module cpu_sys_cmd_xbar_mux
(
    // ----------------------
    // Sinks
    // ----------------------
    input                       sink0_valid,
    input [90-1   : 0]  sink0_data,
    input [5-1: 0]  sink0_channel,
    input                       sink0_startofpacket,
    input                       sink0_endofpacket,
    output                      sink0_ready,

    input                       sink1_valid,
    input [90-1   : 0]  sink1_data,
    input [5-1: 0]  sink1_channel,
    input                       sink1_startofpacket,
    input                       sink1_endofpacket,
    output                      sink1_ready,


    // ----------------------
    // Source
    // ----------------------
    output                      src_valid,
    output [90-1    : 0] src_data,
    output [5-1 : 0] src_channel,
    output                      src_startofpacket,
    output                      src_endofpacket,
    input                       src_ready,

    // ----------------------
    // Clock & Reset
    // ----------------------
    input clk,
    input reset
);
    localparam PAYLOAD_W        = 90 + 5 + 2;
    localparam NUM_INPUTS       = 2;
    localparam SHARE_COUNTER_W  = 1;
    localparam PIPELINE_ARB     = 1;
    localparam ST_DATA_W        = 90;
    localparam ST_CHANNEL_W     = 5;
    localparam PKT_TRANS_LOCK   = 57;

    // ------------------------------------------
    // Signals
    // ------------------------------------------
    wire [NUM_INPUTS - 1 : 0] request;
    wire [NUM_INPUTS - 1 : 0] valid;
    wire [NUM_INPUTS - 1 : 0] grant;
    wire [NUM_INPUTS - 1 : 0] next_grant;
    reg  [NUM_INPUTS - 1 : 0] saved_grant;
    reg  [PAYLOAD_W - 1 : 0]  src_payload;
    wire                      last_cycle;
    reg                       packet_in_progress;
    reg                       update_grant;

    wire [PAYLOAD_W - 1 : 0]  sink0_payload;
    wire [PAYLOAD_W - 1 : 0]  sink1_payload;

    assign valid[0] = sink0_valid;
    assign valid[1] = sink1_valid;

   wire [NUM_INPUTS - 1 : 0] eop;
      assign eop[0]   = sink0_endofpacket;
      assign eop[1]   = sink1_endofpacket;

    // ------------------------------------------
    // ------------------------------------------
    // Grant Logic & Updates
    // ------------------------------------------
    // ------------------------------------------
    reg [NUM_INPUTS - 1 : 0] lock;
    always @* begin
      lock[0] = sink0_data[57];
      lock[1] = sink1_data[57];
    end
    reg [NUM_INPUTS - 1 : 0] locked = '0;
    always @(posedge clk or posedge reset) begin
      if (reset) begin
        locked <= '0;
      end
      else begin
        locked <= next_grant & lock;
      end
    end

    assign last_cycle = src_valid & src_ready & src_endofpacket & ~(|(lock & grant));

    // ------------------------------------------
    // We're working on a packet at any time valid is high, except
    // when this is the endofpacket.
    // ------------------------------------------
    always @(posedge clk or posedge reset) begin
        if (reset) begin
            packet_in_progress <= 1'b0;
        end
        else begin
            if (src_valid)
                packet_in_progress <= 1'b1;
            if (last_cycle)
                packet_in_progress <= 1'b0;
        end
    end


    // ------------------------------------------
    // Shares
    //
    // Special case: all-equal shares _should_ be optimized into assigning a
    // constant to next_grant_share.
    // Special case: all-1's shares _should_ result in the share counter
    // being optimized away.
    // ------------------------------------------
    // Input  |  arb shares  |  counter load value
    // 0      |      1       |  0
    // 1      |      1       |  0
    wire [SHARE_COUNTER_W - 1 : 0] share_0 = 1'd0;
    wire [SHARE_COUNTER_W - 1 : 0] share_1 = 1'd0;

    // ------------------------------------------
    // Choose the share value corresponding to the grant.
    // ------------------------------------------
    reg [SHARE_COUNTER_W - 1 : 0] next_grant_share;
    always @* begin
        next_grant_share =
            share_0 & { SHARE_COUNTER_W {next_grant[0]} } |
            share_1 & { SHARE_COUNTER_W {next_grant[1]} };
    end

    // ------------------------------------------
    // Flag to indicate first packet of an arb sequence.
    // ------------------------------------------

    // ------------------------------------------
    // Compute the next share-count value.
    // ------------------------------------------
    reg [SHARE_COUNTER_W - 1 : 0] p1_share_count;
    reg [SHARE_COUNTER_W - 1 : 0] share_count;
    reg share_count_zero_flag;

    always @* begin
        // Update the counter, but don't decrement below 0.
        p1_share_count = share_count_zero_flag ? '0 : share_count - 1'b1;
    end

    // ------------------------------------------
    // Update the share counter and share-counter=zero flag.
    // ------------------------------------------
    always @(posedge clk or posedge reset) begin
        if (reset) begin
            share_count <= '0;
            share_count_zero_flag <= 1'b1;
        end
        else begin
            if (update_grant) begin
                share_count <= next_grant_share;
                share_count_zero_flag <= (next_grant_share == '0);
            end
            else if (last_cycle) begin
                share_count <= p1_share_count;
                share_count_zero_flag <= (p1_share_count == '0);
            end
        end
    end


    always @* begin
        update_grant = 0;

        // ------------------------------------------
        // The pipeline delays grant by one cycle, so
        // we have to calculate the update_grant signal
        // one cycle ahead of time.
        //
        // Possible optimization: omit the first clause
        //    "if (!packet_in_progress & ~src_valid) ..."
        //   cost: one idle cycle at the the beginning of each 
        //     grant cycle.
        //   benefit: save a small amount of logic.
        // ------------------------------------------
        if (!packet_in_progress & !src_valid)
            update_grant = 1;
        if (last_cycle && share_count_zero_flag)
            update_grant = 1;
    end

    wire save_grant;
    assign save_grant = update_grant;
    assign grant      = saved_grant;

    always @(posedge clk, posedge reset) begin
        if (reset)
            saved_grant <= '0;
        else if (save_grant)
            saved_grant <= next_grant;
    end

    // ------------------------------------------
    // ------------------------------------------
    // Arbitrator
    // ------------------------------------------
    // ------------------------------------------

    // ------------------------------------------
    // Create a request vector that stays high during
    // the packet for unpipelined arbitration.
    //
    // The pipelined arbitration scheme does not require
    // request to be held high during the packet.
    // ------------------------------------------
    reg  [NUM_INPUTS - 1 : 0] prev_request;
    always @(posedge clk, posedge reset) begin
        if (reset)
            prev_request <= '0;
        else
            prev_request <= request & ~(valid & eop);
    end

    assign request = (PIPELINE_ARB == 1) ? valid | locked :
                                           prev_request | valid | locked;


    altera_merlin_arbitrator
    #(
        .NUM_REQUESTERS(NUM_INPUTS),
        .SCHEME        ("round-robin"),
        .PIPELINE      (1)
    ) arb (
        .clk                    (clk),
        .reset                  (reset),
        .request                (request),
        .grant                  (next_grant),
        .save_top_priority      (src_valid),
        .increment_top_priority (update_grant)
    );

    // ------------------------------------------
    // ------------------------------------------
    // Mux
    //
    // Implemented as a sum of products.
    // ------------------------------------------
    // ------------------------------------------

    assign sink0_ready = src_ready && grant[0];
    assign sink1_ready = src_ready && grant[1];

    assign src_valid = |(grant & valid);

    always @* begin
        src_payload =
            sink0_payload & {PAYLOAD_W {grant[0]} } |
            sink1_payload & {PAYLOAD_W {grant[1]} };
    end

    // ------------------------------------------
    // Mux Payload Mapping
    // ------------------------------------------

    assign sink0_payload = {sink0_channel,sink0_data,
        sink0_startofpacket,sink0_endofpacket};
    assign sink1_payload = {sink1_channel,sink1_data,
        sink1_startofpacket,sink1_endofpacket};

    assign {src_channel,src_data,src_startofpacket,src_endofpacket} = src_payload;

endmodule