According to Synopsis DesignWare EQoS Databook, the Clock Domain Cross
synchronization error is introduced tue to the clock(GMII Tx/Rx clock)
being different at the capture as compared to the PTP
clock(clk_ptp_ref_i) that is used to generate the time.

The CDC synchronization error is almost equal to 2 times the clock
period of the PTP clock(clk_ptp_ref_i).

On a Intel Tigerlake platform (with Marvell 88E2110 external PHY):

Before applying this patch (with CDC synchronization error):
ptp4l[64.044]: rms    8 max   13 freq +30877 +/-  11 delay   216 +/-   0
ptp4l[65.047]: rms   13 max   20 freq +30869 +/-  17 delay   213 +/-   0
ptp4l[66.050]: rms   12 max   20 freq +30857 +/-  11 delay   213 +/-   0
ptp4l[67.052]: rms   11 max   22 freq +30849 +/-  10 delay   215 +/-   0
ptp4l[68.055]: rms   10 max   16 freq +30853 +/-  13 delay   215 +/-   0
ptp4l[69.057]: rms    7 max   13 freq +30848 +/-   9 delay   216 +/-   0
ptp4l[70.060]: rms    8 max   13 freq +30846 +/-  10 delay   216 +/-   0
ptp4l[71.063]: rms    9 max   15 freq +30836 +/-   8 delay   218 +/-   0

After applying this patch (CDC syncrhonization error is taken care of):
ptp4l[61.516]: rms  773 max  824 freq +31526 +/- 158 delay   200 +/-   0
ptp4l[62.519]: rms  427 max  596 freq +31668 +/-  39 delay   198 +/-   0
ptp4l[63.522]: rms  113 max  206 freq +31482 +/-  57 delay   198 +/-   0
ptp4l[64.525]: rms   40 max   56 freq +31316 +/-  29 delay   200 +/-   0
ptp4l[65.528]: rms   47 max   56 freq +31255 +/-  17 delay   200 +/-   0
ptp4l[66.531]: rms   26 max   36 freq +31246 +/-   9 delay   200 +/-   0
ptp4l[67.534]: rms   12 max   18 freq +31254 +/-  12 delay   202 +/-   0
ptp4l[68.537]: rms    7 max   12 freq +31263 +/-  10 delay   202 +/-   0
Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: Wong Vee Khee <vee.khee.wong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>