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

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>

3 files changed, 33 insertions, 27 deletions

diff --git a/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c b/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c
index e6fea23112..020031e3f4 100644
--- a/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c
+++ b/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c
@@ -340,13 +340,13 @@ GetTimeInNanoSecond (
   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.

 

@@ -366,22 +366,28 @@ InternalCalculateTscFrequency (
   InterruptState = SaveAndDisableInterrupts ();

 

   TimerAddr = InternalAcpiGetAcpiTimerIoPort ();

-  Ticks = IoRead32 (TimerAddr) + (ACPI_TIMER_FREQUENCY / 10000);    // Set Ticks to 100us in the future

+  //

+  // Compute the number of ticks to wait to measure TSC frequency.

+  // Use 363 * 9861 = 3579543 Hz which is within 2 Hz of ACPI_TIMER_FREQUENCY.

+  // 363 counts is a calibration time of 101.4 uS.

+  //

+  Ticks = IoRead32 (TimerAddr) + 363;

 

   StartTSC = AsmReadTsc ();                                         // Get base value for the TSC

   //

-  // Wait until the ACPI timer has counted 100us.

+  // Wait until the ACPI timer has counted 101.4 us.

   // Timer wrap-arounds are handled correctly by this function.

-  // When the current ACPI timer value is greater than 'Ticks', the while loop will exit.

+  // When the current ACPI timer value is greater than 'Ticks',

+  // the while loop will exit.

   //

   while (((Ticks - IoRead32 (TimerAddr)) & BIT23) == 0) {

     CpuPause();

   }

-  EndTSC = AsmReadTsc ();                                           // TSC value 100us later

+  EndTSC = AsmReadTsc ();                                           // TSC value 101.4 us later

 

   TscFrequency = MultU64x32 (

-                   (EndTSC - StartTSC),                             // Number of TSC counts in 100us

-                   10000                                            // Number of 100us in a second

+                   (EndTSC - StartTSC),                             // Number of TSC counts in 101.4 us

+                   9861                                             // Number of 101.4 us in a second

                    );

 

   SetInterruptState (InterruptState);

diff --git a/PcAtChipsetPkg/Library/AcpiTimerLib/BaseAcpiTimerLib.c b/PcAtChipsetPkg/Library/AcpiTimerLib/BaseAcpiTimerLib.c
index 8819ebcfcc..29521f8b22 100644
--- a/PcAtChipsetPkg/Library/AcpiTimerLib/BaseAcpiTimerLib.c
+++ b/PcAtChipsetPkg/Library/AcpiTimerLib/BaseAcpiTimerLib.c
@@ -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.

 

diff --git a/PcAtChipsetPkg/Library/AcpiTimerLib/DxeAcpiTimerLib.c b/PcAtChipsetPkg/Library/AcpiTimerLib/DxeAcpiTimerLib.c
index 7f7b0f8f62..b141c680fb 100644
--- a/PcAtChipsetPkg/Library/AcpiTimerLib/DxeAcpiTimerLib.c
+++ b/PcAtChipsetPkg/Library/AcpiTimerLib/DxeAcpiTimerLib.c
@@ -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.