[SRU][Artful][PATCH 2/3] x86/tsc: Fix erroneous TSC rate on Skylake Xeon

Joseph Salisbury joseph.salisbury at canonical.com
Thu Feb 1 18:59:28 UTC 2018

From: Len Brown <len.brown at intel.com>

BugLink: http://bugs.launchpad.net/bugs/1744988

The INTEL_FAM6_SKYLAKE_X hardcoded crystal_khz value of 25MHZ is

 - 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

[ 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 at intel.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Cc: peterz at infradead.org
Cc: Prarit Bhargava <prarit at redhat.com>
Cc: stable at vger.kernel.org
Link: https://lkml.kernel.org/r/ff6dcea166e8ff8f2f6a03c17beab2cb436aa779.1513920414.git.len.brown@intel.com

(cherry picked from commit b511203093489eb1829cb4de86e8214752205ac6)
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
 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 8f5cf85..0be7a5f 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -602,7 +602,6 @@ unsigned long native_calibrate_tsc(void)
 			crystal_khz = 24000;	/* 24.0 MHz */
 			crystal_khz = 25000;	/* 25.0 MHz */

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