[tip:x86/timers] x86/tsc: Mark TSC frequency determined by CPUID as known

[tip-bot for Bin Gao]

Commit-ID:  4ca4df0b7eb06df264b2919759957f6d6ea1822e
Gitweb:     http://git.kernel.org/tip/4ca4df0b7eb06df264b2919759957f6d6ea1822e
Author:     Bin Gao <bin.gao@xxxxxxxxxxxxxxx>
AuthorDate: Tue, 15 Nov 2016 12:27:22 -0800
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 18 Nov 2016 10:58:30 +0100

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: Bin Gao <bin.gao@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1479241644-234277-3-git-send-email-bin.gao@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 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 d2c4ee4..e58c319 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;
 }