retroDE_ps2/rtl/gif_gs/gs_lpddr_fb_writer.sv

// retroDE_ps2 — gs_lpddr_fb_writer (Ch317)
//
// FIRST LPDDR-backed-framebuffer step: a write sink that takes the GS tile-FLUSH
// pixel stream (PSMCT16, one pixel per emit) and commits it to an LPDDR-style
// framebuffer, modelling the real EMIF AXI4 write path so the addressing / data /
// stride / burst behaviour can be proven in sim before wiring the hard EMIF.
//
// SCOPE (Ch317, deliberately tight — see doc 0010 Ch317):
//   * Tile color/Z stay ON-CHIP; texture stays local. ONLY the framebuffer FLUSH
//     is redirected here.
//   * Address gen is the simple linear `fb_base + (screen_y*pitch + screen_x)*bpp`
//     — which the GS already produces on `raster_pixel_fb_addr_q` for PSMCT16
//     ((fbp<<11) + (pixel_index<<1)), so we consume that byte address directly.
//   * PSMCT16 (2 bytes/pixel) — lower bandwidth, already-proven format.
//   * BURSTS: the flush emits a tile-row's 16 pixels at contiguous +2 byte
//     addresses, then jumps by `pitch` to the next row. The burst engine COALESCES
//     a contiguous +2 run into one burst, capped at MAX_BURST_BYTES (the doc 0008
//     4 KiB-boundary AXI rule). Real per-tile-row burst = 16 beats = 32 bytes.
//   * A staging FIFO decouples the 1-pixel/cycle emit from the burst engine and
//     surfaces under/overflow — the realistic shape a hard EMIF AXI master needs.
//   * Backing memory `fbmem` is byte-addressed and TB-readable for the
//     write/readback PROOF (a later rung swaps it for the EMIF AXI master +
//     LPDDR scanout). At enable=0 the whole module is inert (no writes, counters 0).
//
// COUNTERS (Codex acceptance — bandwidth/diag): bytes_written, burst_count,
// busy_cycles (engine draining), fifo_overflow/underflow, fifo_occ_max. The TB
// computes effective GB/s off bytes_written / (busy_cycles * clk_period).

module gs_lpddr_fb_writer #(
    parameter int FB_BYTES        = 8192,   // backing FB size (64x64 PSMCT16 = 8 KiB)
    parameter int FIFO_DEPTH      = 32,     // pixel staging FIFO depth (power-of-2)
    parameter int MAX_BURST_BYTES = 4096    // AXI4 4 KiB-boundary cap (doc 0008 lesson)
) (
    input  logic        clk,
    input  logic        rst_n,
    input  logic        enable,             // LPDDR_FB_ENABLE; 0 → fully inert

    // GS tile-flush pixel stream (PSMCT16, one pixel per emit)
    input  logic        px_emit,
    input  logic [31:0] px_addr,            // linear FB byte address (raster_pixel_fb_addr_q)
    input  logic [15:0] px_pix16,           // raster_pixel_color_q[15:0]

    // diagnostics / proof (read hierarchically by the TB; no functional consumers)
    output logic [31:0] bytes_written,
    output logic [31:0] burst_count,
    output logic [31:0] busy_cycles,
    output logic [31:0] fifo_overflow_count,
    output logic [31:0] fifo_underflow_count,
    output logic [15:0] fifo_occ
);
    localparam int ADDR_W = $clog2(FB_BYTES);
    localparam int PTR_W  = (FIFO_DEPTH > 1) ? $clog2(FIFO_DEPTH) : 1;

    // ---- byte-addressed backing framebuffer (the LPDDR model) ----
    logic [7:0] fbmem [0:FB_BYTES-1];

    // ---- staging FIFO of {addr, pix16} ----
    logic [31:0] fifo_addr [0:FIFO_DEPTH-1];
    logic [15:0] fifo_pix  [0:FIFO_DEPTH-1];
    logic [PTR_W-1:0] wptr, rptr;
    logic [PTR_W:0]   count;                 // 0..FIFO_DEPTH (PTR_W+1 bits)
    // count==FIFO_DEPTH sets the top bit (FIFO_DEPTH is a power of 2 == 1<<PTR_W),
    // so count[PTR_W] alone is the full flag. (Do NOT compare against a PTR_W-wide
    // literal — PTR_W'(FIFO_DEPTH) truncates FIFO_DEPTH to 0 and reads empty as full.)
    wire  fifo_full  = count[PTR_W];
    wire  fifo_empty = (count == '0);

    // ---- burst engine state (coalesce contiguous +2 runs) ----
    logic        in_burst;                   // currently extending a burst
    logic [31:0] last_addr;                  // last byte address written
    logic [31:0] burst_bytes;                // bytes in the current burst so far

    logic              do_push, do_pop;
    logic [31:0]       a;          // popped byte address
    logic [15:0]       p;          // popped pixel
    logic              contig;
    always_comb begin
        do_push = px_emit && !fifo_full;
        do_pop  = !fifo_empty;                 // drain one entry/cycle when available
        a       = fifo_addr[rptr];
        p       = fifo_pix [rptr];
        contig  = in_burst && (a == last_addr + 32'd2)
                            && (burst_bytes + 32'd2 <= 32'(MAX_BURST_BYTES));
    end

    always_ff @(posedge clk or negedge rst_n) begin
        if (!rst_n) begin
            wptr <= '0; rptr <= '0; count <= '0;
            in_burst <= 1'b0; last_addr <= '0; burst_bytes <= '0;
            bytes_written <= '0; burst_count <= '0; busy_cycles <= '0;
            fifo_overflow_count <= '0; fifo_underflow_count <= '0; fifo_occ <= '0;
        end else if (enable) begin
            // ---- push side: one flushed pixel per emit ----
            if (px_emit && fifo_full)
                fifo_overflow_count <= fifo_overflow_count + 32'd1;  // dropped — proof must show 0
            if (do_push) begin
                fifo_addr[wptr] <= px_addr;
                fifo_pix [wptr] <= px_pix16;
                wptr <= wptr + PTR_W'(1);
            end

            // ---- drain side: pop one entry/cycle, commit fbmem, coalesce bursts ----
            if (do_pop) begin
                // commit the 2 PSMCT16 bytes at the linear address
                if (a < 32'(FB_BYTES))       fbmem[a[ADDR_W-1:0]]           <= p[7:0];
                if ((a + 1) < 32'(FB_BYTES)) fbmem[a[ADDR_W-1:0] + 1'b1]    <= p[15:8];
                bytes_written <= bytes_written + 32'd2;
                busy_cycles   <= busy_cycles + 32'd1;
                rptr <= rptr + PTR_W'(1);
                if (contig)
                    burst_bytes <= burst_bytes + 32'd2;     // extend current burst
                else begin
                    burst_count <= burst_count + 32'd1;     // start a NEW burst
                    burst_bytes <= 32'd2;
                end
                in_burst  <= 1'b1;
                last_addr <= a;
            end else if (in_burst) begin
                in_burst <= 1'b0;                            // FIFO drained → close burst
            end

            // single count update (push and pop net correctly)
            if (do_push && !do_pop)      count <= count + 1'b1;
            else if (!do_push && do_pop) count <= count - 1'b1;
            // (both or neither → unchanged)

            if (16'(count) > fifo_occ) fifo_occ <= 16'(count);
            // fifo_underflow_count: the engine never pops empty (do_pop gated on
            // !fifo_empty), so it stays 0 here — surfaced for the future EMIF rung
            // where an external AXI master could request beyond the staged data.
        end
    end
endmodule : gs_lpddr_fb_writer