linux-kernel - Re: [RFC] lapic calibration results fix

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Date:	Sun, 13 Jul 2008 20:23:37 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] lapic calibration results fix

[Cyrill Gorcunov - Sun, Jul 13, 2008 at 03:37:34PM +0400]
| Ingo, Maciej,
| 
| it seems I found a bit strange code snippet in apic_32.c:setup_boot_APIC_clock.
| We set local_apic_timer_verify_ok = 1 before checking the results. I think
| the following patch make sense. Please take a look on.
| 
| ---
| We set lapic flag that clocksource calibration is
| fine too early. Fix it.
| 
| Signed-off-by: Cyrill Gorcunov <gorcunov@...il.com>
| ---
| 
| Index: linux-2.6.git/arch/x86/kernel/apic_32.c
| ===================================================================
| --- linux-2.6.git.orig/arch/x86/kernel/apic_32.c	2008-07-13 15:25:20.000000000 +0400
| +++ linux-2.6.git/arch/x86/kernel/apic_32.c	2008-07-13 15:31:03.000000000 +0400
| @@ -489,8 +489,6 @@ void __init setup_boot_APIC_clock(void)
|  		    calibration_result / (1000000 / HZ),
|  		    calibration_result % (1000000 / HZ));
|  
| -	local_apic_timer_verify_ok = 1;
| -
|  	/*
|  	 * Do a sanity check on the APIC calibration result
|  	 */
| @@ -504,6 +502,8 @@ void __init setup_boot_APIC_clock(void)
|  		return;
|  	}
|  
| +	local_apic_timer_verify_ok = 1;
| +
|  	/* We trust the pm timer based calibration */
|  	if (!pm_referenced) {
|  		apic_printk(APIC_VERBOSE, "... verify APIC timer\n");

Letme clarify a bit situation why I propose this path (it's not that
clear from its description).

While calibrating apic timer we use different 'flags' signaling if it
was success/fail. On 64bit platform lapic_timer_setup() does check
for CLOCK_EVT_FEAT_DUMMY bit which is set by default and cleared on
successfully calibrated timer. So 64bit is fine now. On 32bit we
use a different approach - local_apic_timer_verify_ok flag _BUT_
as I see it being set too early _before_ calibration result is checked
so we could have the following - kernel reports user that APIC timer
is too slow and disabled but local_apic_timer_verify_ok remains set
to 1 instead of 0.

		- Cyrill -
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