[PATCH 2/4] x86/tsc: mark TSC frequency determined by CPUID as known

[Bin Gao <bin.gao@linux.intel.com>]

Unlike TSC calibration where we determin TSC frequency by another timer
with known frequency, 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 so we set the X86_FEATURE_TSC_KNOWN_FREQ flag to skip the whole
software calibration process.

We had a 24 hours test on one of the CPUID 0x15 capable platforms. With
PIT calibrated frequency, we got more than 3 seconds drift whereas with
CPUID determined frequency we only got less than 0.5 second drift. This
makes us believe that we should prefer CPUID determined frequency over
software calibrated frequency.

Signed-off-by: Bin Gao <bin.gao@intel.com>
---
 arch/x86/kernel/tsc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 3ba146e..f1a7fb5 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -702,6 +702,13 @@ unsigned long native_calibrate_tsc(void)
 		}
 	}
 
+	/*
+	 * TSC frequency determined by CPUID is a "hardware reported"
+	 * frequency and is the most accurate one so far we have. This
+	 * is considered a known frequency.
+	 */
+	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+
 	return crystal_khz * ebx_numerator / eax_denominator;
 }
 
-- 
1.9.1