fpga: lib: Add axis_packetize module

This module takes an AXI-Stream without TLAST and outputs the
same AXI-Stream with TLAST based on the provided packet size
input.

2 files changed, 162 insertions, 0 deletions

diff --git a/fpga/usrp3/lib/axi/Makefile.srcs b/fpga/usrp3/lib/axi/Makefile.srcs
index bf8b3dbd2..598af9ef3 100644
--- a/fpga/usrp3/lib/axi/Makefile.srcs
+++ b/fpga/usrp3/lib/axi/Makefile.srcs
@@ -34,4 +34,5 @@ axis_upsizer.v \
 axis_downsizer.v \
 axis_width_conv.v \
 axis_split.v \
+axis_packetize.v \
 ))
diff --git a/fpga/usrp3/lib/axi/axis_packetize.v b/fpga/usrp3/lib/axi/axis_packetize.v
new file mode 100644
index 000000000..6ee018d9a
--- /dev/null
+++ b/fpga/usrp3/lib/axi/axis_packetize.v
@@ -0,0 +1,161 @@
+//
+// Copyright 2020 Ettus Research, A National Instruments Brand
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+//
+// Module: axis_packetize
+//
+// Description:
+//
+//   This module takes in an axi_stream without packet boundaries (i.e.,
+//   without tlast) and groups the data into packets by adding tlast at the
+//   appropriate time. The size of the packet is controlled by the "size"
+//   input, which is sampled at the beginning of each packet to be output. The
+//   packet_size input indicates the number of i_tdata words to group into a
+//   packet. The i_tlength output indicates the length of the packet being
+//   output.
+//
+//   The gate input causes data transfers to be stopped at the end of the
+//   current packet when the gate input is asserted. It is not legal to
+//   deassert tvalid once it has been asserted until the next transfer is
+//   completed, so this module monitors the state of the AXI-Stream protocol
+//   so that no protocol violations occur.
+//
+//   Note that the current transfer may still complete after gate has been
+//   asserted, so the downstream logic must be able to account for at least
+//   one more transfer.
+//
+// Parameters:
+//
+//   DATA_W       : Width of the tdata signals.
+//   SIZE_W       : The width of the packet size port. This dictates the
+//                  maximum packet size.
+//   FLUSH        : Controls whether or not the input should be stalled or
+//                  flushed. That is, when FLUSH=0, the input data is stalled
+//                  whenever the gate is on (i_tready becomes 0). When
+//                  FLUSH=1, the input data is dropped whenever the gate is on
+//                  (i_tready becomes 1).
+//   DEFAULT_SIZE : The default packet size to use, if it doesn't need to be
+//                  changed at run time.
+//
+
+
+module axis_packetize #(
+  parameter DATA_W       = 32,
+  parameter SIZE_W       = 16,
+  parameter FLUSH        = 0,
+  parameter DEFAULT_SIZE = 2**SIZE_W-1
+) (
+  input wire clk,
+  input wire rst,
+
+  input wire              gate,       // Stop or "gate" packet output
+  input wire [SIZE_W-1:0] size,       // Size to use for the next packet
+
+  // Input data stream
+  input  wire [DATA_W-1:0] i_tdata,
+  input  wire              i_tvalid,
+  output wire              i_tready,
+
+  // Output data stream
+  output wire [DATA_W-1:0] o_tdata,
+  output reg               o_tlast,
+  output wire              o_tvalid,
+  input  wire              o_tready,
+  output wire [SIZE_W-1:0] o_tuser     // Current packet's size
+);
+
+  reg              start_of_packet = 1;    // Next sample is start of a packet
+  reg [SIZE_W-1:0] word_count      = 0;    // Count of output words
+  reg [SIZE_W-1:0] current_size    = DEFAULT_SIZE;  // Current packet size
+
+  reg gating     = 1'b1;     // Indicate if output is blocked
+  reg mid_packet = 1'b0;     // Indicate if we're in the middle of a packet
+
+  //---------------------------------------------------------------------------
+  // Packet Size Logic
+  //---------------------------------------------------------------------------
+
+  assign o_tuser = current_size;
+
+  always @(posedge clk) begin
+    if (rst) begin
+      start_of_packet <= 1;
+      word_count      <= 0;
+      current_size    <= DEFAULT_SIZE;
+      o_tlast         <= 0;
+    end else begin
+      if (gating) begin
+        // Wait until we're enabled. Setup for the start of the next packet.
+        start_of_packet <= 1;
+        current_size    <= size;
+        word_count      <= 0;
+        o_tlast         <= (size == 1);
+      end else if (o_tvalid && o_tready) begin
+        start_of_packet <= 1'b0;
+        word_count      <= word_count + 1;
+        if (o_tlast) begin
+          // This is the last sample, so restart everything for a new packet.
+          o_tlast         <= (size == 1);
+          current_size    <= size;
+          word_count      <= 0;
+          start_of_packet <= 1'b1;
+        end else if (word_count == current_size-2) begin
+          // This is the second to last sample, so we assert tlast for the
+          // last sample.
+          o_tlast <= 1'b1;
+        end
+      end else if (start_of_packet) begin
+        // We're waiting for the start of the next packet. Keep checking the
+        // size input until the next packet starts.
+        current_size <= size;
+        word_count   <= 0;
+        o_tlast      <= (size == 1);
+      end
+    end
+  end
+
+  //---------------------------------------------------------------------------
+  // Handshake Monitor
+  //---------------------------------------------------------------------------
+  
+  // We start out gated to allow a clock cycle for the length to be loaded.
+
+  // Monitor the state of the handshake so we know when it's OK to
+  // enable/disable data transfer.
+  always @(posedge clk) begin
+    if (rst) begin
+      gating     = 1'b1;
+      mid_packet = 1'b0;
+    end else begin
+      // Keep track of if we are in the middle of a packet or not. Note that
+      // mid_packet will be 0 for the first transfer of a packet.
+      if (o_tvalid && o_tready) begin
+        if (o_tlast) begin
+          mid_packet = 1'b0;
+        end else begin
+          mid_packet = 1'b1;
+        end
+      end
+
+      if (gating) begin
+        // We can stop gating any time
+        if (!gate) gating <= 0;
+      end else begin
+        // Only start gating between packets or at the end of a packet
+        if ((mid_packet && !o_tvalid) || (o_tvalid && o_tready && o_tlast)) begin
+          gating <= gate;
+        end
+      end
+    end
+  end
+
+  //---------------------------------------------------------------------------
+  // Data Pass-Through
+  //---------------------------------------------------------------------------
+
+  assign o_tdata  = i_tdata;
+  assign o_tvalid = i_tvalid && !gating;
+  assign i_tready = FLUSH ? (o_tready || gating) : (o_tready && !gating);
+
+endmodule