hw/ip/otbn/rtl/otbn_rnd.sv

// Copyright lowRISC contributors.
// Licensed under the Apache License, Version 2.0, see LICENSE for details.
// SPDX-License-Identifier: Apache-2.0

`include "prim_assert.sv"

/**
 * OTBN random number coordination
 *
 * This module implements the RND, RND_PREFETCH and URND CSRs/WSRs. The EDN (entropy distribution
 * network) provides the bits for random numbers. RND gives direct access to EDN bits. URND provides
 * bits from a PRNG that is seeded with bits from the EDN.
 */

////////////////////////////////////////////////////////////////////////////////////////////////////
// IMPORTANT NOTE:                                                                                //
//                                   DO NOT USE THIS BLINDLY!                                     //
//                                                                                                //
// This is an initial prototype of the random number functionality in OTBN. Details are still     //
// under discussion and subject to change. It has not yet been verified this provides the         //
// necessary guarantees required for the various uses of random numbers in OTBN software.         //
////////////////////////////////////////////////////////////////////////////////////////////////////

module otbn_rnd import otbn_pkg::*;
#(
  parameter urnd_prng_seed_t       RndCnstUrndPrngSeed      = RndCnstUrndPrngSeedDefault
) (
  input logic clk_i,
  input logic rst_ni,

  input  logic            rnd_req_i,
  input  logic            rnd_prefetch_req_i,
  output logic            rnd_valid_o,
  output logic [WLEN-1:0] rnd_data_o,

  // Request URND PRNG reseed from the EDN
  input  logic            urnd_reseed_req_i,
  // Remains asserted whilst reseed is in progress
  output logic            urnd_reseed_busy_o,
  // When asserted PRNG state advances. It is permissible to advance the state whilst
  // reseeding.
  input  logic            urnd_advance_i,
  // URND data from PRNG
  output logic [WLEN-1:0] urnd_data_o,

  // Entropy distribution network (EDN)
  output logic                    edn_rnd_req_o,
  input  logic                    edn_rnd_ack_i,
  input  logic [EdnDataWidth-1:0] edn_rnd_data_i,

  output logic                    edn_urnd_req_o,
  input  logic                    edn_urnd_ack_i,
  input  logic [EdnDataWidth-1:0] edn_urnd_data_i
);

  logic rnd_valid_q, rnd_valid_d;
  logic [WLEN-1:0] rnd_data_q, rnd_data_d;
  logic rnd_data_en;
  logic rnd_req_complete;
  logic edn_rnd_req_complete;
  logic edn_rnd_req_start;

  logic edn_rnd_req_q, edn_rnd_req_d;

  ////////////////////////
  // RND Implementation //
  ////////////////////////

  assign rnd_req_complete = rnd_req_i & rnd_valid_o;
  assign edn_rnd_req_complete = edn_rnd_req_o & edn_rnd_ack_i;

  assign rnd_data_en = edn_rnd_req_complete;
  // RND becomes valid when EDN request completes and provides new bits. Valid is cleared when OTBN
  // reads RND
  assign rnd_valid_d = (rnd_valid_q & ~rnd_req_complete) | edn_rnd_req_complete;
  assign rnd_data_d = edn_rnd_data_i;

  // Start an EDN request when there is a prefetch or an attempt at reading RND when RND data is not
  // available. Signalling `edn_rnd_req_start` whilst there is an outstanding request has no effect
  // and is harmless.
  assign edn_rnd_req_start = rnd_prefetch_req_i | (rnd_req_i & ~rnd_valid_q);

  // Assert `edn_rnd_req_o` when a request is started and keep it asserted until the request is done
  assign edn_rnd_req_d = (edn_rnd_req_q | edn_rnd_req_start) & ~edn_rnd_req_complete;

  assign edn_rnd_req_o = edn_rnd_req_q;

  always_ff @(posedge clk_i) begin
    if (rnd_data_en) begin
      rnd_data_q <= rnd_data_d;
    end
  end

  always_ff @(posedge clk_i or negedge rst_ni) begin
    if (!rst_ni) begin
      rnd_valid_q   <= 1'b0;
      edn_rnd_req_q <= 1'b0;
    end else begin
      rnd_valid_q   <= rnd_valid_d;
      edn_rnd_req_q <= edn_rnd_req_d;
    end
  end

  assign rnd_valid_o = rnd_valid_q;
  assign rnd_data_o  = rnd_data_q;

  /////////////////////////
  // PRNG Implementation //
  /////////////////////////

  logic edn_urnd_req_complete;
  logic edn_urnd_req_q, edn_urnd_req_d;

  assign edn_urnd_req_complete = edn_urnd_req_o & edn_urnd_ack_i;
  assign edn_urnd_req_d = (edn_urnd_req_q | urnd_reseed_req_i) & ~edn_urnd_req_complete;

  assign edn_urnd_req_o = edn_urnd_req_q;
  assign urnd_reseed_busy_o = edn_urnd_req_q;

  always_ff @(posedge clk_i or negedge rst_ni) begin
    if (!rst_ni) begin
      edn_urnd_req_q <= 1'b0;
    end else begin
      edn_urnd_req_q <= edn_urnd_req_d;
    end
  end

  logic xoshiro_seed_en;

  assign xoshiro_seed_en = edn_urnd_req_complete;

  prim_xoshiro256pp #(
    .OutputDw   (WLEN),
    .DefaultSeed(RndCnstUrndPrngSeed)
  ) u_xoshiro256pp(
    .clk_i,
    .rst_ni,
    .seed_en_i    (xoshiro_seed_en),
    .seed_i       (edn_urnd_data_i),
    .xoshiro_en_i (urnd_advance_i),
    .entropy_i    ('0),
    .data_o       (urnd_data_o),
    .all_zero_o   ()
  );

  `ASSERT(rnd_clear_on_req_complete, rnd_req_complete |=> ~rnd_valid_q)
endmodule