[enhancement] revise channel fifo depth syntax

[proppy]

What's hard to do? (limit 100 words)

The way we set channel fifo depth doesn't feel coherent w/ the rest of DSLX syntax

Depth is currently set as the second arg of the channel type declaration, ex:

        let (data_s, data_r) = chan<u8, u32:1>("data");

But it doesn't need to be set on the type of the lhs let binding.

i.e: in this example: the type for data_r is chan<u8> in not chan<u8, u32:1> in.

Current best alternative workaround (limit 100 words)

None.

Your view of the "best case XLS enhancement" (limit 100 words)

• if we want to make this a "property" of the channel construction we should probably use a regular argument maybe with a builtin FifoConfig type ideally inferred from a struct literal:

        let (data_s, data_r) = chan<u8>("data", { depth: u32:1 });

• if we want to make this a "property" of the channel type we should propagate it all the way thru thru all proc and let bindings.

[proppy]

@grebe you add an idea around using an attribute for this, could you expand?

[grebe]

Perhaps channels should have some configuration via attributes, e.g.

proc A {
  #[fifo(depth=1)]
  ch0: chan<u32>
  ...
}

This is a bit tricky in practice as a channel member is only one end of the channel, so you need some rules around how to resolve conflicts.

Another option is to have different chandecl syntaxes, e.g.

let (x, y): (chan<u32>, chan<u32>) = fifo("name", DEPTH);
let (y, z): (chan<u32>, chan<u32>) = fifo_flopped("name", DEPTH, REGISTER_PUSH_OUTPUTS, REGISTER_POP_OUTPUTS);

[ericastor]

FWIW, I like the second option suggested in @grebe's comment; I think it's nice if a chan is the thing that can be passed to send/receive, where only the interface is specified (which would include things like flow control) - and otherwise the details on the channel implementation are left to the point where we create the channel (e.g., an explicit fifo call rather than a chan constructor).