On 12/14/2017 10:46 PM, Len Brown wrote:
> From: Len Brown <len.brown@xxxxxxxxx>
>
> Linux-4.9 added INTEL_FAM6_SKYLAKE_X to native_calibrate_tsc(),
> allowing the kernel to calculate the TSC frequency,
> instead of calibrating:
>
> commit 6baf3d61821f
> ("x86/tsc: Add additional Intel CPU models to the crystal quirk list")
>
> There are several problems with doing this.
>
> The first is that while SKX servers use a 25 MHz crystal,
> SKX workstations (with same model #) use a 24 MHz crystal.
> This results in a -4.0% time drift rate on SKX workstations.
>
> But even SKX servers have a problem.
> SKX subjects the crystal to an EMI reduction circuit that
> reduces its actual value by (approximately) -0.25%.
> This results in -1 second per 10 minute time drift
> as compared to network time on SKX servers.
>
> This also results in an even more subtle symptom on systems
> that use the LAPIC timer (versus the TSC deadline timer).
> Clock ticks scheduled with the LAPIC timer arrive a few usec
> before the time they are expected (according to the slow TSC).
> This causes Linux to poll-idle, when it should be in an idle
> power saving state. The idle and clock code do not graciously
> recover from this error, sometimes resulting in significant polling
> and significant power impact.
>
> So stop using native_calibrate_tsc() for INTEL_FAM6_SKYLAKE_X.
> native_calibrate_tsc() will return 0, boot will run with
> tsc_khz = cpu_khz, and the TSC refined calibration will
> update tsc_khz to correct for the difference.
>
> Yes, a future patch can detect 24 vs 25 MHz crystals,
> and it can estimate a correction for the EMI circuit impact.
> However, that would effect boot-time only, as we'd probably
> choose to keep the refined TSC calibration, because the EMI
> correction estimate may not be precise.
>
> Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
> Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.9+
> ---
> arch/x86/kernel/tsc.c | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 49d772672367..19ca0004caf6 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -602,7 +602,6 @@ unsigned long native_calibrate_tsc(void)
> case INTEL_FAM6_KABYLAKE_DESKTOP:
> crystal_khz = 24000; /* 24.0 MHz */
> break;
> - case INTEL_FAM6_SKYLAKE_X:
> case INTEL_FAM6_ATOM_DENVERTON:
> crystal_khz = 25000; /* 25.0 MHz */
> break;
>
Len, I lifted this code directly from tools/power/x86/turbostat/turbostat.c:4155
-- is that code incorrect too?
P.