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 100us period as determined by the ACPI timer. The ACPI timer is
  24   used because it counts at a known frequency.
  25   The TSC is sampled, followed by waiting for ACPI_TIMER_FREQUENCY / 10000
  26   clocks of the ACPI timer, or 100us. The TSC is then sampled again. The
  27   difference multiplied by 10000 is the TSC frequency. There will be a small
  28   error because of the overhead of reading the ACPI timer. An attempt is
  29   made to determine and 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 }