implement circular routerHEADmaster

+ Not great latency, but has a fairly high fmax and low resource usage
  on my FPGA, so go with this for now. Likely better suited for data
  path, unsure what I'll do for the control path.

1 files changed, 160 insertions, 0 deletions

diff --git a/src/router.sv b/src/router.sv
new file mode 100644
index 0000000..b496ab9
--- /dev/null
+++ b/src/router.sv
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+/* Idea: circular router, where packets are added to a ring and popped off one
+ * when they encounter a node which they match. Relatively high minimum latency,
+ * but simple implementation and *should* be fairly cheap in hardware,
+ * let's see I guess.
+ *
+ * Skid buffers on inputs.
+ *
+ * Should fairly easily allow a form of broadcasting as well by just checking
+ * that visited is all ones, but that's not currently implemented.
+ *
+ * Should presumably also add masking so that users can check when
+ * a device matches, for now it's just a fixed integer sequence.
+ */
+
+module router #(
+	/* number of ports */
+	parameter unsigned N = 4,
+
+	/* index of this router */
+	parameter unsigned I = 0,
+
+	/* arbitrary packet payload type */
+	parameter type data_t = logic,
+
+	/* structure with to/from address info, should maybe add some macro for
+	* generating such a type? */
+	parameter type addr_t = logic
+)(
+	input clk_i,
+	input rstx_i,
+
+	/* TODO SV interfaces? */
+
+	/* first two are up/down wrt. I, in that order, then come
+	 * devices, referred to by index + 2 */
+	input  logic [N+2-1:0] valid_i,
+	output logic [N+2-1:0] ready_o,
+	input  addr_t[N+2-1:0] addr_i,
+	input  data_t[N+2-1:0] data_i,
+
+	output logic [N+2-1:0] valid_o,
+	input  logic [N+2-1:0] ready_i,
+	output addr_t[N+2-1:0] addr_o,
+	output data_t[N+2-1:0] data_o
+);
+
+localparam NW = $bits(addr_i[0].dst.node);
+localparam DW = $bits(addr_i[0].dst.dev);
+
+typedef struct packed {
+	logic alive;
+	logic[N+2-1:0] visited;
+	addr_t addr;
+	data_t data;
+} meta_t;
+
+meta_t[N+2-1:0] ring_d, ring_q;
+
+logic [N+2-1:0] send, sent, recv;
+meta_t[N+2-1:0] handle, insert;
+
+typedef struct packed {
+	addr_t addr;
+	data_t data;
+} skid_t;
+
+logic [N+2-1:0] skid_valid_i, skid_ready_o;
+addr_t[N+2-1:0] skid_addr_i;
+data_t[N+2-1:0] skid_data_i;
+
+/* skid buffers kind of make this file more difficult to read, should I start
+ * doing something like *_skid where I wrap a central block in skip buffers,
+ * presumably always on the input side or something along those lines? */
+for (genvar i = 0; i < N + 2; ++i) begin :skidbuffers
+	skid_t in, out;
+	assign in = '{addr: addr_i[i], data: data_i[i]};
+	assign skid_addr_i[i] = out.addr;
+	assign skid_data_i[i] = out.data;
+
+	skid_buffer #(
+		.W($bits(skid_t))
+	) skid_i (
+		.clk_i,
+		.rstx_i,
+
+		.valid_i(valid_i[i]),
+		.ready_o(ready_o[i]),
+		.data_i (in        ),
+
+		.valid_o(skid_valid_i[i]),
+		.ready_i(skid_ready_o[i]),
+		.data_o (out            )
+	);
+end
+
+for (genvar i = 0; i < N + 2; ++i)
+always_comb begin :gen_ring_knot
+	/* default values */
+	handle[i] = ring_d[i];
+	send[i]   = 1'b0;
+
+	/* first, try to send out an incoming packet */
+	if (ring_d[i].alive) begin
+		/* packet wasn't accepted by anyone, send it back */
+		if (&handle[i].visited)
+			handle[i].addr.dst = handle[i].addr.src;
+
+		/* up */
+		if (i == 0)
+			send[i] = handle[i].addr.dst.node > I[0+:NW];
+		/* down */
+		else if (i == 1)
+			/* slightly silly Verilator warning silencing */
+			send[i] = I == 0 ? 0 : handle[i].addr.dst.node < I[0+:NW];
+		/* dev */
+		else
+			send[i] = handle[i].addr.dst.node == I[0+:NW]
+				& handle[i].addr.dst.dev  == (i - 2);
+	end
+
+	valid_o[i] = send[i];
+	addr_o[i]  = handle[i].addr;
+	data_o[i]  = handle[i].data;
+	sent[i]    = valid_o[i] && ready_i[i];
+
+	/* mark this packet having visited us. If we did try to send it, but
+	 * the receiver wasn't ready, don't set the visited bit so the packet can
+	 * come around a second time in the future to try again. */
+	if (send[i])
+		handle[i].visited[i] |= sent[i];
+	else
+		handle[i].visited[i] |= 1'b1;
+
+	if (sent[i])
+		handle[i].alive = 1'b0;
+
+	/* next, try to insert a new packet into the ring */
+	skid_ready_o[i] = !handle[i].alive;
+	recv[i] = skid_valid_i[i] && skid_ready_o[i];
+
+	insert[i] = '{
+		alive: recv[i],
+		/* sending to oneself is nonsensical, so we automatically visit
+		 * the starting node */
+		visited: 1 << i,
+		addr: skid_addr_i[i],
+		data: skid_data_i[i]
+	};
+
+	/* send either the new packet or the existing one to the next node in
+	 * the ring */
+	ring_q[(i + 1) % (N + 2)] = recv[i] ? insert[i] : handle[i];
+end
+
+for (genvar i = 0; i < N + 2; ++i)
+always_ff @(posedge clk_i or negedge rstx_i)
+if (!rstx_i) ring_d[i] <= '0;
+else         ring_d[i] <= ring_q[i];
+
+endmodule // router