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

[Len Brown]

From: Len Brown <len.brown@xxxxxxxxx>

Linux-4.9 added INTEL_FAM6_SKYLAKE_X to native_calibrate_tsc():

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.

While SKX servers do have a 25  MHz crystal, but they too have a problem.
All SKX 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.

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.

This patch restores correctness.  Without it, all three of the
issues above occur.

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 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;
-- 
2.14.0-rc0