xConnect channels between logical cores on same tile

[erlingrj]

Hi there.

As far as I understand the xConnect on XS2 we have 4 physical links from each Tile onto the Switch. Each Switch has 8 physical links that can connect it to other on-chip or off-chip tiles. My question is, what happens when two logical cores on the same tile open e.g. a streaming channel between them. Please see the attached PNG where I indicate 2 possibilities:
1. A channel is formed through the switch itself and will consume of the available bandwidth of the switch
2. A channel is formed directly between the logical cores and the available bandwidth of the switch remains unchanged

I want to use the Streaming UART library which allows for baud rates >115200. However it requires a streaming channel being opened and AFAIK the Switch can only support 4 concurrent streaming channels. My application needs >4 UART receiver which makes it infeasible. I would need to implement a fast_uart that uses shared memory rather than a streaming channel


Thanks for your help.

[CousinItt]

The data does say that there are four links from each tile to the switch, but the build process reports 32 chanends per tile. Use the -report switch to see your usage. I'm not sure of the details but I am guessing that each channel doesn't consume all of the resources of a link, presumably a certain range of tokens are allocated for each channel.

I tried a quick example using 6 streaming UART receiver tasks and a single task to take all their data, all on the same tile. It built quite happily, consuming two chanends per receiver, and I was able to start it in the simulator, even if it didn't do much.

If you don't need super-high speeds, it might also be a possibility to base your own driver on the UART library, or just tweak it a bit. That might allow you to get more than just one receiver per core. I've never really understood why the standard (non-streaming) UART was limited to 115200 bps.

For more on the links see here: https://www.xcore.com/viewtopic.php?f=47&t=7946

[erlingrj]

Hi Cousin! Thanks for your reply and the links. It seems as if data can be routed between chanends without leaving the tile. This makes sense and makes things easier for me. I will verify this when I get a dev kit.

[CousinItt]

No problem. I've just been looking at this again and I think I have a better understanding. Each tile has 32 chanends (according to the data sheets for the devices I've looked at) and I think the switch is only involved in communication between tiles. There appears to be a local switch on each tile which allows for communications between its cores as you say, but this isn't really explained anywhere that I've been able to find. Please ignore what I said about tokens. If a streaming channel is made between two tiles (whether or not in the same package) then I think that would tie up a link. More info below.

https://www.xmos.ai/download/xCONNECT-A ... e(1.0).pdf

[akp]

Yes, I am pretty sure that streaming channels where the chanends are on the same tile don't consume inter-tile switch resources. They just use the on-tile FIFO resource.