linux-kernel - Re: [PATCH -tip] x86: apic LVTT - use APIC

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Message-ID: <20080712060504.GB14714@elte.hu>
Date:	Sat, 12 Jul 2008 08:05:04 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -tip] x86: apic LVTT - use APIC_DIVISOR on 64bit mode


* Cyrill Gorcunov <gorcunov@...il.com> wrote:

> We use APIC_TDR_DIV_16 while setting APIC_TDCR (divisor) it happened 
> this moment is hidden from __setup_APIC_LVTT caller. So we better 
> increment caller 'clocks' value to not change current behaviour and 
> use APIC_DIVISOR (as already done in 32bit version).
> 
> The main benefit - unified procedure code for 32/64bit modes.

> Please review carefully. Any comments are welcome.
> I'm _not_ 100% sure if this patch is safe.

hm, how about the other caller to __setup_APIC_LVTT(), 
lapic_timer_setup():

        case CLOCK_EVT_MODE_ONESHOT:
                __setup_APIC_LVTT(calibration_result,
                                  mode != CLOCK_EVT_MODE_PERIODIC, 1);
                break;

that will affect high-res timers. Wont calibration_result have the same 
value after your patch as well, but only a 16th of it will be used in 
__setup_APIC_LVTT(), causing 16 times shorter timeouts than intended?

I.e. are we fixing a bug, are we changing nothing, or are we introducing 
a bug? :-)

	Ingo
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