a012df5ec6
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>
62 lines
1.8 KiB
C
62 lines
1.8 KiB
C
/** @file
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ACPI Timer implements one instance of Timer Library.
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Copyright (c) 2013 - 2016, Intel Corporation. All rights reserved.<BR>
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This program and the accompanying materials
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are licensed and made available under the terms and conditions of the BSD License
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which accompanies this distribution. The full text of the license may be found at
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http://opensource.org/licenses/bsd-license.php
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THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
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WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
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**/
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#include <Base.h>
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#include <Library/TimerLib.h>
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#include <Library/BaseLib.h>
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/**
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Calculate TSC frequency.
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The TSC counting frequency is determined by comparing how far it counts
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during a 101.4 us period as determined by the ACPI timer.
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The ACPI timer is used because it counts at a known frequency.
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The TSC is sampled, followed by waiting 363 counts of the ACPI timer,
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or 101.4 us. The TSC is then sampled again. The difference multiplied by
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9861 is the TSC frequency. There will be a small error because of the
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overhead of reading the ACPI timer. An attempt is made to determine and
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compensate for this error.
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@return The number of TSC counts per second.
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**/
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UINT64
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InternalCalculateTscFrequency (
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VOID
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);
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//
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// Cached performance counter frequency
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//
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UINT64 mPerformanceCounterFrequency = 0;
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/**
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Internal function to retrieves the 64-bit frequency in Hz.
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Internal function to retrieves the 64-bit frequency in Hz.
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@return The frequency in Hz.
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**/
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UINT64
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InternalGetPerformanceCounterFrequency (
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VOID
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)
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{
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if (mPerformanceCounterFrequency == 0) {
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mPerformanceCounterFrequency = InternalCalculateTscFrequency ();
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}
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return mPerformanceCounterFrequency;
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}
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