//
// Copyright 2018 Ettus Research, A National Instruments Company
//
// SPDX-License-Identifier: LGPL-3.0-or-later
//
// Module: chdr_compute_tkeep
// Description:
//  This module monitors an AXI-Stream CHDR bus and uses the
//  packet size field in the CHDR header to compute a tkeep
//  trailer signal to indicate the the valid bytes when
//  tlast is asserted.
//
// Parameters:
//   - CHDR_W: Width of the CHDR bus in bits
//   - ITEM_W: Width of the item bus in bits (must be a multiple of 8)
//
// Signals:
//   - axis_* : AXI-Stream CHDR bus

module chdr_compute_tkeep #(
  parameter CHDR_W = 256,
  parameter ITEM_W = 32
)(
  input  wire                       clk,
  input  wire                       rst,
  input  wire [CHDR_W-1:0]          axis_tdata,
  input  wire                       axis_tlast,
  input  wire                       axis_tvalid,
  input  wire                       axis_tready,
  output wire [(CHDR_W/ITEM_W)-1:0] axis_tkeep
);

  `include "rfnoc_chdr_utils.vh"

  generate if (CHDR_W > ITEM_W) begin 

    localparam CHDR_W_BYTES = CHDR_W/8;
    localparam ITEM_W_BYTES = ITEM_W/8;
    localparam KEEP_W       = CHDR_W_BYTES/ITEM_W_BYTES;

    // Binary to thermometer decoder
    // 2'd0 => 4'b1111 (special case)
    // 2'd1 => 4'b0001
    // 2'd2 => 4'b0011
    // 2'd3 => 4'b0111
    function [KEEP_W-1:0] bin2thermo;
      input [$clog2(KEEP_W)-1:0] bin;
      bin2thermo = ~((~1)<<((bin-1)%KEEP_W));
    endfunction
  
    // Read the packet length and figure out the number
    // of trailing items
    wire [15:0]       pkt_len    = chdr_get_length(axis_tdata[63:0]);
    wire [KEEP_W-1:0] len_thermo = bin2thermo(pkt_len[$clog2(CHDR_W_BYTES)-1:$clog2(ITEM_W_BYTES)]);
    reg  [KEEP_W-1:0] reg_len_thermo = 'h0;
    reg               is_header = 1'b1;

    always @(posedge clk) begin
      if (rst) begin
        is_header <= 1'b1;
      end else if (axis_tvalid & axis_tready) begin
        is_header <= axis_tlast;
        if (is_header) begin
          reg_len_thermo <= len_thermo;
        end
      end
    end

    // tkeep indicates trailing items, so for lines with tlast == 0,
    // tkeep is all 1's.
    assign axis_tkeep = (~axis_tlast) ? {KEEP_W{1'b1}} :
      (is_header ? len_thermo : reg_len_thermo);

  end else if (CHDR_W == ITEM_W) begin

    // Only one item per CHDR word. So always keep it.
    assign axis_tkeep = 1'b1;

  end else begin

    // Illegal. A item must be smaller than the CHDR_W
    illegal_parameter_value item_w_cannot_be_larger_than_chdr_w();

  end endgenerate

endmodule // chdr_compute_tkeep