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

[Star Zeng <star.zeng@intel.com>]

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>
---
 PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c | 32 +++++++++++++---------
 .../Library/AcpiTimerLib/BaseAcpiTimerLib.c        | 14 +++++-----
 .../Library/AcpiTimerLib/DxeAcpiTimerLib.c         | 14 +++++-----
 3 files changed, 33 insertions(+), 27 deletions(-)

diff --git a/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c b/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c
index e6fea231123d..020031e3f4a5 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 8819ebcfccef..29521f8b220b 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 7f7b0f8f6294..b141c680fb82 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.
 
-- 
2.7.0.windows.1