Messages in this thread | | | Date | Tue, 8 Jul 2008 15:38:45 +0400 | From | "Cyrill Gorcunov" <> | Subject | Re: [Q] Is 64bit LVTT screwed |
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On Tue, Jul 8, 2008 at 2:08 AM, Maciej W. Rozycki <macro@linux-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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