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

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/08/2016 04:26, Star Zeng wrote:
> 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.
>  
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

On top of this you can add the logic to wait for the beginning of a tick
before calling rdtsc.

Paolo