module axi_register_slice #(

  parameter DATA_WIDTH = 32,
  parameter FORWARD_REGISTERED = 0,
  parameter BACKWARD_REGISTERED = 0)(

  input clk,
  input resetn,

  input s_axi_valid,
  output s_axi_ready,
  input [DATA_WIDTH-1:0] s_axi_data,

  output m_axi_valid,
  input m_axi_ready,
  output [DATA_WIDTH-1:0] m_axi_data
);

/*
 s_axi_data  -> bwd_data     -> fwd_data(1)  -> m_axi_data
 s_axi_valid -> bwd_valid    -> fwd_valid(1) -> m_axi_valid
 s_axi_ready <- bwd_ready(2) <- fwd_ready <- m_axi_ready

 (1) FORWARD_REGISTERED inserts a set of FF before m_axi_data and m_axi_valid
 (2) BACKWARD_REGISTERED insters a FF before s_axi_ready
*/

wire [DATA_WIDTH-1:0] bwd_data_s;
wire bwd_valid_s;
wire bwd_ready_s;
wire [DATA_WIDTH-1:0] fwd_data_s;
wire fwd_valid_s;
wire fwd_ready_s;

generate if (FORWARD_REGISTERED == 1) begin

reg fwd_valid = 1'b0;
reg [DATA_WIDTH-1:0] fwd_data = 'h00;

assign fwd_ready_s = ~fwd_valid | m_axi_ready;
assign fwd_valid_s = fwd_valid;
assign fwd_data_s = fwd_data;

always @(posedge clk) begin
  if (~fwd_valid | m_axi_ready)
    fwd_data <= bwd_data_s;
end

always @(posedge clk) begin
  if (resetn == 1'b0) begin
    fwd_valid <= 1'b0;
  end else begin
    if (bwd_valid_s)
      fwd_valid <= 1'b1;
    else if (m_axi_ready)
      fwd_valid <= 1'b0;
  end
end

end else begin
assign fwd_data_s = bwd_data_s;
assign fwd_valid_s = bwd_valid_s;
assign fwd_ready_s = m_axi_ready;
end
endgenerate

generate if (BACKWARD_REGISTERED == 1) begin

reg bwd_ready = 1'b1;
reg [DATA_WIDTH-1:0] bwd_data = 'h00;

assign bwd_valid_s = ~bwd_ready | s_axi_valid;
assign bwd_data_s = bwd_ready ? s_axi_data : bwd_data;
assign bwd_ready_s = bwd_ready;

always @(posedge clk) begin
  if (bwd_ready)
    bwd_data <= s_axi_data;
end

always @(posedge clk) begin
  if (resetn == 1'b0) begin
    bwd_ready <= 1'b1;
  end else begin
    if (fwd_ready_s)
      bwd_ready <= 1'b1;
    else if (s_axi_valid)
      bwd_ready <= 1'b0;
  end
end

end else begin
assign bwd_valid_s = s_axi_valid;
assign bwd_data_s = s_axi_data;
assign bwd_ready_s = fwd_ready_s;
end endgenerate

assign m_axi_data = fwd_data_s;
assign m_axi_valid = fwd_valid_s;
assign s_axi_ready = bwd_ready_s;

endmodule