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Subject[tip:x86/urgent] x86/tsc: Future-proof native_calibrate_tsc()
Commit-ID:  da4ae6c4a0b8dee5a5377a385545d2250fa8cddb
Author: Len Brown <>
AuthorDate: Fri, 22 Dec 2017 00:27:54 -0500
Committer: Thomas Gleixner <>
CommitDate: Sun, 14 Jan 2018 12:14:50 +0100

x86/tsc: Future-proof native_calibrate_tsc()

If the crystal frequency cannot be determined via CPUID(15).crystal_khz or
the built-in table then native_calibrate_tsc() will still set the
X86_FEATURE_TSC_KNOWN_FREQ flag which prevents the refined TSC calibration.

As a consequence such systems use cpu_khz for the TSC frequency which is
incorrect when cpu_khz != tsc_khz resulting in time drift.

Return early when the crystal frequency cannot be retrieved without setting
the X86_FEATURE_TSC_KNOWN_FREQ flag. This ensures that the refined TSC
calibration is invoked.

[ tglx: Steam-blastered changelog. Sigh ]

Fixes: 4ca4df0b7eb0 ("x86/tsc: Mark TSC frequency determined by CPUID as known")
Signed-off-by: Len Brown <>
Signed-off-by: Thomas Gleixner <>
Cc: Bin Gao <>

arch/x86/kernel/tsc.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 8ea117f..ce4b711 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -612,6 +612,8 @@ unsigned long native_calibrate_tsc(void)

+ if (crystal_khz == 0)
+ return 0;
* TSC frequency determined by CPUID is a "hardware reported"
* frequency and is the most accurate one so far we have. This
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