Hi David,
The SRIO peripheral on the C6678 has 4 SerDes lanes available for use. More description can be found in section 2.3.1.4 SerDes Configuration Example and 3.17.10 PLM Port(n) Path Control Register of the SRIO Users Guide (http://www.ti.com/lit/pdf/sprugw1). It should be noted that table 2-4 is pending an update and that the modes for configuration 4 should be as depicted in the table below:
As you can see, the 4 lanes can form various port widths. Based on the example setup you provided (DSP0 connected to DSP1 and DSP2), you could use mode 3 and have 2 2x ports. One 2x port connecting DSP0 to DSP1 and the other 2x port connecting DSP0 to DSP2. As shown below:
DSP0 <--- lane A & lane B ---> DSP1
DSP0 <--- lane C & lane D ---> DSP2
Other configurations are possible as well depending on what your system requirements are. For example you could use mode 0, 1 or 2 and have additional ports available to connect to other devices.
For further information on throughput capabilities of SRIO, have a look at our Keystone I Throughput Guide (http://www.ti.com/lit/pdf/sprabk5).
Let us know if you have any additional questions.
Thanks,
Clinton