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feat(v2): M4 Memory Manager + Prefetch Engine, real V1 PSRAM backend

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Implements M4: memory_manager.v (arbitration/buffering/forwarding/
latency hiding/double buffering, §12) + prefetch_engine.v
(double-buffered tile fetch, §13), sitting on the REAL, UNMODIFIED V1
PSRAM backend chain (int8_memory_access.v -> memory_interface.v ->
psram_controller.v, per §15's explicit mandate not to touch the
controller).

Verified fully end-to-end with Verilator: real neural_processor (M1)
fed entirely by memory_manager, computing against PSRAM-resident X/W
tiles (double-buffered prefetch across up to 5 tiles) and writing its
result back to PSRAM -- checked via an independent PSRAM read-back,
with poison bytes around the operand regions to catch addressing
errors. 3/3 jobs pass (1/3/5-tile configurations).

Three real RTL bugs found and fixed during integration (full
diagnostic trail in errors.log ERR-0006): prefetch_engine had no
single-in-flight-request discipline, letting a queued request corrupt
the bank bookkeeping of a fetch already running; the fix's own
!pf_busy guard had a one-cycle blind spot (pf_busy lags pf_start by a
clock) that needed an explicit !pf_start term; and a state-based mux
for the shared backend port was off by one cycle, silently dropping
the PSRAM result write entirely.

Real synthesis: 0 CHECK problems, 851 LUT4/789 FF/108 CCU2C/0 DSP
(expected, no multiplication in this module). Real place&route (via a
synthesis-only timing harness, needed for the same TRELLIS_IO pin-
budget reason as M2's array): Fmax 165.86 MHz, PASS at 80MHz.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013xXuuRUWZScuo1DeYJxs3v
This commit is contained in:
michele
2026-09-05 14:39:29 +02:00
co-authored by Claude Sonnet 5
parent 5f0d7f101c
commit 175f697ae1
20 changed files with 137935 additions and 2 deletions
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hardware/v2/rtl/prefetch_engine.v
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`timescale 1ns/1ps
// ================================================================
// FPGA-Neural V2 -- Prefetch Engine (M4, docs/v2-description.md §13)
//
// Fetches ONE tile (P_IN activation bytes + P_IN weight bytes) from
// the byte-level Memory Backend Interface into a pair of output
// registers, sequentially (2*P_IN single-byte transactions -- the
// same byte-at-a-time convention hardware/v1/rtl/neuron_memory.v
// already uses against the same backend, reused unmodified here).
//
// This module fetches exactly one tile per fetch_start pulse; the
// double-buffering strategy itself (§13: compute tile N while
// prefetching tile N+1, swap, repeat) is memory_manager.v's
// responsibility -- it retargets this single engine at whichever
// bank currently needs refilling, so no internal arbitration between
// multiple fetch engines sharing the backend port is ever needed.
//
// The backend port (mem_req/mem_wr/mem_addr/mem_wdata/mem_rdata/
// mem_ready) matches hardware/v1/rtl/int8_memory_access.v's contract
// exactly -- this engine can sit directly on top of that unmodified
// V1 module (which itself sits on memory_interface.v ->
// psram_controller.v, also unmodified, per §15).
// ================================================================
module prefetch_engine #(
parameter DATA_WIDTH = 8,
parameter P_IN = 8,
parameter ADDR_WIDTH = 23
)(
input wire clk,
input wire rst,
input wire fetch_start,
input wire [ADDR_WIDTH-1:0] x_addr, // base addr of this tile's P_IN X bytes
input wire [ADDR_WIDTH-1:0] w_addr, // base addr of this tile's P_IN W bytes
output reg fetch_busy,
output reg fetch_done, // one-cycle pulse
output reg signed [DATA_WIDTH*P_IN-1:0] tile_x,
output reg signed [DATA_WIDTH*P_IN-1:0] tile_w,
output reg mem_req,
output reg mem_wr,
output reg [ADDR_WIDTH-1:0] mem_addr,
output reg signed [7:0] mem_wdata,
input wire signed [7:0] mem_rdata,
input wire mem_ready
);
localparam ST_IDLE = 2'd0;
localparam ST_READ_X = 2'd1;
localparam ST_READ_W = 2'd2;
localparam ST_DONE = 2'd3;
reg [1:0] state;
reg [$clog2(P_IN+1)-1:0] byte_idx;
always @(posedge clk) begin
if (rst) begin
state <= ST_IDLE;
byte_idx <= 0;
fetch_busy <= 1'b0;
fetch_done <= 1'b0;
mem_req <= 1'b0;
mem_wr <= 1'b0;
mem_addr <= {ADDR_WIDTH{1'b0}};
mem_wdata <= 8'sd0;
end else begin
mem_req <= 1'b0;
fetch_done <= 1'b0;
case (state)
ST_IDLE: begin
if (fetch_start) begin
fetch_busy <= 1'b1;
byte_idx <= 0;
mem_req <= 1'b1;
mem_wr <= 1'b0;
mem_addr <= x_addr;
state <= ST_READ_X;
end
end
ST_READ_X: begin
if (mem_ready) begin
tile_x[byte_idx*DATA_WIDTH +: DATA_WIDTH] <= mem_rdata;
if (byte_idx == P_IN[$clog2(P_IN+1)-1:0] - 1'b1) begin
byte_idx <= 0;
mem_req <= 1'b1;
mem_wr <= 1'b0;
mem_addr <= w_addr;
state <= ST_READ_W;
end else begin
byte_idx <= byte_idx + 1'b1;
mem_req <= 1'b1;
mem_wr <= 1'b0;
mem_addr <= x_addr + byte_idx + 1'b1;
end
end
end
ST_READ_W: begin
if (mem_ready) begin
tile_w[byte_idx*DATA_WIDTH +: DATA_WIDTH] <= mem_rdata;
if (byte_idx == P_IN[$clog2(P_IN+1)-1:0] - 1'b1) begin
state <= ST_DONE;
end else begin
byte_idx <= byte_idx + 1'b1;
mem_req <= 1'b1;
mem_wr <= 1'b0;
mem_addr <= w_addr + byte_idx + 1'b1;
end
end
end
ST_DONE: begin
fetch_busy <= 1'b0;
fetch_done <= 1'b1;
state <= ST_IDLE;
end
default: state <= ST_IDLE;
endcase
end
end
endmodule