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>
diff --git a/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c b/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c
index e6fea23..020031e 100644
--- a/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c
+++ b/PcAtChipsetPkg/Library/AcpiTimerLib/AcpiTimerLib.c
@@ -340,13 +340,13 @@
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.
@@ -366,22 +366,28 @@
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 8819ebc..29521f8 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 7f7b0f8..b141c68 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.
