如何在AXI和AXI4-Stream的Master和Slave间插入一级pipeline

        之前在FPGA中做AXI4和AXI4-Stream接口控制逻辑时遇到过一个问题，如何在Master和Slave接口之间插入一级pipeline register。刚开始接触时认为这个问题很简单，结果搞了几天都没写出功能完全正确的代码。设计的难点主要在于Master和Slave之间的ready和valid两个握手信号的处理。因为在传输过程中valid和ready都有随时拉高和拉低的可能。在拉高或者拉低时，如何正确处理正在传输的数据以及其他控制信号是设计的关键。

        Xilinx提供了AXI Register Slice和AXI4-Stream Register Slice两个IP核可以用来实现这个功能。后来看到了github上ADI公司提供的一段verilog代码：https://github.com/analogdevicesinc/hdl/blob/master/library/axi_dmac/axi_register_slice.v

// ***************************************************************************
// ***************************************************************************
// Copyright 2014 - 2017 (c) Analog Devices, Inc. All rights reserved.
//
// In this HDL repository, there are many different and unique modules, consisting
// of various HDL (Verilog or VHDL) components. The individual modules are
// developed independently, and may be accompanied by separate and unique license
// terms.
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// The user should read each of these license terms, and understand the
// freedoms and responsibilities that he or she has by using this source/core.
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`timescale 1ns/100ps

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

        经过分析终于明白了其中的设计思想。