FX3: 32 bit protocol

[wallento]

To gain more bandwidth we can use another signaling protocol on the FX3 (and other 32 bit wide physical interfaces).

Problem

GLIP is inteded to provide a 16 bit interface. If we currently have a 32 bit interface (FX3) only the lower 16 bit are used. Instead there should be some (de-) serialization. The problem is how to distinct a valid item from an invalid item on the upper 16 bit (the lower 16 bit are always set apparently).

Possible solution

A solution we chose to generate a side channel in the UART backend is to use a special word to signal no-data and to duplicate actual occurences of this item.

This requires some more fiddling and discussion.

[imphil]

small correction here (agree otherwise): we use a 16 bit firmware for 16 bit FIFO width, so no bandwidth is wasted (e.g. on the USB bus). We're just limited by the maximum frequency of fx3_pclk, which doesn't allow using the full USB 3 bandwidth but only ~ 100 MByte/s bidirectional.