x86/tsc: Mark TSC frequency determined by CPUID as known

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CPUs/SoCs with CPUID leaf 0x15 come with a known frequency and will report
the frequency to software via CPUID instruction. This hardware provided
frequency is the "real" frequency of TSC.

Set the X86_FEATURE_TSC_KNOWN_FREQ flag for such systems to skip the
software calibration process.

A 24 hours test on one of the CPUID 0x15 capable platforms was
conducted. PIT calibrated frequency resulted in more than 3 seconds drift
whereas the CPUID determined frequency showed less than 0.5 second
drift.

Signed-off-by: Bin Gao <bin.gao@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1479241644-234277-3-git-send-email-bin.gao@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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Bin Gao 2016-11-15 12:27:22 -08:00 • committed by Thomas Gleixner

parent 47c95a46d0

commit 4ca4df0b7e

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arch/x86/kernel/tsc.c

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