servo not updating on slave (after ~49.7days)

Servo stuck on slave WRS. Looks like regular, but update counter is not increased. On master's side, the port is marked as EXT_OFF.

wr_mon's output on slave:

WR Switch Sync Monitor v6.0-82-g1debde40 [q = quit]

WR time (TAI)    : 2021-06-24 14:46:33.006139   Leap seconds:  37
Switch time (UTC): 2021-06-24 14:45:56.006130   TAI-UTC     : +37.000009
TimingMode: BC    PLL locking state: LOCKED  
----- HAL ---|---------------------------------- PPSI --------------------------------------------------------
 Iface| Freq |Inst|     Name     |   Config   | MAC of peer port  |    PTP/EXT/PDETECT States    | Pro | VLANs
------+------+----+--------------+------------+-------------------+------------------------------+-----+------
 wri1 |      |  0 |wri1-1-wr-raw |slave       | XX:XX:XX:XX:XX:XX | SLAVE     /IDLE      /EXT_ON | V-W | 31
 wri2 |      |  1 |wri2-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| R-W | 
 wri3 |      |  2 |wri3-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| R-W | 
 wri4 |      |  3 |wri4-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| R-W | 
 wri5 |      |  4 |wri5-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| R-W | 
 wri6 |      |  5 |wri6-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| R-W | 
 wri7 |      |  6 |wri7-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| R-W | 
 wri8 |      |  7 |wri8-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| R-W | 
*wri9 |      |  8 |wri9-1-wr-raw |master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri10|      |  9 |wri10-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri11|      | 10 |wri11-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri12|      | 11 |wri12-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri13|      | 12 |wri13-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri14|      | 13 |wri14-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri15|      | 14 |wri15-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri16|      | 15 |wri16-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri17|      | 16 |wri17-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
*wri18|      | 17 |wri18-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
Pro - Protocol mapping: V-Ethernet over VLAN; U-UDP; R-Ethernet

--------------------------- Synchronization status ----------------------------
Servo state:          wri1:White-Rabbit: TRACK_PHASE

 +- Timing parameters ---------------------------------------------------------
 | meanDelay        :           80.519 nsec
 | delayMS          :           80.508 nsec
 | delayMM          :         1063.492 nsec
 | delayAsymmetry   :            0.011 nsec
 | delayCoefficient : -0.000267798264878874 fpa : -1235001513901056
 | ingressLatency   :          225.959 nsec
 | egressLatency    :          224.295 nsec
 | semistaticLatency:            0.000 nsec
 | offsetFromMaster :            0.005 nsec
 | Phase setpoint   :          -15.047 nsec
 | Skew             :           -0.012 nsec
 | Update counter   :          4212013 times    <--------------------------- does not increase
 | Master PHY delays TX:          224.879 nsec  RX:          227.321 nsec
 | Slave  PHY delays TX:          224.295 nsec  RX:          225.959 nsec

wr_mon's output on master:

WR Switch Sync Monitor v6.0-82-g1debde40 [q = quit]

WR time (TAI)    : 2021-06-23 00:02:38.000217   Leap seconds:  37
Switch time (UTC): 2021-06-23 00:02:01.000210   TAI-UTC     : +37.000007
TimingMode: BC    PLL locking state: LOCKED  
----- HAL ---|---------------------------------- PPSI --------------------------------------------------------
 Iface| Freq |Inst|     Name     |   Config   | MAC of peer port  |    PTP/EXT/PDETECT States    | Pro | VLANs
------+------+----+--------------+------------+-------------------+------------------------------+-----+------
 wri1 | Lock |  0 |wri1-1-wr-raw |slave       | XX:XX:XX:XX:XX:XX | SLAVE     /IDLE      /EXT_ON | V-W | 31
*wri2 |      |  1 |wri2-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | DISABLED  /IDLE      /NONE   | V-W | 31
*wri3 |      |  2 |wri3-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | DISABLED  /IDLE      /NONE   | V-W | 31
 wri4 |      |  3 |wri4-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_ON | V-W | 31
 wri5 |      |  4 |wri5-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_OFF| V-W | 31    <--------- slave connected to this port, note the EXT_OFF
 wri6 |      |  5 |wri6-1-wr-raw |master      | XX:XX:XX:XX:XX:XX | MASTER    /IDLE      /EXT_ON | V-W | 31
*wri7 |      |  6 |wri7-1-wr-raw |master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri8 |      |  7 |wri8-1-wr-raw |master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri9 |      |  8 |wri9-1-wr-raw |master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri10|      |  9 |wri10-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri11|      | 10 |wri11-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri12|      | 11 |wri12-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri13|      | 12 |wri13-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri14|      | 13 |wri14-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri15|      | 14 |wri15-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri16|      | 15 |wri16-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri17|      | 16 |wri17-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | V-W | 31
*wri18|      | 17 |wri18-1-wr-raw|master      | 00:00:00:00:00:00 | DISABLED  /IDLE      /NONE   | R-W | 
Pro - Protocol mapping: V-Ethernet over VLAN; U-UDP; R-Ethernet

--------------------------- Synchronization status ----------------------------
Servo state:          wri1:White-Rabbit: TRACK_PHASE

 +- Timing parameters ---------------------------------------------------------
 | meanDelay        :         2527.601 nsec
 | delayMS          :         2527.263 nsec
 | delayMM          :         5973.412 nsec
 | delayAsymmetry   :            0.338 nsec
 | delayCoefficient : -0.000267798264878874 fpa : -1235001513901056
 | ingressLatency   :          225.959 nsec
 | egressLatency    :          224.295 nsec
 | semistaticLatency:            0.000 nsec
 | offsetFromMaster :            0.002 nsec
 | Phase setpoint   :            1.839 nsec
 | Skew             :            0.003 nsec
 | Update counter   :           495091 times
 | Master PHY delays TX:          227.016 nsec  RX:          240.940 nsec
 | Slave  PHY delays TX:          224.295 nsec  RX:          225.959 nsec


-------------------------------- Temperatures ---------------------------------
FPGA: 58.69 PLL: 55.06 PSL: 32.62 PSR: 37.75

When servo is stuck it is necessary to restart PPSI or do link down/up (by ifconfig or unplug/plug fiber).

Only firmware v6.0 is affected with this bug.

Related to #239 (closed), #235 (closed). The details are described in ppsi#32.