retroDE_ps2/sim/tb/gif_gs/tb_gs_scanout_restart.sv

// retroDE_ps2 — tb_gs_scanout_restart (Ch443c)
//
// Focused test for the frame-restart latency repair in gs_lpddr_scanout_lb's AXI
// prefetch FSM. On a vsync (frame_start) that lands MID-ROW, the FSM must, after the
// current single-beat response is accepted, abandon the remainder of the now-obsolete
// row (no next old-row AR, no publish/increment) and restart the prefetch at
// V_SOURCE_START. Proves:
//   (1) the in-flight response is accepted normally (no deadlock);
//   (2) NO further old-row AR issues after the restart trigger;
//   (3) the restart's first AR targets row V_SOURCE_START (=32);
//   (4) rows 32 and 33 become resident before the first active (in_window) consumption;
//   (5) no post-restart underflow on the displayed frame.
`timescale 1ns/1ps

module tb_gs_scanout_restart;
    logic emif_clk = 0; always #2.5 emif_clk = ~emif_clk;   // 200 MHz
    logic video_clk = 0; always #4  video_clk = ~video_clk; // 125 MHz
    logic rst_n;

    localparam int N_ROWS = 48, VSS = 32, ROW_BEATS = 2, STRIDE = 64;
    localparam int ROW32_ADDR = VSS * STRIDE;      // 2048
    localparam int ROW33_ADDR = (VSS+1) * STRIDE;  // 2112

    logic        enable, frame_start, in_window;
    logic [11:0] pixel_x, pixel_y;
    logic [7:0]  r,g,b; logic line_valid, underflow; logic [31:0] rd_errs;
    logic        diag_rderr_nz, diag_valid; logic [29:0] diag_first; logic [6:0] diag_stat;
    logic [29:0] araddr; logic [1:0] arburst; logic [6:0] arid;
    logic [7:0]  arlen; logic [2:0] arsize; logic arvalid, arready;
    logic [255:0] rdata; logic [1:0] rresp; logic rlast, rvalid, rready;

    gs_lpddr_scanout_lb #(.FB_BASE(30'd0), .STRIDE_BYTES(STRIDE), .ROW_BEATS(ROW_BEATS),
                          .N_ROWS(N_ROWS), .PSMCT32(1'b1), .V_SOURCE_START(VSS)) dut (
        .axi_clk(emif_clk), .axi_rst_n(rst_n), .enable(enable),
        .video_clk(video_clk), .frame_start(frame_start),
        .pixel_x(pixel_x), .pixel_y(pixel_y), .in_window(in_window),
        .r(r), .g(g), .b(b), .line_valid(line_valid), .underflow(underflow), .rd_errs(rd_errs),
        .diag_rderr_nz(diag_rderr_nz), .diag_valid(diag_valid), .diag_first(diag_first), .diag_stat(diag_stat),
        .araddr(araddr), .arburst(arburst), .arid(arid), .arlen(arlen), .arsize(arsize),
        .arvalid(arvalid), .arready(arready), .rdata(rdata), .rresp(rresp),
        .rlast(rlast), .rvalid(rvalid), .rready(rready)
    );

    // behavioral EMIF responder: grant AR, return OKAY R one cycle later (fast).
    logic pend; logic [29:0] pend_addr;
    assign arready = arvalid && !pend && !rvalid;
    always_ff @(posedge emif_clk or negedge rst_n) begin
        if (!rst_n) begin pend<=0; rvalid<=0; rlast<=0; rresp<=0; rdata<=0; end
        else begin
            if (arready) begin pend<=1; pend_addr<=araddr; end
            if (pend && !rvalid) begin rvalid<=1; rlast<=1; rresp<=0; rdata<={8{2'b01,pend_addr}}; pend<=0; end
            else if (rvalid && rready) begin rvalid<=0; rlast<=0; end
        end
    end

    int errors = 0;
    task automatic chk(input string s, input logic c);
        if (!c) begin $error("[restart] FAIL: %s", s); errors++; end
        else     $display("[restart]  ok : %s", s);
    endtask

    // ---- monitors (emif domain) ----
    // capture, at the restart trigger, whether we were mid-fetch and which row.
    logic        restart_seen, restart_midfetch;
    logic [$clog2(N_ROWS):0] row_at_restart;
    // track the FIRST araddr issued (arvalid rising) AFTER the restart trigger.
    logic        arv_prev, armed_first_ar, got_first_ar;
    logic [29:0] first_ar_after_restart;
    // flag ANY old-row AR after restart (araddr not equal to a fresh row-32-onwards fetch
    // before we've seen the row-32 restart AR).
    always_ff @(posedge emif_clk or negedge rst_n) begin
        if (!rst_n) begin
            restart_seen<=0; restart_midfetch<=0; row_at_restart<=0;
            arv_prev<=0; armed_first_ar<=0; got_first_ar<=0; first_ar_after_restart<=0;
        end else begin
            arv_prev <= arvalid;
            // restart trigger = fs_edge_e inside the DUT
            // lstate_t = { L_IDLE=0, L_AR=1, L_R=2, L_C=3 }
            if (dut.fs_edge_e && !restart_seen) begin
                restart_seen     <= 1'b1;
                restart_midfetch <= (dut.lst == 2'd1) || (dut.lst == 2'd2);  // L_AR or L_R
                row_at_restart   <= dut.cur_row;
                armed_first_ar   <= 1'b1;
            end
            // first arvalid rising edge after the restart trigger
            if (armed_first_ar && arvalid && !arv_prev && !got_first_ar) begin
                got_first_ar          <= 1'b1;
                first_ar_after_restart<= araddr;
            end
        end
    end

    task automatic pulse_fs();
        @(posedge video_clk) frame_start <= 1'b1;
        repeat (3) @(posedge video_clk); frame_start <= 1'b0;
        repeat (2) @(posedge video_clk);
    endtask

    initial begin
        rst_n=0; enable=0; frame_start=0; in_window=0; pixel_x=0; pixel_y=VSS[11:0];
        repeat (8) @(posedge emif_clk); rst_n=1;
        repeat (6) @(posedge emif_clk);

        // ---- warm the prefetch: enable, first frame, advance disp_row so rows load ----
        enable=1;
        pulse_fs();
        // climb pixel_y from VSS upward (in_window=0 -> no underflow) so the prefetch
        // actively fetches rows 32,33,34,...  Inject the mid-row restart partway through.
        for (int y=VSS; y<VSS+8; y++) begin
            @(posedge video_clk) pixel_y <= y[11:0];
            repeat (6) @(posedge video_clk);
            if (y == VSS+4) begin
                // MID-ROW vsync: assert frame_start while the FSM is actively fetching.
                @(posedge video_clk) frame_start <= 1'b1;
                repeat (3) @(posedge video_clk); frame_start <= 1'b0;
            end
        end
        repeat (40) @(posedge emif_clk);

        // ---- (1)+(2)+(3): restart abandoned the old row and re-fetched from row 32 ----
        chk("restart trigger observed",            restart_seen === 1'b1);
        chk("restart landed mid-fetch (L_AR/L_R)",  restart_midfetch === 1'b1);
        chk("first AR after restart = row 32",      first_ar_after_restart === 30'(ROW32_ADDR));

        // ---- (4): rows 32 and 33 resident before active consumption ----
        // advance disp_row to let the restarted prefetch reach next_fetch >= 34.
        for (int y=VSS; y<VSS+4; y++) begin
            @(posedge video_clk) pixel_y <= y[11:0];
            repeat (10) @(posedge video_clk);
        end
        repeat (40) @(posedge emif_clk);
        chk("next_fetch advanced past rows 32,33 (>=34)", dut.next_fetch >= ($clog2(N_ROWS)+1)'(VSS+2));
        chk("line_valid set (rows resident)",              line_valid === 1'b1);

        // ---- (5): now consume actively at row 32 -> NO underflow ----
        @(posedge video_clk) begin in_window <= 1'b1; pixel_y <= VSS[11:0]; pixel_x <= 12'd0; end
        repeat (60) @(posedge video_clk);
        chk("no post-restart underflow at row 32", underflow === 1'b0);

        if (errors==0) $display("[tb_gs_scanout_restart] PASS");
        else           $display("[tb_gs_scanout_restart] FAIL (%0d errors)", errors);
        $finish;
    end

    initial begin #400000; $error("[tb_gs_scanout_restart] TIMEOUT"); $finish; end
endmodule : tb_gs_scanout_restart