Merge branch 'PHC-freq-fine-tuning'

Richard Cochran says:

====================
PHC frequency fine tuning

This series expands the PTP Hardware Clock subsystem by adding a
method that passes the frequency tuning word to the the drivers
without dropping the low order bits.  Keeping those bits is useful for
drivers whose frequency resolution is higher than 1 ppb.

The appended script (below) runs a simple demonstration of the
improvement.  This test needs two Intel i210 PCIe cards installed in
the same PC, with their SDP0 pins connected by copper wire.  Measuring
the estimated offset (from the ptp4l servo) and the true offset (from
the PPS) over one hour yields the following statistics.

|        |   Est. Before |    Est. After |   True Before |    True After |
|--------+---------------+---------------+---------------+---------------|
| min    | -5.200000e+01 | -1.600000e+01 | -3.100000e+01 | -1.000000e+00 |
| max    | +5.700000e+01 | +2.500000e+01 | +8.500000e+01 | +4.000000e+01 |
| pk-pk: | +1.090000e+02 | +4.100000e+01 | +1.160000e+02 | +4.100000e+01 |
| mean   | +6.472222e-02 | +1.277778e-02 | +2.422083e+01 | +1.826083e+01 |
| stddev | +1.158006e+01 | +4.581982e+00 | +1.207708e+01 | +4.981435e+00 |

Here the numbers in units of nanoseconds, and the ~20 nanosecond PPS
offset is due to input/output delays on the i210's external interface
logic.

With the series applied, both the peak to peak error and the standard
deviation improve by a factor of more than two.  These two graphs show
the improvement nicely.

  http://linuxptp.sourceforge.net/fine-tuning/fine-est.png

  http://linuxptp.sourceforge.net/fine-tuning/fine-tru.png
====================

Signed-off-by: David S. Miller <davem@davemloft.net>