"WARNING: TX SGDMA TImeout" and no data sent

Regarding FIFO:

Sorry, you are correct that there is no '# of bytes in FIFO' register or anything nearly so nice. What I did was ping the TSE until the error bit (statistics counter offset 0x2b) went true.

Since you normally need an arp transmit/reply before you can send any packet to a system, I hardcoded the IP level MAC address into the system from which I was pinging so that no ARP was needed. On Linux use 'arp -i <interface> -s <ipaddr> <macaddr>'). I suppose it might be interesting to watch the arp broadcasts and see if they fill up the FIFO...