From 76029b9bc1242a9292905cae6f71e080955ceaf2 Mon Sep 17 00:00:00 2001
From: Kimplul <kimi.h.kuparinen@gmail.com>
Date: Mon, 22 Jun 2026 00:50:26 +0300
Subject: add simple round-robin arbiter

+ Should check how well it synthesises with OpenROAD or something,
  Vivado seemed to kind of choke on it and it was way slower/bigger than
  I was expecting.
---
 src/arb.sv | 101 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 101 insertions(+)
 create mode 100644 src/arb.sv

(limited to 'src')

diff --git a/src/arb.sv b/src/arb.sv
new file mode 100644
index 0000000..15d89b3
--- /dev/null
+++ b/src/arb.sv
@@ -0,0 +1,101 @@
+module first_set #(
+	parameter WIDTH = 32
+)(
+	input  reg[WIDTH-1 :0] cand,
+	output reg[WIDTH-1 :0] selected
+);
+
+/* base case */
+if (WIDTH == 1) begin :base
+	assign selected  = cand;
+end
+
+/* recursive case */
+else begin :recursive
+	localparam SLICE_WIDTH = WIDTH / 2;
+	logic[SLICE_WIDTH-1:0] left, right;
+	logic pref_left, pref_right;
+	logic empty_left, empty_right;
+
+	first_set #(
+		.WIDTH(SLICE_WIDTH)
+	) first_set_left (
+		.cand(cand[SLICE_WIDTH +: SLICE_WIDTH]),
+		.selected (left)
+	);
+
+	first_set #(
+		.WIDTH(SLICE_WIDTH)
+	) first_set_right (
+		.cand(cand[0 +: SLICE_WIDTH]),
+		.selected (right)
+	);
+
+	assign selected = right != '0
+			? {{SLICE_WIDTH{1'b0}}, right}
+			: {left, {SLICE_WIDTH{1'b0}}}
+			;
+end
+
+always_comb begin
+	assert ($onehot0(selected));
+end
+
+`ifdef NOSUCHDEF
+/* assertions that verilator don't support, apparently */
+assert property (WIDTH >= 1)
+else $error("input width too narrow");
+
+assert property (1 << $clog2(WIDTH) == WIDTH)
+else $error("width not pow2");
+
+`endif
+
+endmodule // arb_recursive
+
+module arb #(
+	parameter WIDTH = 32
+)(
+	input clk,
+	input rstx,
+
+	input [WIDTH-1:0] cand,
+	output[WIDTH-1:0] selected
+);
+
+localparam W2 = 1 << $clog2(WIDTH);
+localparam DEPTH = W2 == 1 ? 1 : $clog2(W2);
+
+logic[W2-1:0] w2_cand, w2_selected, mask, next_mask, rot_selected;
+
+/* if there are candidates within the mask of 'above preferred index', use them,
+ * otherwise invert the mask and use candidates 'below' preferred index. */
+assign w2_cand = ((cand & mask) == '0) ? cand & ~mask : cand & mask;
+
+first_set #(
+	.WIDTH(W2)
+) first_set_i (
+	.cand    (w2_cand),
+	.selected(w2_selected)
+);
+
+assign selected = w2_selected;
+
+assign rot_selected = {
+	w2_selected[0 +: W2-1],
+	w2_selected[W2-1]
+};
+
+assign next_mask = ~(rot_selected - 1) | rot_selected;
+
+always_ff @(posedge clk or negedge rstx)
+if (!rstx) begin
+	mask  <= ~0;
+end else begin
+	if (selected != 0 && mask != next_mask)
+		mask <= next_mask;
+	else
+		mask <= mask;
+end
+
+endmodule // arb
-- 
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