This article explains the meaning of the syslog : fpc0 [RERROR]pfpga_pcie_i2c_read_delay:169: i2c read error addr: 0x50 0

This is syslog seen intermittently on Lumentum CWDM4 100G xcvr. The i2c read failure is seen randomly and does not have any functional impact. 

There have been some issues seen previously on the optic from same vendor and there is one potential issue fixed for Ultimate platform for which similar fix we might need. 

Lumentum vendor XCVR takes more time to respond between successive reads and sometimes reads fail.

On PTX1K, QFX10002-60C and PTX10002-60C platforms, Issue seen on the devices seem to be happening on LUMENTUM vendor 100G-CWDM4 xcvr type.  Below error logs coming continuously post upgrade of the device.
RE0  fpc0 [RERROR]pfpga_pcie_i2c_read_delay:169: i2c read error addr: 0x50 seg: 0x51 off: 0x0 len: 0x20
RE0 fpc0 [RERROR]pfpga_pcie_i2c_read_delay:169: i2c read error addr: 0x50 seg: 0x51 off: 0x0 len: 0x20

FPC0(RE0 vty)# show qsfp 1 identifier        
  Identifier:             QSFP28
  Connector type:         LC
  Encoding:               Reserved
  Vendor name:            LUMENTUM        
  Vendor OUI:             00019C
  Vendor PN:              LQ210CR-CPA1    
  Vendor Rev:             01
  Vendor SN:              FH13572505D5    
  Juniper PN:             NON-JNPR
  Transceiver code:
    10G/40G/100G Ethernet:     100G-CWDM4
  Device description:     QSFP-100G-CWDM4

This PR could also lead to I2C write error as well, as showed in below logs:

addr: 0x50 seg: 0x23 off: 0x0 len: 0x20
:29581: Apr 14 14:53:25.444  RE0 fpc0 [WERROR]pfpga_pcie_i2c_read_delay:165: i2c write error addr: 0x50 seg: 0x23 off: 0x0 len: 0x20
:29594: Apr 14 17:44:59.564  RE0 fpc0 [WERROR]pfpga_pcie_i2c_read_delay:165: i2c write error addr: 0x50 seg: 0x23 off: 0x0 len: 0x20
:29594: Apr 14 17:44:59.564  RE0 fpc0 [WERROR]pfpga_pcie_i2c_read_delay:165: i2c write error addr: 0x50 seg: 0x23 off: 0x0 len: 0x20
addr: 0x50 seg: 0x23 off: 0x0 len: 0x20
:29581: Apr 14 14:53:25.444  RE0 fpc0 [WERROR]pfpga_pcie_i2c_read_delay:165: i2c write error addr: 0x50 seg: 0x23 off: 0x0 len: 0x20
:29594: Apr 14 17:44:59.564  RE0 fpc0 [WERROR]pfpga_pcie_i2c_read_delay:165: i2c write error addr: 0x50 seg: 0x23 off: 0x0 len: 0x20
:29594: Apr 14 17:44:59.564  RE0 fpc0 [WERROR]pfpga_pcie_i2c_read_delay:165: i2c write error addr: 0x50 seg: 0x23 off: 0x0 len: 0x20

Lumentum vendor xcvr takes more time to respond between successive reads and sometimes reads fail. Increase the time between successive reads.

These logs are intermittent informational logs and would not impact any functionality. We can safely ignore these logs unless the logs are continuously seen on every read or write cycle. Also, this is seen on Lumentum xcvr more frequently.

https://gnats.juniper.net/web/default/1760100#external_tab