fix: show snow when no frontend owns the screen

README.md

@@ -7,8 +7,11 @@ the core itself: 352x240p60 (NTSC) by default, with 720x480i60 and 352x288p50
 (PAL) selectable by the ARM-side launcher through a DDR control block (see
 [DDR contract](rtl/native_video_reader.sv)). There are no video options in the OSD — the
 mode and the H/V centering trims are owned by the launcher. Idle video stays
-black during frontend handoff; upstream-style snow appears behind the stock
-OSD. Native frontend frames take priority over both backgrounds.
+black during frontend handoff. Upstream-style snow appears behind the stock
+OSD, and whenever Main sets `status[10]` to report that no frontend owns or
+is about to take the screen (frontend off, quit, or given up), so a setup
+with no frontend and no OSD, such as kiosk mode, still shows snow. Native
+frontend frames take priority over both backgrounds.
 
 Native writers must use the v2 magic and publish a live frame counter. The
 legacy 320-pixel, no-magic DDR contract is no longer accepted.

files.qip

@@ -13,3 +13,4 @@ set_global_assignment -name SYSTEMVERILOG_FILE sys/mister_magik_bootstrap_black.
 set_global_assignment -name SYSTEMVERILOG_FILE rtl/zaparoo_bootstrap_video.sv
 set_global_assignment -name SYSTEMVERILOG_FILE rtl/zaparoo_snow_phase.sv
 set_global_assignment -name SYSTEMVERILOG_FILE rtl/zaparoo_pixel_enable.sv
+set_global_assignment -name SYSTEMVERILOG_FILE rtl/zaparoo_snow_enable.sv

menu.sv

@@ -320,6 +320,8 @@ end
 
 wire FB  = status[5];
 wire [2:0] led = status[8:6];
+// Set by Main while no frontend owns or is about to take the screen.
+wire snow_allowed = status[10];
 
 // Pixel clock: CLK_VIDEO = 27.000 MHz (the universal SD video clock).
 // ce_pix /4 = 6.75 MHz gives exactly 15734.27 Hz (NTSC, 429-px line) and
@@ -402,26 +404,27 @@ zaparoo_snow_phase snow_motion (
 	.new_frame(native_new_frame), .vcount(native_vcount), .phase(snow_phase)
 );
 
-// OSD status comes from the HDMI domain. Main disables OSD before handing
-// video to the frontend, so startup stays black without a new bus command.
-(* altera_attribute = "-name SYNCHRONIZER_IDENTIFICATION FORCED_IF_ASYNCHRONOUS" *)
-reg [1:0] snow_osd_sync = 0;
 always @(posedge CLK_VIDEO) begin
-	if (RESET | ~vid_locked) begin
-		snow_sample <= 0;
-		snow_osd_sync <= 0;
-	end else begin
-		if (ce_pix) snow_sample <= snow_random[2:0];
-		snow_osd_sync <= {snow_osd_sync[0], OSD_STATUS};
-	end
+	if (RESET | ~vid_locked) snow_sample <= 0;
+	else if (ce_pix) snow_sample <= snow_random[2:0];
 end
 
-// Black handoff, snow behind stock OSD, or native frontend RGB. All three
-// share the same native sync/DE; no video-mode switch is needed.
+// Snow shows behind the stock OSD, and whenever Main reports that no frontend
+// will draw. Main disables the OSD and leaves status[10] clear before handing
+// video to the frontend, so that startup stays black.
+wire show_snow;
+zaparoo_snow_enable snow_enable (
+	.clk(CLK_VIDEO), .reset(RESET | ~vid_locked),
+	.osd_status(OSD_STATUS), .snow_allowed(snow_allowed),
+	.show_snow(show_snow)
+);
+
+// Black handoff, snow, or native frontend RGB. All three share the same
+// native sync/DE; no video-mode switch is needed.
 zaparoo_bootstrap_video bootstrap_video (
 	.native_active(native_active),
 	.native_rgb({native_r, native_g, native_b}),
-	.show_snow(snow_osd_sync[1]),
+	.show_snow(show_snow),
 	.snow_rgb({snow_pixel, snow_pixel, snow_pixel}),
 	.de_in(native_de), .hs_in(native_hs), .vs_in(native_vs),
 	.rgb_out({VGA_R, VGA_G, VGA_B}),

rtl/zaparoo_bootstrap_video.sv

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
-// Native CRT frames take priority; idle stays black except behind stock OSD.
+// Native CRT frames take priority; idle is black unless show_snow is set
+// (see zaparoo_snow_enable).
 `timescale 1ns/1ps
 module zaparoo_bootstrap_video (
     input wire native_active,

rtl/zaparoo_snow_enable.sv

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Decides when idle video shows snow instead of handoff black. Both inputs
+// come from other clock domains and are synchronized here.
+//   osd_status:   the stock OSD is up, which always sits on snow.
+//   snow_allowed: Main reports that no frontend owns or is about to take the
+//                 screen. Clear at reset, so startup stays black until Main
+//                 says otherwise, and under a Main that never sets it.
+`timescale 1ns/1ps
+module zaparoo_snow_enable (
+    input wire clk,
+    input wire reset,
+    input wire osd_status,
+    input wire snow_allowed,
+    output wire show_snow
+);
+    (* altera_attribute = "-name SYNCHRONIZER_IDENTIFICATION FORCED_IF_ASYNCHRONOUS" *)
+    reg [1:0] osd_sync = 0;
+    (* altera_attribute = "-name SYNCHRONIZER_IDENTIFICATION FORCED_IF_ASYNCHRONOUS" *)
+    reg [1:0] allowed_sync = 0;
+    always @(posedge clk) begin
+        if (reset) begin
+            osd_sync <= 0;
+            allowed_sync <= 0;
+        end
+        else begin
+            osd_sync <= {osd_sync[0], osd_status};
+            allowed_sync <= {allowed_sync[0], snow_allowed};
+        end
+    end
+    assign show_snow = osd_sync[1] | allowed_sync[1];
+endmodule

tb/bootstrap_video_tb.sv

@@ -29,14 +29,14 @@ module bootstrap_video_tb;
         native_active = 0; #1;
         assert(rgb_out == 0) else $fatal(1, "writer stop did not restore black");
         show_snow = 1; de_in = 1; #1;
-        assert(rgb_out == snow_rgb) else $fatal(1, "OSD did not reveal snow");
+        assert(rgb_out == snow_rgb) else $fatal(1, "show_snow did not reveal snow");
         de_in = 0; #1;
         assert(rgb_out == 0) else $fatal(1, "snow painted outside DE");
         native_active = 1; #1;
         assert(rgb_out == native_rgb) else $fatal(1, "snow covered native frontend");
         native_active = 0; show_snow = 0; de_in = 1; #1;
-        assert(rgb_out == 0) else $fatal(1, "OSD close did not restore black");
-        $display("PASS: black handoff, OSD snow, native CRT priority and timing");
+        assert(rgb_out == 0) else $fatal(1, "clearing show_snow did not restore black");
+        $display("PASS: black handoff, snow, native CRT priority and timing");
         $finish;
     end
 endmodule

tb/run.sh

@@ -19,6 +19,8 @@ run_test bootstrap_video_tb \
 
 run_test snow_phase_tb ../rtl/zaparoo_snow_phase.sv snow_phase_tb.sv
 
+run_test snow_enable_tb ../rtl/zaparoo_snow_enable.sv snow_enable_tb.sv
+
 run_test pixel_enable_tb ../rtl/zaparoo_pixel_enable.sv pixel_enable_tb.sv
 
 run_test scanout_tb \

tb/snow_enable_tb.sv

@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+`timescale 1ns/1ps
+module snow_enable_tb;
+    reg clk = 0;
+    always #5 clk = ~clk;
+    reg reset = 1;
+    reg osd_status = 0;
+    reg snow_allowed = 0;
+    wire show_snow;
+    zaparoo_snow_enable dut (.*);
+
+    // Two synchronizer stages: an input change is visible after two edges.
+    task settle;
+        begin
+            repeat (2) @(posedge clk);
+            #1;
+        end
+    endtask
+
+    initial begin
+        // Reset wins over both sources, so a video restart begins black.
+        osd_status = 1; snow_allowed = 1;
+        repeat (4) begin
+            @(posedge clk); #1;
+            assert(show_snow === 1'b0) else $fatal(1, "snow during reset");
+        end
+        @(negedge clk); reset = 0; osd_status = 0; snow_allowed = 0;
+        settle;
+        assert(show_snow === 1'b0) else $fatal(1, "idle did not start black");
+
+        // Every combination: either source reveals snow, neither keeps black.
+        for (integer i = 0; i < 4; i = i + 1) begin
+            @(negedge clk); {osd_status, snow_allowed} = i[1:0];
+            @(posedge clk); #1;
+            settle;
+            assert(show_snow === (osd_status | snow_allowed))
+                else $fatal(1, "snow source mismatch: osd=%0d allowed=%0d", osd_status, snow_allowed);
+        end
+
+        // No frontend and no OSD (kiosk): snow stays up across an OSD cycle.
+        @(negedge clk); osd_status = 0; snow_allowed = 1;
+        settle;
+        assert(show_snow === 1'b1) else $fatal(1, "no snow without frontend or OSD");
+        @(negedge clk); osd_status = 1;
+        settle;
+        @(negedge clk); osd_status = 0;
+        repeat (4) begin
+            @(posedge clk); #1;
+            assert(show_snow === 1'b1) else $fatal(1, "OSD close dropped allowed snow");
+        end
+
+        // A frontend taking the screen returns to black within the sync depth.
+        @(negedge clk); snow_allowed = 0;
+        settle;
+        assert(show_snow === 1'b0) else $fatal(1, "frontend takeover did not restore black");
+
+        // One edge after a change the output still holds: no path skips a stage.
+        @(negedge clk); snow_allowed = 1;
+        @(posedge clk); #1;
+        assert(show_snow === 1'b0) else $fatal(1, "snow bypassed the synchronizer");
+
+        $display("PASS: snow enable reset, OSD and no-frontend sources, takeover black");
+        $finish;
+    end
+    initial begin
+        #20000; $fatal(1, "snow enable test timeout");
+    end
+endmodule