Commit da4ae6c4 authored Dec 22, 2017 by Len Brown Committed by Thomas Gleixner Jan 14, 2018

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: 4ca4df0b ("x86/tsc: Mark TSC frequency determined by CPUID as known")
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: peterz@infradead.org
Cc: Bin Gao <bin.gao@intel.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/0fe2503aa7d7fc69137141fc705541a78101d2b9.1513920414.git.len.brown@intel.com

parent 9f15b912

arch/x86/kernel/tsc.c

+2 −0

Original line number	Original line	Diff line number	Diff line
	@@ -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"		* TSC frequency determined by CPUID is a "hardware reported"
	* frequency and is the most accurate one so far we have. This		* frequency and is the most accurate one so far we have. This

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