media_settings.json are not decoded properly on ports with unused lanes

[abohanyang]

I am adding the media_settings.json file for x86_64-arista_7800r3a_36d2_lc. This platform has 36 QSFP-DD ports. The media_settings.json contains the tuning data for lane0-lane7 on each QSFP-DD port. But this makes the 36x100G SKU unhappy because the xcvrd sets an 8-lane tuning data for a 4-lane 100G port. The Jupiter SAI will fail port serdes creation because the number of port serdes does not match the number of tuning parameters.

root@s1:~# sonic-db-cli -n asic1 APPL_DB hgetall "PORT_TABLE:Ethernet240"
{'admin_status': 'up', 'alias': 'Ethernet31/1', 'asic_port_name': 'Eth240-ASIC1', 'coreId': '0', 'corePortId': '25', 'description': 'Ethernet240-connected-to-nv419@eth56/1', 'fec': 'rs', 'index': '31', 'lanes': '40,41,42,43', 'mtu': '9100', 'numVoq': '8', 'pfc_asym': 'off', 'role': 'Ext', 'speed': '100000', 'tpid': '0x8100', 'oper_status': 'up', 'main': '0x4e,0x4e,0x4b,0x4b,0x4b,0x4e,0x4b,0x4e', 'post1': '-0x16,-0x16,-0x14,-0x14,-0x14,-0x16,-0x14,-0x16', 'post2': '0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0', 'post3': '0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0', 'pre1': '-0x5,-0x5,-0x5,-0x5,-0x5,-0x5,-0x5,-0x5', 'pre2': '0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0'}

The problem is that the method "get_media_val_str" in sonic-platform-daemons/sonic-xcvrd/xcvrd/xcvrd.py cannot handle this case properly. I am thinking about how to handle this issue. Should we fix the logic in get_media_val_str, read the number of lanes from config_db? or make an SKU-specific media_settings.json file?