cpu/lib/axil_wb_bridge.sv

// TODO: improve throughput. Currently limited to 1 write every 3 cycles and read every 2 cycles (plus wb slave latency)

module axil_wb_bridge #(
    parameter ADDR_WIDTH = 8,
    parameter DATA_WIDTH = 32
)(
    input logic clk,
    input logic reset,

    ///// AXI4-Lite /////

    // Write address
    input logic axil_awvalid,
    output logic axil_awready,
    input logic [ADDR_WIDTH-1:0] axil_awaddr,
    input logic [2:0] axil_awprot,

    // Write data
    input logic axil_wvalid,
    output logic axil_wready,
    input logic [DATA_WIDTH-1:0] axil_wdata,
    input logic [DATA_WIDTH/8 - 1:0] axil_wstrb,

    // Write response
    output logic axil_bvalid,
    input logic axil_bready,
    output logic [1:0] axil_bresp,

    // Read address
    input logic axil_arvalid,
    output logic axil_arready,
    input logic [ADDR_WIDTH-1:0] axil_araddr,
    input logic [2:0] axil_arprot,

    // Read data
    output logic axil_rvalid,
    input logic axil_rready,
    output logic [DATA_WIDTH-1:0] axil_rdata,
    output logic [1:0] axil_rresp,

    ///// Wishbone /////

    output logic [ADDR_WIDTH-1:0] wb_adr_o,
    input logic [DATA_WIDTH-1:0] wb_dat_i,
    output logic [DATA_WIDTH-1:0] wb_dat_o,
    output logic wb_we_o,
    output logic wb_sel_o,
    output logic wb_stb_o,
    input logic wb_ack_i,
    output logic wb_cyc_o
);

localparam AXIL_RESP_OKAY = 2'b00;
// localparam AXIL_RESP_EXOKAY = 2'b01; // Only valid for full AXI, not AXI-Lite
localparam AXIL_RESP_SLVERR = 2'b10;
localparam AXIL_RESP_DECERR = 2'b11; // Indicates there is no slave at the transaction address

localparam STATE_IDLE = 0;
localparam STATE_READ = 1;
localparam STATE_WRITE = 2;

logic [ADDR_WIDTH-1:0] waddr;
logic [ADDR_WIDTH-1:0] raddr;
logic [DATA_WIDTH-1:0] wdata;
logic [DATA_WIDTH-1:0] rdata;
logic waddr_legal;
logic raddr_legal;
logic wdata_legal;

logic waddr_valid = 0;
logic raddr_valid = 0;
logic wdata_valid = 0;
logic rdata_valid = 0;
logic w_complete = 0;
logic [1:0] state_wb = STATE_IDLE;


always_ff @(posedge clk) begin
    if (reset) begin
        waddr_valid <= 0;
        waddr_legal <= 1; // unnecessary
        wdata_valid <= 0;
        wdata_legal <= 1; // unnecessary
        w_complete <= 0;
        raddr_valid <= 0;
        raddr_legal <= 1; // unnecessary
        rdata_valid <= 0;
        state_wb <= STATE_IDLE;
    end else begin
        // axil write address
        if (axil_awvalid && axil_awready) begin
            waddr <= axil_awaddr;
            waddr_valid <= 1;
            waddr_legal <= 1;
            // TODO: use write protection
        end

        // axil write data
        if (axil_wvalid && axil_wready) begin
            wdata <= axil_wdata;
            wdata_valid <= 1;
            if (&axil_wstrb) begin
                // wishbone only supports full width writes
                wdata_legal <= 1;
            // end else if (!|axil_wstrb) begin
            //     // TODO: optionally discard and ignore access with axil_wstrb = 4'b0000
            end else begin
                // invalid write strobe combination
                wdata_legal <= 0;
            end
        end

        if (
            ((axil_wvalid && axil_wready) || wdata_valid) &&
            ((axil_awvalid && axil_awready) || waddr_valid) &&
            !w_complete
            // TODO: check for wdata_legal (and combinatorial wdata_legal)
        ) begin
            if (state_wb == STATE_IDLE) begin
                state_wb <= STATE_WRITE;
            end
        end

        // axil write response
        if (axil_bvalid && axil_bready) begin
            // reset write logic for next transfer
            waddr_valid <= 0;
            wdata_valid <= 0;
            w_complete <= 0;
        end

        // axil read address
        if (axil_arvalid && axil_arready) begin
            raddr <= axil_araddr;
            raddr_valid <= 1;
            raddr_legal <= 1;
            // TODO: use read protection

            if (state_wb == STATE_IDLE) begin
                // if write and read arrive on the same cycle, read takes priority
                state_wb <= STATE_READ;
            end
        end

        // axil read data
        if (axil_rvalid && axil_rready) begin
            // read is complete. Get ready for next read
            raddr_valid <= 0;
            rdata_valid <= 0;
        end

        // wb
        case (state_wb)
            STATE_IDLE: begin
                // do nothing
            end
            STATE_WRITE: begin
                if (wb_cyc_o && wb_sel_o && wb_stb_o && wb_ack_i) begin
                    w_complete <= 1;
                    state_wb <= STATE_IDLE;
                    // TODO: switch directly to STATE_READ if we can
                end
            end
            STATE_READ: begin
                if (wb_cyc_o && wb_sel_o && wb_stb_o && wb_ack_i) begin
                    rdata <= wb_dat_i;
                    rdata_valid <= 1;
                    state_wb <= STATE_IDLE;
                    // TODO: switch directly to STATE_WRITE if we can
                    // This will also ensure continuous reads don't inhibit all writes
                end
            end
            default: begin
                // bh_assert_equal(state_wb, 0, "Wishbone master encountered unknown state");
                state_wb <= STATE_IDLE;
            end
        endcase
    end
end

always_comb begin
    // axil write address
    axil_awready = !waddr_valid;

    // axil write data
    axil_wready = !wdata_valid;

    // axil write response
    axil_bvalid = waddr_valid && wdata_valid && w_complete;
    axil_bresp = (waddr_legal && wdata_legal) ? AXIL_RESP_OKAY : AXIL_RESP_SLVERR;

    // axil read address
    axil_arready = !raddr_valid;

    // axil read data
    axil_rvalid = rdata_valid;
    axil_rdata = rdata;
    axil_rresp = raddr_legal ? AXIL_RESP_OKAY : AXIL_RESP_SLVERR;

    // wishbone
    wb_we_o = state_wb == STATE_WRITE;
    wb_dat_o = wdata;
    wb_adr_o = (state_wb == STATE_WRITE) ? waddr : raddr;
    wb_cyc_o = state_wb == STATE_WRITE || state_wb == STATE_READ;
    wb_sel_o = 1; // TODO: figure out if this is right
    wb_stb_o = 1; // TODO: figure out if this is right
end

endmodule