LiteEthStream2UDPTX: fix for end of buffer tags

[hcab14]

Please have a look. For it works, i.e. I can use the entire GBit ethernet bandwidth (using a butterstick).

I wonder how one could make a CI test for this.

[rowanG077]

Why wasn't it possible to reach GBit ethernet speeds before this? If I read the code correctly this will double the bram usage. Which I'm not a fan of.

[hcab14]

It was possible to reach GBit speeds before but the last tags were not properly dealt with.

In two cases it did work as expected:

1. last=0: the FIFO was always fully filled and then its contents were sent in one UDP packet
2. last=1: FIFO content was sent one by one

But, e.g., if I wanted to send UDP packets with two FIFO entries, i.e., last was made to toggle it did not work because last is checked only in the IDLE state and not during the SEND state.

[rowanG077]

I understand. I had this issue too.

But I think double buffering is not the correct path here due to such waste of resources. I dealt with it by building a custom FIFO and using the UDP raw interface.What I would advise here is to replace the SyncFIFO with the PacketFIFO. It should be possible to send full packets only. I didn't go this route because I'm not proficient in migen/litex.

The drawback is that the user will have to specify the length of the packet up front.

[enjoy-digital]

Thanks @hcab14, @rowanG077, while discussing with @hcab14 about the issue (on IRC IIRC), I also thought about replacing the SyncFIFO with a proper PacketFIFO. I'll try to have a closer look at this soon. We should indeed try to minimize resource usage.