linux-kernel - Re: [Q] Is 64bit LVTT screwed

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Date:	Tue, 8 Jul 2008 15:38:45 +0400
From:	"Cyrill Gorcunov" <gorcunov@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Q] Is 64bit LVTT screwed

On Tue, Jul 8, 2008 at 2:08 AM, Maciej W. Rozycki <macro@...ux-mips.org> wrote:
[...]
>>
>> but in both cases we use "divide by 16" in divide register. The only
>> explanation I imagine - for 64bit mode we are required to 'stuck'
>> for a bit longer (by 16 times longer to be precise). Am I right?
>> Or there is another reason why we dont use APIC_DIVISOR here. Actually,
>> as I see it not fair to a caller. For 64bit mode APIC timer is requested
>> to count 250000000 ticks but in real it will count 250000000 * 16.
>> Not sure who is right there. I think the better would be to
>> use 4000000000 and APIC_DIVISOR in 64bit mode. How do you think?
>
>  The APIC clock varies across systems so the timer setup includes
> calibration.  Which means both variations end up with the correct
> interrupt rate probably, except using different divisors.  We used to
> print the APIC clock rate and that would be off with one of the above, but
> I don't we do this anymore.  We had a similar problem with the 82489DX
> where the prescaler was bypassed altogether resulting in an incorrect
> clock rate printed, but the rate of timer interrupt was correctly
> calibrated regardless.
>
>  This is of course merely an explanation why the code you quoted does not
> explode spectacularly.  It does not make it correct.  If the prescaler is
> indeed set to 16 for 64-bit systems, then APIC_DIVISOR should obviously be
> used in the calculation above too.  And if not, then I think the prescaler
> should be set consistently across systems and then the setting reflected
> in the calculations accordingly.  Please note that only values between 2
> and 16 are supported in a uniform way across all the APIC models and using
> low values risks an overflow as clock rates are in the GHz range these
> days already.  The value of 16 seems a reasonable one.
>
>  Maciej
>

Thanks, Maciej, for explanation. Unfortunelly I'm quite busy now so
I stopped merging/research on apic code, as only get spare time - will
let you know my results.

- Cyrill -
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