PcAtChipsetPkg/Library/AcpiTimerLib/BaseAcpiTimerLib.c

   1 /** @file
   2   ACPI Timer implements one instance of Timer Library.
   3
   4   Copyright (c) 2013 - 2016, Intel Corporation. All rights reserved.<BR>
   5   This program and the accompanying materials
   6   are licensed and made available under the terms and conditions of the BSD License
   7   which accompanies this distribution.  The full text of the license may be found at
   8   http://opensource.org/licenses/bsd-license.php
   9
  10   THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
  11   WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
  12
  13 **/
  14
  15 #include <Base.h>
  16 #include <Library/TimerLib.h>
  17 #include <Library/BaseLib.h>
  18
  19 /**
  20   Calculate TSC frequency.
  21
  22   The TSC counting frequency is determined by comparing how far it counts
  23   during a 101.4 us period as determined by the ACPI timer.
  24   The ACPI timer is used because it counts at a known frequency.
  25   The TSC is sampled, followed by waiting 363 counts of the ACPI timer,
  26   or 101.4 us. The TSC is then sampled again. The difference multiplied by
  27   9861 is the TSC frequency. There will be a small error because of the
  28   overhead of reading the ACPI timer. An attempt is made to determine and
  29   compensate for this error.
  30
  31   @return The number of TSC counts per second.
  32
  33 **/
  34 UINT64
  35 InternalCalculateTscFrequency (
  36   VOID
  37   );
  38
  39 /**
  40   Internal function to retrieves the 64-bit frequency in Hz.
  41
  42   Internal function to retrieves the 64-bit frequency in Hz.
  43
  44   @return The frequency in Hz.
  45
  46 **/
  47 UINT64
  48 InternalGetPerformanceCounterFrequency (
  49   VOID
  50   )
  51 {
  52   return InternalCalculateTscFrequency ();
  53 }