ibex/shared/rtl/bus.sv

// Copyright lowRISC contributors.
// Licensed under the Apache License, Version 2.0, see LICENSE for details.
// SPDX-License-Identifier: Apache-2.0

/**
 * Simplistic Ibex bus implementation
 *
 * This module is designed for demo and simulation purposes, do not use it in
 * a real-world system.
 *
 * This implementation doesn't handle the full bus protocol, but makes the
 * following simplifying assumptions.
 *
 * - All devices (slaves) must respond in the next cycle after the request.
 * - Host (master) arbitration is strictly priority based.
 */
module bus #(
  parameter int NrDevices    = 1,
  parameter int NrHosts      = 1,
  parameter int DataWidth    = 32,
  parameter int AddressWidth = 32
) (
  input                           clk_i,
  input                           rst_ni,

  // Hosts (masters)
  input                           host_req_i    [NrHosts],
  output logic                    host_gnt_o    [NrHosts],

  input        [AddressWidth-1:0] host_addr_i   [NrHosts],
  input                           host_we_i     [NrHosts],
  input        [ DataWidth/8-1:0] host_be_i     [NrHosts],
  input        [   DataWidth-1:0] host_wdata_i  [NrHosts],
  output logic                    host_rvalid_o [NrHosts],
  output logic [   DataWidth-1:0] host_rdata_o  [NrHosts],
  output logic                    host_err_o    [NrHosts],

  // Devices (slaves)
  output logic                    device_req_o    [NrDevices],

  output logic [AddressWidth-1:0] device_addr_o   [NrDevices],
  output logic                    device_we_o     [NrDevices],
  output logic [ DataWidth/8-1:0] device_be_o     [NrDevices],
  output logic [   DataWidth-1:0] device_wdata_o  [NrDevices],
  input                           device_rvalid_i [NrDevices],
  input        [   DataWidth-1:0] device_rdata_i  [NrDevices],
  input                           device_err_i    [NrDevices],

  // Device address map
  input        [AddressWidth-1:0] cfg_device_addr_base [NrDevices],
  input        [AddressWidth-1:0] cfg_device_addr_mask [NrDevices]
);

  localparam int unsigned NumBitsHostSel = NrHosts > 1 ? $clog2(NrHosts) : 1;
  localparam int unsigned NumBitsDeviceSel = NrDevices > 1 ? $clog2(NrDevices) : 1;

  logic [NumBitsHostSel-1:0] host_sel_req, host_sel_resp;
  logic [NumBitsDeviceSel-1:0] device_sel_req, device_sel_resp;

  // Master select prio arbiter
  always_comb begin
    host_sel_req = '0;
    for (integer host = NrHosts - 1; host >= 0; host = host - 1) begin
      if (host_req_i[host]) begin
        host_sel_req = NumBitsHostSel'(host);
      end
    end
  end

  // Device select
  always_comb begin
    device_sel_req = '0;
    for (integer device = 0; device < NrDevices; device = device + 1) begin
      if ((host_addr_i[host_sel_req] & cfg_device_addr_mask[device])
          == cfg_device_addr_base[device]) begin
        device_sel_req = NumBitsDeviceSel'(device);
      end
    end
  end

  always_ff @(posedge clk_i or negedge rst_ni) begin
     if (!rst_ni) begin
        host_sel_resp <= '0;
        device_sel_resp <= '0;
     end else begin
        // Responses are always expected 1 cycle after the request
        device_sel_resp <= device_sel_req;
        host_sel_resp <= host_sel_req;
     end
  end

  always_comb begin
    for (integer device = 0; device < NrDevices; device = device + 1) begin
      if (NumBitsDeviceSel'(device) == device_sel_req) begin
        device_req_o[device]   = host_req_i[host_sel_req];
        device_we_o[device]    = host_we_i[host_sel_req];
        device_addr_o[device]  = host_addr_i[host_sel_req];
        device_wdata_o[device] = host_wdata_i[host_sel_req];
        device_be_o[device]    = host_be_i[host_sel_req];
      end else begin
        device_req_o[device]   = 1'b0;
        device_we_o[device]    = 1'b0;
        device_addr_o[device]  = 'b0;
        device_wdata_o[device] = 'b0;
        device_be_o[device]    = 'b0;
      end
    end
  end

  always_comb begin
    for (integer host = 0; host < NrHosts; host = host + 1) begin
      host_gnt_o[host] = 1'b0;
      if (NumBitsHostSel'(host) == host_sel_resp) begin
        host_rvalid_o[host] = device_rvalid_i[device_sel_resp];
        host_err_o[host]    = device_err_i[device_sel_resp];
        host_rdata_o[host]  = device_rdata_i[device_sel_resp];
      end else begin
        host_rvalid_o[host] = 1'b0;
        host_err_o[host]    = 1'b0;
        host_rdata_o[host]  = 'b0;
      end
    end
    host_gnt_o[host_sel_req] = host_req_i[host_sel_req];
  end
endmodule