SNLS543 August 2018 DS90UH949A-Q1
When using older FPD-Link III devices, simultaneous access to serializer or deserializer registers from both local and remote I2C masters may cause incorrect operation, thus restrictions should be imposed on accessing of serializer and deserializer registers. The likelihood of an error occurrence is relatively small, but it is possible for collision on reads and writes to occur, resulting in an read or write error.
Two basic options are recommended. The first is to allow device register access only from one controller. This would allow only the Host controller to access the serializer registers (local) and the deserializer registers (remote). A controller at the deserializer would not be allowed to access the deserializer or serializer registers.
The second basic option is to allow local register access only with no access to remote serializer or deserializer registers. The Host controller would be allowed to access the serializer registers while a controller at the deserializer could access those register only. Access to remote I2C slaves would still be allowed in one direction.
In a very limited case, remote and local access could be allowed to the deserializer registers at the same time. Register access is ensured to work correctly if both local and remote masters are accessing the same deserializer register. This allows a simple method of passing control of the Bidirectional Control Channel from one master to another.