Add optional automatic frame skipping

This PR uses the `RETRO_ENVIRONMENT_SET_AUDIO_BUFFER_STATUS_CALLBACK`
environment to implement optional automatic frame skipping, based on
the implementation in snes9x2005.

libretro/libretro.c

@@ -249,6 +249,19 @@ static retro_input_state_t		input_cb = NULL;
 static retro_audio_sample_batch_t			audio_batch_cb = NULL;
 static retro_environment_t		environ_cb = NULL;
 
+static unsigned frameskip_type = 0;
+static unsigned frameskip_threshold = 0;
+static uint16_t frameskip_counter = 0;
+static bool retro_audio_buff_active = false;
+static unsigned retro_audio_buff_occupancy = 0;
+static bool retro_audio_buff_underrun = false;
+
+/* Maximum number of consecutive frames that can be skipped */
+#define FRAMESKIP_MAX 30
+
+static unsigned retro_audio_latency = 0;
+static bool update_audio_latency = false;
+
 extern s9xcommand_t keymap[1024];
 bool overclock_cycles = false;
 bool reduce_sprite_flicker = false;
@@ -279,15 +292,69 @@ static const uint32_t MAX_SNES_WIDTH_NTSC = ((SNES_NTSC_OUT_WIDTH(256) + 3) / 4)
 
 static void update_geometry();
 
+static void retro_audio_buff_status_cb(bool active, unsigned occupancy, bool underrun_likely)
+{
+	retro_audio_buff_active = active;
+	retro_audio_buff_occupancy = occupancy;
+	retro_audio_buff_underrun = underrun_likely;
+}
+
+static void retro_set_audio_buff_status_cb(void) {
+	if (frameskip_type > 0)
+	{
+		struct retro_audio_buffer_status_callback buf_status_cb;
+		buf_status_cb.callback = retro_audio_buff_status_cb;
+
+		if (!environ_cb(RETRO_ENVIRONMENT_SET_AUDIO_BUFFER_STATUS_CALLBACK, &buf_status_cb))
+		{
+			if (log_cb)
+				log_cb(RETRO_LOG_WARN, "Frameskip disabled - frontend does not support audio buffer status monitoring.\n");
+
+			retro_audio_buff_active = false;
+			retro_audio_buff_occupancy = 0;
+			retro_audio_buff_underrun = false;
+			retro_audio_latency = 0;
+		}
+		else
+		{
+			/* Frameskip is enabled - increase frontend audio latency to
+			 * minimise potential buffer underruns */
+			uint32_t frame_time_usec = Settings.FrameTimeNTSC;
+
+			if (Settings.PAL)
+				frame_time_usec = Settings.FrameTimePAL;
+
+			/* Set latency to 6x current frame time... */
+			retro_audio_latency = (unsigned)(6 * frame_time_usec / 1000);
 
-static void check_variables(void)
+			/* ...then round up to nearest multiple of 32 */
+			retro_audio_latency = (retro_audio_latency + 0x1F) & ~0x1F;
+		}
+	}
+	else
+	{
+		environ_cb(RETRO_ENVIRONMENT_SET_AUDIO_BUFFER_STATUS_CALLBACK, NULL);
+		retro_audio_latency = 0;
+	}
+
+	update_audio_latency = true;
+}
+
+static void check_variables(bool first_run)
 {
 	bool reset_sfx = false;
 	int old_filter = snes_ntsc_filter;
 	struct retro_variable var;
+	bool prev_force_ntsc;
+	bool prev_force_pal;
+	bool prev_frameskip_type;
 
 	var.key = "snes9x_2010_region";
 	var.value = NULL;
+
+	prev_force_ntsc = Settings.ForceNTSC;
+	prev_force_pal = Settings.ForcePAL;
+
 	if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
 	{
 		if (strcmp(var.value, "auto") == 0)
@@ -309,6 +376,28 @@ static void check_variables(void)
 		}
 	}
 
+	var.key = "snes9x_2010_frameskip";
+	var.value = NULL;
+
+	prev_frameskip_type = frameskip_type;
+	frameskip_type = 0;
+
+	if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
+	{
+		if (strcmp(var.value, "auto") == 0)
+			frameskip_type = 1;
+		else if (strcmp(var.value, "manual") == 0)
+			frameskip_type = 2;
+	}
+
+	var.key = "snes9x_2010_frameskip_threshold";
+	var.value = NULL;
+
+	frameskip_threshold = 33;
+
+	if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
+		frameskip_threshold = strtol(var.value, NULL, 10);
+
 	var.key = "snes9x_2010_aspect";
 	var.value = NULL;
 	if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
@@ -435,6 +524,13 @@ static void check_variables(void)
 		else
 			reduce_sprite_flicker = false;
 	}
+	/* Reinitialise frameskipping, if required */
+	if (!first_run &&
+	    ((frameskip_type     != prev_frameskip_type) ||
+	     (Settings.ForceNTSC != prev_force_ntsc)     ||
+	     (Settings.ForcePAL  != prev_force_pal)))
+      retro_set_audio_buff_status_cb();
+
 //
 	if (old_filter != snes_ntsc_filter)
 		snes_ntsc_init(&snes_ntsc, &ntsc_setup);
@@ -873,7 +969,16 @@ void retro_deinit(void)
 	free(GFX.Screen);
 	free(ntsc_screen_buffer);
 #endif
+	/* Reset globals (required for static builds) */
 	libretro_supports_bitmasks = false;
+	frameskip_type             = 0;
+	frameskip_threshold        = 0;
+	frameskip_counter          = 0;
+	retro_audio_buff_active    = false;
+	retro_audio_buff_occupancy = 0;
+	retro_audio_buff_underrun  = false;
+	retro_audio_latency        = 0;
+	update_audio_latency       = false;
 }
 
 void retro_reset (void)
@@ -1068,7 +1173,7 @@ void retro_run(void)
 
 	if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE_UPDATE, &updated) && updated)
 	{
-		check_variables();
+		check_variables(false);
 		update_geometry();
 	}
 
@@ -1090,6 +1195,49 @@ void retro_run(void)
 		Settings.HardDisableAudio = false;
 	}
 
+	if ((frameskip_type > 0) &&
+	    retro_audio_buff_active &&
+	    IPPU.RenderThisFrame)
+	{
+		bool skip_frame;
+		switch (frameskip_type)
+		{
+			case 1: /* auto */
+				skip_frame = retro_audio_buff_underrun;
+				break;
+			case 2: /* manual */
+				skip_frame = (retro_audio_buff_occupancy < frameskip_threshold);
+				break;
+			default:
+				skip_frame = false;
+				break;
+		}
+
+		if (skip_frame)
+		{
+			if (frameskip_counter < FRAMESKIP_MAX)
+			{
+				IPPU.RenderThisFrame = false;
+				frameskip_counter++;
+			}
+			else
+				frameskip_counter = 0;
+		}
+	}
+	else
+		frameskip_counter = 0;
+
+	/* If frameskip/timing settings have changed,
+	 * update frontend audio latency
+	 * > Can do this before or after the frameskip
+	 *   check, but doing it after means we at least
+	 *   retain the current frame's audio output */
+	if (update_audio_latency)
+	{
+		environ_cb(RETRO_ENVIRONMENT_SET_MINIMUM_AUDIO_LATENCY, &retro_audio_latency);
+		update_audio_latency = false;
+  }
+
 	poll_cb();
 	report_buttons();
 
@@ -1288,8 +1436,9 @@ bool retro_load_game(const struct retro_game_info *game)
 		return FALSE;
 	}
 
-	check_variables();
+	check_variables(true);
 
+	retro_set_audio_buff_status_cb();
 	set_system_specs();
 	environ_cb(RETRO_ENVIRONMENT_SET_MEMORY_MAPS, &map);
 
@@ -1316,7 +1465,9 @@ void S9xDeinitUpdate(int width, int height)
 {
 	static int burst_phase = 0;
 
-	if (snes_ntsc_filter)
+	if (!IPPU.RenderThisFrame)
+		video_cb(NULL, width, height, GFX.Pitch);
+	else if (snes_ntsc_filter)
 	{
 		burst_phase = (burst_phase + 1) % 3;
 

libretro/libretro_core_options.h

@@ -143,6 +143,43 @@ struct retro_core_option_definition option_defs_us[] = {
       },
       "enabled"
    },
+   {
+      "snes9x_2010_frameskip",
+      "Frameskip",
+      "Skip frames to avoid audio buffer under-run (crackling). Improves performance at the expense of visual smoothness. 'Auto' skips frames when advised by the frontend. 'Manual' utilises the 'Frameskip Threshold (%)' setting.",
+      {
+         { "disabled", NULL },
+         { "auto",     "Auto" },
+         { "manual",   "Manual" },
+         { NULL, NULL},
+      },
+      "disabled"
+   },
+   {
+      "snes9x_2010_frameskip_threshold",
+      "Frameskip Threshold (%)",
+      "When 'Frameskip' is set to 'Manual', specifies the audio buffer occupancy threshold (percentage) below which frames will be skipped. Higher values reduce the risk of crackling by causing frames to be dropped more frequently.",
+      {
+         { "15", NULL },
+         { "18", NULL },
+         { "21", NULL },
+         { "24", NULL },
+         { "27", NULL },
+         { "30", NULL },
+         { "33", NULL },
+         { "36", NULL },
+         { "39", NULL },
+         { "42", NULL },
+         { "45", NULL },
+         { "48", NULL },
+         { "51", NULL },
+         { "54", NULL },
+         { "57", NULL },
+         { "60", NULL },
+         { NULL, NULL },
+      },
+      "33"
+   },
 
    { NULL, NULL, NULL, {{0}}, NULL },
 };

src/cpuexec.c

@@ -427,17 +427,11 @@ static void S9xEndScreenRefresh (void)
 	if (IPPU.RenderThisFrame)
 	{
 		FLUSH_REDRAW();
-
-		if (!(GFX.DoInterlace && GFX.InterlaceFrame == 0))
-		{
-			S9xDeinitUpdate(IPPU.RenderedScreenWidth, IPPU.RenderedScreenHeight);
-#ifdef LAGFIX
-			finishedFrame = true;
-#endif
-		}
 	}
-	else
+
+	if (!(GFX.DoInterlace && GFX.InterlaceFrame == 0))
 	{
+		S9xDeinitUpdate(IPPU.RenderedScreenWidth, IPPU.RenderedScreenHeight);
 #ifdef LAGFIX
 		finishedFrame = true;
 #endif