x86/tsc: Convert ART in nanoseconds to TSC

Device drivers use get_device_system_crosststamp() to produce precise
system/device cross-timestamps. The PHC clock and ALSA interfaces, for
example, make the cross-timestamps available to user applications.  On
Intel platforms, get_device_system_crosststamp() requires a TSC value
derived from ART (Always Running Timer) to compute the monotonic raw and
realtime system timestamps.

Starting with Intel Goldmont platforms, the PCIe root complex supports the
PTM time sync protocol. PTM requires all timestamps to be in units of
nanoseconds. The Intel root complex hardware propagates system time derived
from ART in units of nanoseconds performing the conversion as follows:

     ART_NS = ART * 1e9 / <crystal frequency>

When user software requests a cross-timestamp, the system timestamps
(generally read from device registers) must be converted to TSC by the
driver software as follows:

    TSC = ART_NS * TSC_KHZ / 1e6

This is valid when CPU feature flag X86_FEATURE_TSC_KNOWN_FREQ is set
indicating that tsc_khz is derived from CPUID[15H]. Drivers should check
whether this flag is set before conversion to TSC is attempted.

Suggested-by: Christopher S. Hall <christopher.s.hall@intel.com>
Signed-off-by: Rajvi Jingar <rajvi.jingar@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: peterz@infradead.org
Link: https://lkml.kernel.org/r/1520530116-4925-1-git-send-email-rajvi.jingar@intel.com