module axis_gmii_rx #
(
    parameter DATA_WIDTH = 8,
    parameter PTP_TS_ENABLE = 0,
    parameter PTP_TS_WIDTH = 96,
    parameter USER_WIDTH = (PTP_TS_ENABLE ? PTP_TS_WIDTH : 0) + 1
)
(
    input  wire                     clk,
    input  wire                     rst,

    /*
     * GMII input
     */
    input  wire [DATA_WIDTH-1:0]    gmii_rxd,
    input  wire                     gmii_rx_dv,
    input  wire                     gmii_rx_er,

    /*
     * AXI output
     */
    output wire [DATA_WIDTH-1:0]    m_axis_tdata,
    output wire                     m_axis_tvalid,
    output wire                     m_axis_tlast,
    output wire [USER_WIDTH-1:0]    m_axis_tuser,

    /*
     * PTP
     */
    input  wire [PTP_TS_WIDTH-1:0]  ptp_ts,

    /*
     * Control
     */
    input  wire                     clk_enable,
    input  wire                     mii_select,

    /*
     * Status
     */
    output wire                     start_packet,
    output wire                     error_bad_frame,
    output wire                     error_bad_fcs
);

// bus width assertions
initial begin
    if (DATA_WIDTH != 8) begin
        $error("Error: Interface width must be 8");
        $finish;
    end
end

localparam [7:0]
    ETH_PRE = 8'h55,
    ETH_SFD = 8'hD5;

localparam [2:0]
    STATE_IDLE = 3'd0,
    STATE_PAYLOAD = 3'd1,
    STATE_WAIT_LAST = 3'd2;

reg [2:0] state_reg = STATE_IDLE, state_next;

// datapath control signals
reg reset_crc;
reg update_crc;

reg mii_odd = 1'b0;
reg mii_locked = 1'b0;

reg [DATA_WIDTH-1:0] gmii_rxd_d0 = {DATA_WIDTH{1'b0}};
reg [DATA_WIDTH-1:0] gmii_rxd_d1 = {DATA_WIDTH{1'b0}};
reg [DATA_WIDTH-1:0] gmii_rxd_d2 = {DATA_WIDTH{1'b0}};
reg [DATA_WIDTH-1:0] gmii_rxd_d3 = {DATA_WIDTH{1'b0}};
reg [DATA_WIDTH-1:0] gmii_rxd_d4 = {DATA_WIDTH{1'b0}};

reg gmii_rx_dv_d0 = 1'b0;
reg gmii_rx_dv_d1 = 1'b0;
reg gmii_rx_dv_d2 = 1'b0;
reg gmii_rx_dv_d3 = 1'b0;
reg gmii_rx_dv_d4 = 1'b0;

reg gmii_rx_er_d0 = 1'b0;
reg gmii_rx_er_d1 = 1'b0;
reg gmii_rx_er_d2 = 1'b0;
reg gmii_rx_er_d3 = 1'b0;
reg gmii_rx_er_d4 = 1'b0;

reg [DATA_WIDTH-1:0] m_axis_tdata_reg = {DATA_WIDTH{1'b0}}, m_axis_tdata_next;
reg m_axis_tvalid_reg = 1'b0, m_axis_tvalid_next;
reg m_axis_tlast_reg = 1'b0, m_axis_tlast_next;
reg m_axis_tuser_reg = 1'b0, m_axis_tuser_next;

reg start_packet_reg = 1'b0, start_packet_next;
reg error_bad_frame_reg = 1'b0, error_bad_frame_next;
reg error_bad_fcs_reg = 1'b0, error_bad_fcs_next;

reg [PTP_TS_WIDTH-1:0] ptp_ts_reg = 0, ptp_ts_next;

reg [31:0] crc_state = 32'hFFFFFFFF;
wire [31:0] crc_next;

assign m_axis_tdata = m_axis_tdata_reg;
assign m_axis_tvalid = m_axis_tvalid_reg;
assign m_axis_tlast = m_axis_tlast_reg;
assign m_axis_tuser = PTP_TS_ENABLE ? {ptp_ts_reg, m_axis_tuser_reg} : m_axis_tuser_reg;

assign start_packet = start_packet_reg;
assign error_bad_frame = error_bad_frame_reg;
assign error_bad_fcs = error_bad_fcs_reg;

lfsr #(
    .LFSR_WIDTH(32),
    .LFSR_POLY(32'h4c11db7),
    .LFSR_CONFIG("GALOIS"),
    .LFSR_FEED_FORWARD(0),
    .REVERSE(1),
    .DATA_WIDTH(8),
    .STYLE("AUTO")
)
eth_crc_8 (
    .data_in(gmii_rxd_d4),
    .state_in(crc_state),
    .data_out(),
    .state_out(crc_next)
);

always @* begin
    state_next = STATE_IDLE;

    reset_crc = 1'b0;
    update_crc = 1'b0;

    m_axis_tdata_next = {DATA_WIDTH{1'b0}};
    m_axis_tvalid_next = 1'b0;
    m_axis_tlast_next = 1'b0;
    m_axis_tuser_next = 1'b0;

    start_packet_next = 1'b0;
    error_bad_frame_next = 1'b0;
    error_bad_fcs_next = 1'b0;

    ptp_ts_next = ptp_ts_reg;

    if (!clk_enable) begin
        // clock disabled - hold state
        state_next = state_reg;
    end else if (mii_select && !mii_odd) begin
        // MII even cycle - hold state
        state_next = state_reg;
    end else begin
        case (state_reg)
            STATE_IDLE: begin
                // idle state - wait for packet
                reset_crc = 1'b1;

                if (gmii_rx_dv_d4 && !gmii_rx_er_d4 && gmii_rxd_d4 == ETH_SFD) begin
                    ptp_ts_next = ptp_ts;
                    start_packet_next = 1'b1;
                    state_next = STATE_PAYLOAD;
                end else begin
                    state_next = STATE_IDLE;
                end
            end
            STATE_PAYLOAD: begin
                // read payload
                update_crc = 1'b1;

                m_axis_tdata_next = gmii_rxd_d4;
                m_axis_tvalid_next = 1'b1;

                if (gmii_rx_dv_d4 && gmii_rx_er_d4) begin
                    // error
                    m_axis_tlast_next = 1'b1;
                    m_axis_tuser_next = 1'b1;
                    error_bad_frame_next = 1'b1;
                    state_next = STATE_WAIT_LAST;
                end else if (!gmii_rx_dv) begin
                    // end of packet
                    m_axis_tlast_next = 1'b1;
                    if (gmii_rx_er_d0 || gmii_rx_er_d1 || gmii_rx_er_d2 || gmii_rx_er_d3) begin
                        // error received in FCS bytes
                        m_axis_tuser_next = 1'b1;
                        error_bad_frame_next = 1'b1;
                    end else if ({gmii_rxd_d0, gmii_rxd_d1, gmii_rxd_d2, gmii_rxd_d3} == ~crc_next) begin
                        // FCS good
                        m_axis_tuser_next = 1'b0;
                    end else begin
                        // FCS bad
                        m_axis_tuser_next = 1'b1;
                        error_bad_frame_next = 1'b1;
                        error_bad_fcs_next = 1'b1;
                    end
                    state_next = STATE_IDLE;
                end else begin
                    state_next = STATE_PAYLOAD;
                end
            end
            STATE_WAIT_LAST: begin
                // wait for end of packet

                if (~gmii_rx_dv) begin
                    state_next = STATE_IDLE;
                end else begin
                    state_next = STATE_WAIT_LAST;
                end
            end
        endcase
    end
end

always @(posedge clk) begin
    state_reg <= state_next;

    ptp_ts_reg <= ptp_ts_next;

    m_axis_tdata_reg <= m_axis_tdata_next;
    m_axis_tvalid_reg <= m_axis_tvalid_next;
    m_axis_tlast_reg <= m_axis_tlast_next;
    m_axis_tuser_reg <= m_axis_tuser_next;

    if (clk_enable) begin
        if (mii_select) begin
            mii_odd <= !mii_odd;

            if (mii_locked) begin
                mii_locked <= gmii_rx_dv;
            end else if (gmii_rx_dv && {gmii_rxd[3:0], gmii_rxd_d0[7:4]} == ETH_SFD) begin
                mii_locked <= 1'b1;
                mii_odd <= 1'b1;
            end

            gmii_rxd_d0 <= {gmii_rxd[3:0], gmii_rxd_d0[7:4]};

            if (mii_odd) begin
                gmii_rxd_d1 <= gmii_rxd_d0;
                gmii_rxd_d2 <= gmii_rxd_d1;
                gmii_rxd_d3 <= gmii_rxd_d2;
                gmii_rxd_d4 <= gmii_rxd_d3;

                gmii_rx_dv_d0 <= gmii_rx_dv & gmii_rx_dv_d0;
                gmii_rx_dv_d1 <= gmii_rx_dv_d0 & gmii_rx_dv;
                gmii_rx_dv_d2 <= gmii_rx_dv_d1 & gmii_rx_dv;
                gmii_rx_dv_d3 <= gmii_rx_dv_d2 & gmii_rx_dv;
                gmii_rx_dv_d4 <= gmii_rx_dv_d3 & gmii_rx_dv;

                gmii_rx_er_d0 <= gmii_rx_er | gmii_rx_er_d0;
                gmii_rx_er_d1 <= gmii_rx_er_d0;
                gmii_rx_er_d2 <= gmii_rx_er_d1;
                gmii_rx_er_d3 <= gmii_rx_er_d2;
                gmii_rx_er_d4 <= gmii_rx_er_d3;
            end else begin
                gmii_rx_dv_d0 <= gmii_rx_dv;
                gmii_rx_er_d0 <= gmii_rx_er;
            end
        end else begin
            gmii_rxd_d0 <= gmii_rxd;
            gmii_rxd_d1 <= gmii_rxd_d0;
            gmii_rxd_d2 <= gmii_rxd_d1;
            gmii_rxd_d3 <= gmii_rxd_d2;
            gmii_rxd_d4 <= gmii_rxd_d3;

            gmii_rx_dv_d0 <= gmii_rx_dv;
            gmii_rx_dv_d1 <= gmii_rx_dv_d0 & gmii_rx_dv;
            gmii_rx_dv_d2 <= gmii_rx_dv_d1 & gmii_rx_dv;
            gmii_rx_dv_d3 <= gmii_rx_dv_d2 & gmii_rx_dv;
            gmii_rx_dv_d4 <= gmii_rx_dv_d3 & gmii_rx_dv;

            gmii_rx_er_d0 <= gmii_rx_er;
            gmii_rx_er_d1 <= gmii_rx_er_d0;
            gmii_rx_er_d2 <= gmii_rx_er_d1;
            gmii_rx_er_d3 <= gmii_rx_er_d2;
            gmii_rx_er_d4 <= gmii_rx_er_d3;
        end
    end

    if (reset_crc) begin
        crc_state <= 32'hFFFFFFFF;
    end else if (update_crc) begin
        crc_state <= crc_next;
    end

    start_packet_reg <= start_packet_next;
    error_bad_frame_reg <= error_bad_frame_next;
    error_bad_fcs_reg <= error_bad_fcs_next;

    if (rst) begin
        state_reg <= STATE_IDLE;

        m_axis_tvalid_reg <= 1'b0;

        start_packet_reg <= 1'b0;
        error_bad_frame_reg <= 1'b0;
        error_bad_fcs_reg <= 1'b0;

        crc_state <= 32'hFFFFFFFF;

        mii_locked <= 1'b0;
        mii_odd <= 1'b0;

        gmii_rx_dv_d0 <= 1'b0;
        gmii_rx_dv_d1 <= 1'b0;
        gmii_rx_dv_d2 <= 1'b0;
        gmii_rx_dv_d3 <= 1'b0;
        gmii_rx_dv_d4 <= 1'b0;
    end
end

endmodule