>  static unsigned long hpet_pie_delta;
>  static unsigned long hpet_pie_limit;
> @@ -905,6 +905,14 @@ static unsigned long hpet_pie_limit;
>  static rtc_irq_handler irq_handler;
>  
>  /*
> + * Check that the hpet counter c1 is ahead of the c2
> + */
> +static inline int hpet_cnt_ahead(u32 c1, u32 c2)
> +{
> +	return (s32)(c2 - c1) < 0;
> +}
> +
> +/*
>   * Registers a IRQ handler.
>   */
>  int hpet_register_irq_handler(rtc_irq_handler handler)
> @@ -1075,7 +1083,7 @@ static void hpet_rtc_timer_reinit(void)
>  		hpet_t1_cmp += delta;
>  		hpet_writel(hpet_t1_cmp, HPET_T1_CMP);
>  		lost_ints++;
> -	} while ((s32)(hpet_readl(HPET_COUNTER) - hpet_t1_cmp) > 0);
> +	} while (!hpet_cnt_ahead(hpet_t1_cmp, hpet_readl(HPET_COUNTER)));

These are not equivalent for the case where the values are equal.

Let "A = hpet_t1_cmp" and "B = hpet_readl(HPET_COUNTER)"

Then "!(A > B)" means "(B - A) >= 0" not "(B - A) > 0"

Shouldn't it be:

+	} while (hpet_cnt_ahead(hpet_readl(HPET_COUNTER), hpet_t1_cmp));

Or am I missing something?

>  
>  	if (lost_ints) {
>  		if (hpet_rtc_flags & RTC_PIE)
> --

-- 
Daniel K. Forrest		Space Science and
dan.forrest@...c.wisc.edu	Engineering Center
(608) 890 - 0558		University of Wisconsin, Madison
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