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SubjectRe: [PATCH 1/2] x86/tsc: use CPUID.0x16 to calculate missing crystal frequency
On Mon, Apr 22, 2019 at 06:15:25PM +0800, Daniel Drake wrote:
> Additionally, crystal clock frequency for platforms that were missing
> from the list (e.g. SKYLAKE_X) will now be provided.

Well, SKX isn't exactly 'missing'; it would be very good if we can
confirm the new code is still correct under the below mentioned
conditions.

---

commit b511203093489eb1829cb4de86e8214752205ac6
Author: Len Brown <len.brown@intel.com>
Date: Fri Dec 22 00:27:55 2017 -0500

x86/tsc: Fix erroneous TSC rate on Skylake Xeon

The INTEL_FAM6_SKYLAKE_X hardcoded crystal_khz value of 25MHZ is
problematic:

- SKX workstations (with same model # as server variants) use a 24 MHz
crystal. This results in a -4.0% time drift rate on SKX workstations.

- SKX servers subject the crystal to an EMI reduction circuit that reduces its
actual frequency by (approximately) -0.25%. This results in -1 second per
10 minute time drift as compared to network time.

This issue can also trigger a timer and power problem, on configurations
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 measurable power impact.

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.

[ tglx: Sanitized change log ]

Fixes: 6baf3d61821f ("x86/tsc: Add additional Intel CPU models to the crystal quirk list")
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: peterz@infradead.org
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/ff6dcea166e8ff8f2f6a03c17beab2cb436aa779.1513920414.git.len.brown@intel.com

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index ce4b71119c36..3bf4df7f52d7 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;
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