x86/tsc: Mark TSC frequency determined by CPUID as known
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: NBin 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.comSigned-off-by: NThomas Gleixner <tglx@linutronix.de>
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