From: Len Brown len.brown@intel.com
If native_calibrate_tsc() can not discover the TSC frequency, via CPUID or via built-in table, it must return without setting X86_FEATURE_TSC_KNOWN_FREQ. Otherwise, X86_FEATURE_TSC_KNOWN_FREQ will prevent TSC refined calibration.
This patch allows Linux to correctly support future Intel hardware, that has (cpu_khz != tsc_khz), and support for CPUID.15 without support for CPUID.15.crystal_khz.
This patch is needed since X86_FEATURE_TSC_KNOWN_FREQ was added in Linux-4.10:
commit 4ca4df0b7eb0 ("x86/tsc: Mark TSC frequency determined by CPUID as known")
If not applied, such systems will run with tsc_khz = cpu_khz, which may result in under-stated TSC rate, and time-of-day drift.
Signed-off-by: Len Brown len.brown@intel.com Cc: Bin Gao bin.gao@intel.com Cc: stable@vger.kernel.org # v4.10+ --- arch/x86/kernel/tsc.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 8ea117f8142e..ce4b71119c36 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -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" * frequency and is the most accurate one so far we have. This