Risc-V communication with host

[JulioSDDR]

Is there any abstraction layer to make a serial communication between the fomu and the host pc? I have been taking a look at the implementation that is used at the micropython port, but it is a bit too low level.

[xobs]

There isn't one yet.

One option might be to adapt dummyusb to provide a simple Tx/Rx interface.

[apullin]

I wanted to ping this ancient thread with the same question as initially proposed:

Is there an example of bare metal firmware for the pre-built LiteX/VexRISC core that demonstrates using the UART to communicate with the host?
I don't think there's an out-of-the-box example of that available.

There's a number of sub-questions there - details that I am not quite sure of...

There's already a Micropython port, and that can definitely communicate over USB-serial - so I looked there first.
As far as I can tell ... that requires running the TinyUSB lib to provide a USB-CDC, and then the actual UART driver for micropy is in @mithro 's fork of Micropython, here:
https://github.com/mithro/micropython/blob/7a4245d24b1f185d6a7a1a863349e148a2c74c5a/litex/uart.c

But then I am not sure where uart_rxtx_write actually gets implemented.
And it's a bit unclear how that code works or what it is doing.

For that solution - is it ultimately going through TinyUSB?
Unclear if any UART that LiteX might add is actually being used, or if it is just a character stream with CDC transport.

I should/will try copying that driver in the micropy port, rolling it into an otherwise empty firmware derived from riscv-blink example, do some stdio redirection, and get the minimum working example that I am looking for.
Although, per everything above, I wanted to check if this would be expected to work, or if I am misunderstanding a component.

Beyond that, there seems to be some related discussion here:
#421

but they might be chasing a different kind of solution.

Given that people are therein proposing a gateware implementation of USB-CDC, would it potentially be possible to roll a bitstream that required zero C-code level support, and just followed the standard RISC-V low-level code of hitting a couple of UART registers to pend/send characters (e.g. here), and then the gateware would invisibly get it to the host? That would be a pretty wonderful solution. Outside of my skillset to implement, sadly.

[xobs]

It's ultimately going through TinyUSB, yes.

If you want USB-CDC without TinyUSB, you have two options:

1. Implement it yourself using eptri. This hardware block is documented at https://rm.fomu.im/usb.html and is what TinyUSB uses.
2. Implement it as a replacement hardware block. There was an implementation someone did as part of a C64 implementation which you could adapt: https://github.com/jamesacraig/6502-on-fomu/blob/master/fomu_usb_cdc.py

[apullin]

@xobs Ah, ok, great. I will try roll a minimum working example of the blink bare metal example incorporating the CDC config and driver taken from the micropython port. That should only be a matter of software, without having to tweak anything else.

I think that other thread was also proposing a similar solution, of a driverless hardware block. Beyond my abilities for now, but that'll be next steps.