mlkem-sync/sync_rtl/top/keccak_arbiter.v

// keccak_arbiter.v - Fixed-priority arbiter for sharing a single keccak_core
//
// Allows N consumers (sha3_chain, sample_cbd, sample_ntt, sha3_top) to
// share one keccak_core instance. Consumer 0 has highest priority — used
// for sha3_chain which needs fast turnaround during KeyGen.
//
// State machine:
//   IDLE: Wait for any cons_valid_i. Grant to highest-priority consumer
//         (lowest index). Assert kc_valid_i to start permutation.
//   BUSY: Hold grant until kc_valid_o fires (permutation done), then
//         return to IDLE. cons_valid_o pulses for the granted consumer.
//
// Parameters:
//   NUM_CONSUMERS = 4
//
// Interface:
//   Keccak side:  kc_state_i/o, kc_valid_i/o, kc_ready_i/o (single instance)
//   Consumer side: packed per-consumer valid/ready/state vectors

module keccak_arbiter #(
    parameter NUM_CONSUMERS = 4
) (
    input                               clk,
    input                               rst_n,

    // ── Keccak core side (single keccak_core instance) ──────────────
    output      [1599:0]                kc_state_i,
    output                              kc_valid_i,
    input                               kc_ready_o,
    input       [1599:0]                kc_state_o,
    input                               kc_valid_o,
    output                              kc_ready_i,

    // ── Consumer side (N copies, packed) ────────────────────────────
    input       [NUM_CONSUMERS*1600-1:0] cons_state_i,
    input       [NUM_CONSUMERS-1:0]      cons_valid_i,
    output      [NUM_CONSUMERS-1:0]      cons_ready_o,
    output      [NUM_CONSUMERS*1600-1:0] cons_state_o,
    output      [NUM_CONSUMERS-1:0]      cons_valid_o,
    /* verilator lint_off UNUSEDSIGNAL */
    input       [NUM_CONSUMERS-1:0]      cons_ready_i
    /* verilator lint_on UNUSEDSIGNAL */
);

    // ================================================================
    // State machine
    // ================================================================
    localparam ST_IDLE = 1'b0;
    localparam ST_BUSY = 1'b1;

    reg state_r, state_next;

    // ================================================================
    // Grant logic
    // ================================================================
    localparam GRANT_W = $clog2(NUM_CONSUMERS);

    reg  [GRANT_W-1:0] grant_r;          // registered grant (held during BUSY)
    reg  [GRANT_W-1:0] grant_comb;       // priority-encoded index (combinational)

    // Any consumer requesting
    wire any_valid;
    assign any_valid = |cons_valid_i;

    // Priority encoder: consumer 0 has highest priority (lowest index).
    // Reverse iteration so last assignment wins → lowest-index priority.
    integer i;
    always @(*) begin
        grant_comb = {GRANT_W{1'b0}};
        for (i = NUM_CONSUMERS - 1; i >= 0; i = i - 1) begin
            if (cons_valid_i[i]) begin
                /* verilator lint_off WIDTHTRUNC */
                grant_comb = i;  // intentional truncation: i ∈ [0,NUM_CONSUMERS-1] fits GRANT_W
                /* verilator lint_on WIDTHTRUNC */
            end
        end
    end

    // Active grant: combinational in IDLE, registered (held) in BUSY
    wire [GRANT_W-1:0] active_grant;
    assign active_grant = (state_r == ST_IDLE) ? grant_comb : grant_r;

    // ================================================================
    // FSM next-state logic
    // ================================================================
    always @(*) begin
        state_next = state_r;
        case (state_r)
            ST_IDLE: if (kc_ready_o && any_valid) state_next = ST_BUSY;
            ST_BUSY: if (kc_valid_o)              state_next = ST_IDLE;
            default:                               state_next = ST_IDLE;
        endcase
    end

    // ================================================================
    // Sequential logic
    // ================================================================
    always @(posedge clk or negedge rst_n) begin
        if (!rst_n) begin
            state_r <= ST_IDLE;
            grant_r <= {GRANT_W{1'b0}};
        end else begin
            state_r <= state_next;
            // Capture grant on IDLE→BUSY transition
            if (state_r == ST_IDLE && state_next == ST_BUSY)
                grant_r <= grant_comb;
        end
    end

    // ================================================================
    // Keccak core side
    // ================================================================
    // Route selected consumer's state to keccak
    assign kc_state_i = cons_state_i[active_grant * 1600 +: 1600];

    // Start permutation when IDLE, keccak ready, and any consumer wants access
    assign kc_valid_i = (state_r == ST_IDLE) && kc_ready_o && any_valid;

    // Always accept keccak output (keccak_core's ready_i)
    assign kc_ready_i = 1'b1;

    // ================================================================
    // Consumer side (generated per consumer)
    // ================================================================
    genvar g;
    generate
        for (g = 0; g < NUM_CONSUMERS; g = g + 1) begin : gen_consumer
            // cons_ready_o: this consumer is granted AND keccak is ready
            assign cons_ready_o[g] = (active_grant == g)
                                   && any_valid
                                   && (state_r == ST_IDLE)
                                   && kc_ready_o;

            // cons_valid_o: this consumer was granted AND keccak finished
            assign cons_valid_o[g] = (grant_r == g) && kc_valid_o;

            // cons_state_o: broadcast keccak output to all consumers.
            // Each consumer latches only when its own valid_o is high.
            assign cons_state_o[g*1600 +: 1600] = kc_state_o;
        end
    endgenerate

endmodule