Hi Xining,
Control symbols are physical layer message elements used to manage link maintenance, packet
delimiting, packet acknowledgment, error reporting, and error recovery. They are protected by their own 5-bit CRC, which provides error detection on the control symbols.
When error-free control symbols are being exchanged, that means the ports have been initialized and are communicating. For more details on control symbols and control symbol protection, please see the Rapid IO Part 6: Serial Physical Layer Specification. This can be found in revision 1.2 for TI's implementation on the C6455.
RapidIO specifation page: http://www.rapidio.org/rapidio-specifications/#tab1955
Additionally, you can see details of the 'port initialization' sequence and state machines in the same document.
Thanks,
Clinton
PS: It looks like the RapidIO website is not functioning properly. You may need to request the documents from them directly.