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Re: [PATCH 1/2] x86/time: use relative counts in calibration loops
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 13 Jan 2022 10:37:53 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Thu, 13 Jan 2022 09:38:16 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 12.01.2022 09:55, Jan Beulich wrote:
> @@ -504,11 +501,8 @@ static s64 __init init_pmtimer(struct pl
>
> count = inl(pmtmr_ioport) & mask;
> start = rdtsc_ordered();
> - target = count + CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
> - if ( target < count )
> - while ( (inl(pmtmr_ioport) & mask) >= count )
> - continue;
> - while ( (inl(pmtmr_ioport) & mask) < target )
> + target = CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
> + while ( (elapsed = (inl(pmtmr_ioport) & mask) - count) < target )
I think this is wrong, and instead needs to be
while ( (elapsed = (inl(pmtmr_ioport) - count) & mask) < target )
There no similar issue with HPET as there we always have full 32 bits
available.
Roger - you gave your R-b. If you agree, I'd like to retain that with
the fix in place. But I'm not going to commit either variant ahead of
hearing back from you.
Jan
> continue;
>
> return (rdtsc_ordered() - start) * CALIBRATE_FRAC;
>
>
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