The PHY provides only 39b timestamp. With current timing
implementation, we discard lower 7b, leaving 32b timestamp.
The driver reconstructs the full 64b timestamp by correlating the
32b timestamp with cached_time for performance. The reconstruction
algorithm does both forward & backward interpolation.

The 32b timeval has overflow duration of 2^32 counts ~= 4.23 second.
Due to interpolation in both direction, its now ~= 2.125 second
IIRC, going with at least half a duration, the cached_time is updated
with periodic thread of 1 second (worst-case) periodicity.

But the 1 second periodicity is based on System-timer.
With PPB adjustments, if the 1588 timers increments at say
double the rate, (2s in-place of 1s), the Nyquist rate/half duration
sampling/update of cached_time with 1 second periodic thread will
lead to incorrect interpolations.

Hence we should restrict the PPB adjustments to at least half duration
of cached_time update which translates to 500,000,000 PPB.

Since the periodicity of the cached-time system thread can vary,
it is good to have some buffer time and considering practicality of
PPB adjustments, limiting the max_adj to 100,000,000.
Signed-off-by: Siddaraju DH <siddaraju.dh@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>