fpga: rfnoc: Add option to sample sideband info at start of packet

The axis_data_to_chdr block previously only sampled the sideband
information at the end of the packet. This adds a parameter that
controls if the sideband information should be sampled at the beginning
of the packet or the end of the packet. In the former case, large
internal packet buffers are not required.

1 files changed, 117 insertions, 58 deletions

diff --git a/fpga/usrp3/lib/rfnoc/core/axis_data_to_chdr.v b/fpga/usrp3/lib/rfnoc/core/axis_data_to_chdr.v
index 6a5b3ce05..2ff5a1ee7 100644
--- a/fpga/usrp3/lib/rfnoc/core/axis_data_to_chdr.v
+++ b/fpga/usrp3/lib/rfnoc/core/axis_data_to_chdr.v
@@ -11,27 +11,42 @@
 //   with sideband information for packet flags and timestamp). A CHDR packet 
 //   will be generated for each data packet that is input.
 //
-//   The sideband information (e.g., timestamp, flags) must be input coincident 
-//   with the AXI-Stream data input and will be sampled coincident with the 
-//   last word of data in the packet (i.e., when tlast is asserted).
+//   The sideband information (e.g., timestamp, flags) must be input with the
+//   AXI-Stream data input and will be sampled coincident with either the first
+//   word or the last word of data in the packet (i.e., when tlast is
+//   asserted), depending on the SIDEBAND_AT_END parameter setting.
+//
+//   If sideband information is sampled at the beginning of a packet then
+//   tlength must be provided.
+//
+//   If sideband information is sampled at the end of the packet then tlength
+//   will be ignored, and the entire packet will be buffered before the CHDR
+//   output can begin. Also, in this mode, the payload FIFO size will be
+//   coerced to be at least as big as the MTU. This mode is intended for cases
+//   when the sideband information is not known until the end of the packet
+//   (e.g., the length).
 //
 //   This module also performs an optional clock crossing and data width 
 //   conversion from a user requested width for the payload bus to CHDR_W.
 //
 // Parameters:
 //
-//   CHDR_W         : Width of the input CHDR bus in bits
-//   ITEM_W         : Width of the output item bus in bits
-//   NIPC           : The number of output items delivered per cycle
-//   SYNC_CLKS      : Are the CHDR and data clocks synchronous to each other?
-//   MTU            : Log2 of the maximum packet size in CHDR words
-//   INFO_FIFO_SIZE : Log2 of the info FIFO size. This determines the number of 
-//                    packets that can be simultaneously buffered in the 
-//                    payload FIFO.
-//   PYLD_FIFO_SIZE : Log2 of the payload FIFO size. The actual FIFO size will 
-//                    be the maximum of 2**MTU or 2**PYLD_FIFO_SIZE, since the 
-//                    FIFO must be at least one MTU so that we can calculate 
-//                    the packet length in the header.
+//   CHDR_W          : Width of the input CHDR bus in bits
+//   ITEM_W          : Width of the output item bus in bits
+//   NIPC            : The number of output items delivered per cycle
+//   SYNC_CLKS       : Are the CHDR and data clocks synchronous to each other?
+//   MTU             : Log2 of the maximum packet size in CHDR words
+//   INFO_FIFO_SIZE  : Log2 of the info FIFO size. This determines the number
+//                     of packets that can be simultaneously buffered in the
+//                     payload FIFO.
+//   PYLD_FIFO_SIZE  : Log2 of the payload FIFO size. The actual FIFO size will 
+//                     be the maximum of 2**MTU or 2**PYLD_FIFO_SIZE, since the 
+//                     FIFO must be at least one MTU so that we can calculate 
+//                     the packet length in the header.
+//   SIDEBAND_AT_END : If 0 then the sideband information is sampled coincident
+//                     with the first word of the input packet. If 1, then the
+//                     sideband information is sampled with the last word of the
+//                     input packet (the word in which tlast is asserted).
 //
 // Signals:
 //
@@ -41,13 +56,14 @@
 //
 
 module axis_data_to_chdr #(
-  parameter CHDR_W         = 256,
-  parameter ITEM_W         = 32,
-  parameter NIPC           = 2,
-  parameter SYNC_CLKS      = 0,
-  parameter MTU            = 10,
-  parameter INFO_FIFO_SIZE = 5,
-  parameter PYLD_FIFO_SIZE = MTU
+  parameter CHDR_W          = 256,
+  parameter ITEM_W          = 32,
+  parameter NIPC            = 2,
+  parameter SYNC_CLKS       = 0,
+  parameter MTU             = 10,
+  parameter INFO_FIFO_SIZE  = 5,
+  parameter PYLD_FIFO_SIZE  = MTU,
+  parameter SIDEBAND_AT_END = 1
 )(
   // Clock, reset and settings
   input  wire                     axis_chdr_clk,
@@ -68,6 +84,7 @@ module axis_data_to_chdr #(
   // Payload sideband info
   input  wire [63:0]              s_axis_ttimestamp,
   input  wire                     s_axis_thas_time,
+  input  wire [15:0]              s_axis_tlength,
   input  wire                     s_axis_teov,
   input  wire                     s_axis_teob,
   // Flush signals
@@ -77,11 +94,11 @@ module axis_data_to_chdr #(
   output wire                     flush_done
 );
 
-  // Make sure the payload FIFO is large enough to store an entire packet's 
-  // worth of payload data. This will ensure that we can buffer the entire 
-  // packet to calculate its length.
-  localparam PAYLOAD_FIFO_SIZE = PYLD_FIFO_SIZE > MTU ? 
-                                 PYLD_FIFO_SIZE : MTU;
+  // If we are sampling the sideband information at the end of the packet then
+  // the payload FIFO must be large enough to store an entire packet's worth of
+  // payload data. We coerce the size here if needed.
+  localparam PAYLOAD_FIFO_SIZE =
+   (!SIDEBAND_AT_END || PYLD_FIFO_SIZE > MTU) ? PYLD_FIFO_SIZE : MTU;
 
 
   // ---------------------------------------------------
@@ -93,12 +110,24 @@ module axis_data_to_chdr #(
 
 
   //---------------------------------------------------------------------------
-  // Timestamp and Flags Capture
+  // Start of Packet Register
   //---------------------------------------------------------------------------
-  //
-  // The timestamp and flags that we use for each packet is that of the last 
-  // data word. Here, we capture this information at the end of the packet. 
-  //
+
+  // The sop register will indicate if the next word on s_axis_t* is the first
+  // word of a packet.
+  reg start_of_packet = 1'b1;
+
+  always @(posedge axis_data_clk) begin
+    if (axis_data_rst) begin
+      start_of_packet <= 1'b1;
+    end else if (s_axis_tvalid && s_axis_tready) begin
+      start_of_packet <= s_axis_tlast;
+    end
+  end
+
+
+  //---------------------------------------------------------------------------
+  // Timestamp and Flags Capture
   //---------------------------------------------------------------------------
 
   reg [63:0] packet_timestamp;
@@ -107,7 +136,10 @@ module axis_data_to_chdr #(
   reg        packet_eob;
 
   always @(posedge axis_data_clk) begin
-    if (s_axis_tvalid & s_axis_tready & s_axis_tlast) begin
+    if (s_axis_tvalid && s_axis_tready &&
+      (( SIDEBAND_AT_END && s_axis_tlast) || 
+       (!SIDEBAND_AT_END && start_of_packet))
+    ) begin
       packet_timestamp <= s_axis_ttimestamp;
       packet_has_time  <= s_axis_thas_time;
       packet_eov       <= s_axis_teov;
@@ -121,43 +153,70 @@ module axis_data_to_chdr #(
   //---------------------------------------------------------------------------
   //
   // Here We track the state of the incoming packet to determine the payload 
-  // length.
+  // length, if needed.
   //
   //---------------------------------------------------------------------------
 
-  localparam HDR_LEN = CHDR_W/8;      // Length of CHDR header word in bytes
-
-  reg [15:0] packet_length;
-  reg [15:0] length_count       = HDR_LEN;
   reg        in_pkt_info_tvalid = 0;
   wire       in_pkt_info_tready;
+  reg [15:0] packet_length;
 
-  always @(posedge axis_data_clk) begin : pkt_length_counter
-    if (axis_data_rst) begin
-      length_count       <= HDR_LEN;
-      in_pkt_info_tvalid <= 1'b0;
-    end else begin : pkt_length_counter_main
-      // Calculate the length of this word in bytes, taking tkeep into account
-      integer i;
-      integer num_bytes;
-      num_bytes = 0;
-      for (i = 0; i < NIPC; i = i + 1) begin
-        num_bytes = num_bytes + (s_axis_tkeep[i]*(ITEM_W/8));
+  localparam HDR_LEN = CHDR_W/8;      // Length of CHDR header word in bytes
+
+  generate
+    if (!SIDEBAND_AT_END) begin : gen_sample_sop
+      //-------------------------------
+      //  Sample at Start of Packet
+      //-------------------------------
+
+      always @(posedge axis_data_clk) begin
+        if (axis_data_rst) begin
+          in_pkt_info_tvalid <= 1'b0;
+        end else begin
+          in_pkt_info_tvalid <= 1'b0;
+          if (s_axis_tvalid && s_axis_tready && start_of_packet) begin
+            packet_length      <= s_axis_tlength + HDR_LEN;
+            in_pkt_info_tvalid <= 1'b1;
+          end
+        end
       end
 
-      // Update the packet length if the word is accepted
-      in_pkt_info_tvalid <= 1'b0;
-      if (s_axis_tvalid && s_axis_tready) begin
-        if (s_axis_tlast) begin
+    end else begin : gen_sample_eop
+      //-------------------------------
+      //  Sample at End of Packet
+      //-------------------------------
+
+      reg [15:0] length_count = HDR_LEN;
+
+      always @(posedge axis_data_clk) begin : pkt_length_counter
+        if (axis_data_rst) begin
           length_count       <= HDR_LEN;
-          packet_length      <= length_count + num_bytes;
-          in_pkt_info_tvalid <= 1'b1;
-        end else begin
-          length_count <= length_count + num_bytes;
+          in_pkt_info_tvalid <= 1'b0;
+        end else begin : pkt_length_counter_main
+          // Calculate the length of this word in bytes, taking tkeep into account
+          integer i;
+          integer num_bytes;
+          num_bytes = 0;
+          for (i = 0; i < NIPC; i = i + 1) begin
+            num_bytes = num_bytes + (s_axis_tkeep[i]*(ITEM_W/8));
+          end
+
+          // Update the packet length if the word is accepted
+          in_pkt_info_tvalid <= 1'b0;
+          if (s_axis_tvalid && s_axis_tready) begin
+            if (s_axis_tlast) begin
+              length_count       <= HDR_LEN;
+              packet_length      <= length_count + num_bytes;
+              in_pkt_info_tvalid <= 1'b1;
+            end else begin
+              length_count <= length_count + num_bytes;
+            end
+          end
         end
       end
     end
-  end
+  endgenerate
+
 
 
   //---------------------------------------------------------------------------