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

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Date:	Fri, 06 Feb 2009 20:00:00 +0300
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Daniel Forrest <dan.forrest@...c.wisc.edu>
CC:	Ingo Molnar <mingo@...e.hu>,
	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

Daniel Forrest wrote:
> 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?

When comparator it equal to counter (the corner case we're talking about) the
hpet_cnt_ahead will return false and the loop will go on shifting the cmp,
which is what we need here.

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

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