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PcAtChipsetPkg AcpiTimerLib: Wait 363 ACPI timer counts to get TSC Freq

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Compute the number of ticks to wait to measure TSC frequency.
Instead of (ACPI_TIMER_FREQUENCY / 10000) = 357 and 357 * 10000 = 3570000,
use 363 * 9861 = 3579543 Hz which is within 2 Hz of ACPI_TIMER_FREQUENCY.
363 counts is a calibration time of 101.4 uS.

The idea comes from Michael and Paolo.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paul A Lohr <paul.a.lohr@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
This commit is contained in:
Star Zeng
2016-08-17 10:08:31 +08:00
parent 7503cd70fb
commit a012df5ec6
3 changed files with 33 additions and 27 deletions
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14
PcAtChipsetPkg/Library/AcpiTimerLib/BaseAcpiTimerLib.c
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@@ -20,13 +20,13 @@
Calculate TSC frequency.
The TSC counting frequency is determined by comparing how far it counts
during a 100us period as determined by the ACPI timer. The ACPI timer is
used because it counts at a known frequency.
The TSC is sampled, followed by waiting for ACPI_TIMER_FREQUENCY / 10000
clocks of the ACPI timer, or 100us. The TSC is then sampled again. The
difference multiplied by 10000 is the TSC frequency. There will be a small
error because of the overhead of reading the ACPI timer. An attempt is
made to determine and compensate for this error.
during a 101.4 us period as determined by the ACPI timer.
The ACPI timer is used because it counts at a known frequency.
The TSC is sampled, followed by waiting 363 counts of the ACPI timer,
or 101.4 us. The TSC is then sampled again. The difference multiplied by
9861 is the TSC frequency. There will be a small error because of the
overhead of reading the ACPI timer. An attempt is made to determine and
compensate for this error.
@return The number of TSC counts per second.